JP7508346B2 - Method for controlling lithium-ion secondary battery - Google Patents

Description

The present invention relates to a control method for determining the SOC usage range of a lithium-ion secondary battery.

Conventionally, a charge/discharge control method is known that changes the voltage range for charging and discharging depending on the deterioration state of a lithium-ion secondary battery (Patent Document 1). In the charge/discharge control method described in Patent Document 1, battery information related to the charge/discharge state of a lithium-ion battery is acquired, and a Q-dV/dQ curve showing the relationship between the discharge capacity Q of the lithium-ion battery and dV/dQ showing the ratio of the change amount dV of the battery voltage V to the change amount dQ of the discharge capacity Q is calculated based on the battery information, and the deterioration state of the lithium-ion battery is judged based on the Q-dV/dQ curve. Then, based on the result of the judgment of the deterioration state, the voltage range for charging and discharging the lithium-ion battery is changed.

International Publication No. 2015/025402

However, the above charge/discharge control method requires discharging at a low rate to obtain the Q-dV/dQ curve, so there is a problem in that the voltage range for charging and discharging cannot be changed unless a long discharge time can be secured.

The problem that this invention aims to solve is to provide a control method that can determine the SOC usage range of a lithium-ion secondary battery, not just in situations where long charge/discharge times can be ensured.

The present invention solves the above problem by measuring the voltage response to an AC current input or the current response to an AC voltage input when charging or discharging is stopped, and determining the usable range (SOC usable range) of the state of charge (SOC) of a lithium-ion secondary battery based on the difference in response between the discharge voltage response, which is the voltage response on the discharge side, and the charge voltage response, which is the voltage response on the charge side, or the difference in response between the discharge current response, which is the current response on the discharge side, and the charge current response, which is the current response on the charge side.

According to the present invention, the SOC usage range of a lithium-ion secondary battery can be determined, not just in situations where a long charge/discharge time can be ensured.

FIG. 1 is a block diagram showing a battery control system for a secondary battery according to the present embodiment. FIG. 2 shows a perspective view of the secondary battery before assembly and a plan view of the secondary battery. FIG. 3 is a graph showing the characteristics of the capacity retention rate versus the number of cycles of a secondary battery. FIG. 4 is a graph showing the capacity maintenance rate when the SOC usage range is set to 50 to 100% and the capacity maintenance rate when the SOC usage range is set to 0 to 100% for a secondary battery including a Gr/SiO negative electrode. FIG. 5A is a graph showing the responsiveness of the voltage response to an AC current input (triangular wave), and FIG. 5B is a graph showing the characteristics of the voltage amplitude ratio (ΔV _{— d} /ΔV _{— c} ) versus the SOC. FIG. 6A is a graph showing the responsiveness of the voltage response to an AC current input (rectangular wave), and FIG. 6B is a graph showing the characteristics of the voltage amplitude ratio (ΔV _{— d} /ΔV _{— c} ) versus the SOC. FIG. 7 is a flowchart showing the procedure of a control process in the system for determining the SOC usage range in the battery control system for the secondary battery according to the first embodiment. FIG. 8 is a flowchart showing a sub-flow of the control process in step S2 shown in FIG. FIG. 9 is a flowchart showing a procedure of a control process in a system for determining an SOC usage range in a battery control system for a secondary battery according to the second embodiment. FIG. 10 is a flowchart showing a sub-flow of the control process in step S2' shown in FIG. FIG. 11 is a flowchart showing the procedure of a control process in a system for determining an SOC usage range in a battery control system for a secondary battery according to the third embodiment. FIG. 12 is a graph for explaining the relationship between the magnitude of the SOC estimated in the control process of step 32 shown in FIG. Figure 13(a) is a graph showing the measurement results in an embodiment, and is a graph showing the responsiveness of the voltage response, Figure 13(b) is a graph showing the measurement results of the amplitude of the voltage response versus SOC (ΔV _{_d} , ΔV _{_c} ), Figure 13(c) is a graph showing the measurement results of the amplitude ratio of the voltage response versus SOC (ΔV _{_d} /ΔV _{_c} ), and Figure 13(d) is a graph showing the measurement results of the amplitude difference of the voltage response versus SOC (ΔV _{_d} - ΔV _{_c} ).

First Embodiment
An embodiment of a control device and a control method for determining an SOC usage range of a lithium-ion secondary battery according to the present invention will be described with reference to the drawings. Fig. 1 is a block diagram showing a battery control system including an embodiment of a battery control device according to the present invention. As shown in Fig. 1, the battery control system according to this embodiment is a system for controlling charging and discharging of a secondary battery 2 and determining an SOC usage range of the secondary battery 2. The battery control system includes a battery control device 1 and a secondary battery 2. Note that the battery control device 1 may include a charging device and the like that are not shown in Fig. 1.

The battery control device 1 includes a controller 10, a voltage sensor 11, a current sensor 12, and a DC-DC converter 13. The controller 10 is a battery control unit (BCU). The controller 10 manages the state of the secondary battery 2 based on the detected voltage detected by the voltage sensor 11 and/or the detected current detected by the current sensor 12, and determines the SOC usage range of the secondary battery 2 according to the state of the secondary battery 2. The controller 10 is composed of a memory such as a ROM or a RAM, and a processor such as a CPU.

The voltage sensor 11 is a sensor for detecting the voltage between the terminals of the secondary battery 2. The voltage sensor 11 is connected between the wiring connected to the positive and negative electrodes of the secondary battery 2. The current sensor 12 is a sensor for detecting the input/output current of the secondary battery 2. The current sensor 12 is connected to the wiring connected to the positive or negative electrode of the secondary battery 2. The voltage sensor 11 and the current sensor 12 detect the state of the battery, and output the detected value to the controller 10.

The DCDC converter 13 is a power conversion device that converts the voltage input from the secondary battery 2 to a predetermined voltage and outputs power to a load such as a motor. The DCDC converter 13 is also a power conversion device that converts the voltage input from a load such as a motor or a charging device to a predetermined voltage and outputs power to the secondary battery 2. The DCDC converter 13 is controlled by the controller 10. The secondary battery 2 is connected to the input side of the DCDC converter 13, and a load is connected to the output side of the DCDC converter 13. The load is a power grid or the like. In other words, the secondary battery 2 is connected to the load via the DCDC converter 13.

The secondary battery 2 is, for example, a lithium ion secondary battery. An example of this type of secondary battery 2 is one that uses, as the negative electrode active material, an active material having a plurality of charge/discharge regions in which the charge/discharge potential changes stepwise with the insertion/extraction of lithium ions. As such an active material, a graphite-based active material containing a graphite structure is suitable. Therefore, in the embodiment shown below, the present invention is explained by exemplifying a lithium ion secondary battery having a negative electrode containing graphite and a metal active material. In the following explanation, the structure and materials of the secondary battery 2 when it is an electrolyte lithium ion secondary battery are explained, but the secondary battery 2 may be an all-solid-state lithium ion secondary battery. In addition, in the following, a laminated battery (laminated cell) is explained as an example of the secondary battery 2, but the secondary battery 2 may be a square battery (square cell) or a cylindrical battery (cylindrical cell).

Figure 2 shows a perspective view of the secondary battery 2 before assembly and a plan view of the secondary battery 2. As shown in Figure 2 (a), the secondary battery 2 includes a positive electrode layer 21, a negative electrode layer 22, a separator 23, a positive electrode tab 24 connected to the positive electrode layer 21, a negative electrode tab 25 connected to the negative electrode layer 22, an upper exterior member 26, and a lower exterior member 27. The power generating elements, which are the positive electrode layer 21, the negative electrode layer 22, and the separator 23, the positive electrode tab 24, and the negative electrode tab 25 are housed in a sealed state by the upper exterior member 26 and the lower exterior member 27.

The positive electrode layer 21 is formed by preparing a slurry by adding a positive electrode active material, a conductive agent such as carbon black, a binder such as polyvinylidene fluoride, and an organic solvent such as N-methyl-2-pyrrolidone, applying the slurry to a main surface of a part of a positive electrode side current collector which is a metal foil such as aluminum, and then drying and pressing. The positive electrode active material is not particularly limited, and examples thereof include lithium composite oxides such as lithium nickel oxide (LiNiO ₂ ), lithium manganate (LiMn ₂ O ₄ ), and lithium cobalt oxide (LiCoO ₂ ), LiFePO ₄ , and LiMnPO ₄ , etc.

The negative electrode active material layer constituting the negative electrode layer 22 is formed by mixing a negative electrode active material, which is a mixture of at least two types of active material, graphite (carbon) and a metal, with a water-based binder such as styrene butadiene rubber, and water as a solvent to prepare a slurry, which is then applied to a main surface of a portion of the negative electrode side current collector, which is a metal foil such as copper, and then drying and pressing. Examples of metals contained in the negative electrode active material include metals whose main elements are Si, Sn, Sb, Pb, Al, Ge, etc., or compounds of metals of these main elements.

The separator 23 prevents short-circuiting between the positive electrode layer 21 and the negative electrode layer 22, and may also have the function of retaining an electrolyte. This separator is, for example, a microporous membrane made of polyolefins such as polyethylene (PE) and polypropylene (PP), and when an overcurrent flows, the pores in the layer are blocked by the heat generated, thereby blocking the current.

The electrolyte solution contained in the secondary battery 2 is a liquid in which a lithium salt such as lithium fluoroborate (LiBF ₄ ) or lithium hexafluorophosphate (LiPF ₆ ) is dissolved as a solute in an organic liquid solvent. Examples of the organic liquid solvent constituting the electrolyte solution include ester-based solvents such as propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl formate (MF), methyl acetate (MA), and methyl propionate (MP), and these can be used in combination.

The positive electrode tab 24 and the negative electrode tab 25 can be made of aluminum foil, aluminum alloy foil, copper foil, nickel foil, or the like. The positive electrode tab 24 is joined to the current collector of the positive electrode layer 21, and the negative electrode tab 25 is joined to the negative electrode layer 22.

As shown in FIG. 2(a), the positive electrode layer 21 and the negative electrode layer 22 are laminated with a separator 23 interposed therebetween, forming a power generating element. The power generating element is then housed in an upper exterior member 26 and a lower exterior member 27 so that the surfaces of the positive electrode layer 21 and the negative electrode layer 22 are covered. The upper exterior member 26 and the lower exterior member 27 are formed of a flexible material, such as a resin film such as polyethylene or polypropylene, or a resin-metal thin film laminate material in which both sides of a metal foil such as aluminum are laminated with a resin such as polyethylene or polypropylene. Then, an electrolyte is injected into the battery, and the periphery of the exterior members 26 and 27 is heat-sealed and sealed with the positive electrode tab 24 and the negative electrode tab 25 led out to the outside, thereby forming the secondary battery 2.

The secondary battery 2 is electrically connected to a charging device. The charging device connected to the secondary battery 2 is, for example, a device for charging the secondary battery 2 mounted on an electric vehicle or hybrid vehicle. The secondary battery 2 mounted on the vehicle is charged by removing the charging cable of the charging device, attaching the charging gun at the end of the charging cable to the connector of the vehicle's charging port, and then operating the charging start switch. The controller 10 manages the state of charge (State of Charge: SOC) of the secondary battery 2, and controls the DCDC converter 13 and the charging device so that the state of charge of the secondary battery 2 becomes a target state of charge.

The secondary battery 2 is electrically connected to a load such as a motor. The load is a device that operates using the power of the secondary battery 2, such as a motor that drives the vehicle, and auxiliary devices such as an air conditioner and lights. Discharging of the secondary battery 2 is performed under the control of the controller 10 in response to a system request or an external power request. A system request corresponds to a command from an on-board computer such as an ECU while the vehicle is running. With regard to a power request from the outside, for example, when an air conditioner is operated before the vehicle starts running by a command from an external device such as a mobile terminal, with a timer setting, so that the temperature inside the vehicle interior is appropriate when the vehicle starts running, the command from the external device corresponds to an external power request.

The secondary battery 2 installed in an electric vehicle or hybrid vehicle may also be used for vehicle grid integration (VGI). VGI is a technology that connects an electric vehicle or hybrid vehicle equipped with a secondary battery 2 to a power grid and supplies the power stored in the secondary battery 2 to the power grid (load) via the power grid.

Next, the deterioration characteristics and SOC range of the lithium ion secondary battery 2 will be described. When a mixture of graphite and metal active material is used for the negative electrode as in this embodiment, when the secondary battery 2 is charged and discharged in the SOC range in which the contribution of the metal active material is large, the deterioration of the metal active material is promoted. FIG. 3 is a graph showing the characteristics of the capacity maintenance rate versus the number of cycles of the secondary battery 2. The number of cycles is the number of times that the SOC is charged and discharged between 0% and 100%. The capacity maintenance rate represents the degree of deterioration when the battery capacity of the secondary battery 2 in the initial state is set to 1.0. In FIG. 3, graph a shows the characteristics of the secondary battery 2 equipped with a negative electrode (hereinafter also referred to as a Gr negative electrode) that does not contain a metal active material but contains graphite as the negative electrode active material. Graph b shows the characteristics of the secondary battery 2 equipped with a negative electrode (hereinafter also referred to as a Gr/SiO negative electrode) that contains a mixture of graphite and metal as the negative electrode active material. The metal active material is a metal compound (SiO) with Si as the main element. As shown in FIG. 3, as the number of cycles increases, the capacity retention rate of the secondary battery 2 with the Gr/SiO negative electrode becomes lower than the capacity retention rate of the secondary battery 2 with the Gr negative electrode. In other words, the battery capacity of the secondary battery 2 with the Gr/SiO negative electrode decreases more quickly.

The graph in FIG. 4 shows the capacity maintenance rate when the SOC usage range is 50-100% and the capacity maintenance rate when the SOC usage range is 0-100% for a secondary battery 2 having a Gr/SiO negative electrode. The characteristics when the SOC usage range is 50-100% have a cycle number of about 100, and the characteristics when the SOC usage range is 0-100% have a cycle number of about 50. The SOC usage range is the range in which the secondary battery 2 can be used, expressed in SOC, and the secondary battery 2 is charged and discharged so that the SOC of the secondary battery 2 falls within the SOC usage range. As shown in FIG. 4, when the SOC usage range is 50-100%, the capacity maintenance rate when the SOC usage range is 50-100% is greater than the capacity maintenance rate when the SOC usage range is 0-100%, even if the cycle number is about twice as many as when the SOC usage range is 0-100%. In other words, the secondary battery 2 equipped with the Gr/SiO negative electrode can suppress the decrease in battery capacity by avoiding charging and discharging at a low SOC.

In this way, when a secondary battery 2 having a Gr/SiO negative electrode is charged and discharged in a low SOC usage range, the contribution of the metal active material becomes large, and deterioration is accelerated. Therefore, in order to suppress deterioration of the secondary battery 2, it is necessary to charge and discharge in an SOC usage range where the contribution of the metal active material is small. In addition, since the SOC usage range where the contribution of the metal active material is small changes depending on the deterioration state of the secondary battery 2, it is necessary to detect the SOC usage range.

In contrast, in this embodiment, when charging/discharging of the secondary battery 2 is stopped, the voltage response to the AC current input is measured, and the usable range (SOC usable range) of the state of charge (SOC) of the lithium ion secondary battery is determined based on the difference in responsiveness between the discharge voltage response, which is the voltage response on the discharge side, and the charge voltage response, which is the voltage response on the charge side. This makes it possible to detect the SOC range in which the contribution of the metal active material is large, and to avoid charging/discharging in that SOC range. In this embodiment, the value of the voltage amplitude ratio (ΔV _d /ΔV _c ), which is the ratio between the amplitude (ΔV _{_d} ) of the discharge voltage response and the amplitude (ΔV _{_c} ) of the charge voltage response, is used as an index representing the difference in responsiveness between _the discharge voltage response and the charge _voltage response. In other words, the difference in responsiveness between the discharge voltage response and the charge voltage response is the difference in value between the discharge voltage amplitude (ΔV _{_d} ) and the charge voltage amplitude (ΔV _{_c} ), and can be expressed as a ratio (ΔV _{_d} /ΔV _{_c,} ΔV _{_c} /ΔV _{_d} ) or a difference (ΔV _{_d} - ΔV _{_c} , ΔV _{_c} - ΔV _{_d} ).

Figure 5(a) is a graph showing the responsiveness of the voltage response to an AC current input (triangular wave). Graph a in Figure 5(a) shows the AC current input value to the secondary battery 2, and graph b shows the voltage response to the AC current. However, the voltage value shown in Figure 5(a) shows the difference between the voltage value generated by the AC current and the cell voltage of the secondary battery 2 in a resting state. The mass ratio of Gr to SiO in the Gr/SiO negative electrode is 95:5. Figure 5(a) also shows the responsiveness of the voltage response to an AC current input when the SOC is 10%, specifically showing the voltage change when a triangular wave with a frequency of 2 Hz and a current amplitude of 4 mA is input for 5 seconds.

As shown in Fig. 5(a), when an AC current is input to the secondary battery 2, a voltage response to the AC current can be obtained. _{ΔV_d} (n) (n is a positive integer, and in this example, n=4) is the value of the discharge voltage at the apex of the discharge side (downward convex side) in Fig. 5(a), and indicates the amplitude of the discharge voltage response. On the other hand, _{ΔV_c} (n) is the value of the discharge voltage at the apex of the charge side (upward convex side), and indicates the amplitude of the charge voltage response. _{ΔV_c} (n) and _{ΔV_d} (n) are values after taking the difference from the cell voltage in the resting state of the secondary battery 2.

The above discharge voltage amplitude ( _{ΔV_d} ) can be the average value of _{ΔV_d} (n) in FIG. 5(a), and similarly, the average value of _{ΔV_c} (n) in FIG. 5(a) can be used as the charge voltage amplitude ( _{ΔV_c} ). In this example, the charge voltage amplitude ( _{ΔV_c} ) and the discharge voltage amplitude ( _{ΔV_d} ) can be calculated from the _four ΔV_c(n) and _{ΔV_d} (n). Then, the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) can be calculated from the calculated charge voltage amplitude ( _{ΔV_c} ) and discharge voltage amplitude ( _{ΔV_d} ).

FIG. 5(b) is a graph showing the characteristics of the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) with respect to the SOC. Here, in addition to the voltage amplitude ratio (ΔV_d/ _{ΔV_c} ) in the case of SOC (10%) calculated from FIG. 5(a) above, the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) in the case of SOC (30 _% , 50%) is also shown. As shown in FIG. 5(b), the secondary battery 2 having the Gr/SiO negative electrode has a tendency that the value of the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) increases as the value of the SOC decreases. That is, in the SOC range in which SiO contributes greatly to charging and discharging, the discharge voltage amplitude ( _{ΔV_d} ) is larger than the charge voltage amplitude ( _{ΔV_c} ) compared to the SOC range in which Gr contributes.

When the secondary battery 2 is charged and discharged within the SOC usage range where the voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) is greater than the threshold value (1.1), the secondary battery 2 is charged and discharged within the SOC usage range where the contribution of SiO is large, and deterioration of the metal active material is accelerated. That is, the voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) is calculated, and the SOC range where the calculated voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) is less than the voltage amplitude ratio threshold value is determined as the SOC usage range. This makes it possible to detect the SOC range where the contribution of the metal active material is large, and to avoid charging and discharging within that SOC range.

In addition, although the AC current waveform is a triangular wave in Fig. 5(a) and Fig. 5(b), the AC current waveform is not limited to this. The above-mentioned correlation between the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) and the SOC is also valid when the AC current waveform is a square wave, for example. Fig. 6(a) is a graph showing the responsiveness of the voltage response to an AC current input (square wave), and shows the voltage change when the AC current in Fig. 5(a) is a square wave. Fig. 6(b) is a graph showing the characteristics of the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) versus the SOC.

Even in such a case, as shown in FIG. 6B, the secondary battery 2 has a tendency that the voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) increases as the SOC value decreases. In the case shown in FIG. 6B, the voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) is greater than 1.1 in the range of SOC (about 21%) or less. When the secondary battery 2 is charged and discharged within the range of SOC use where the voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) is greater than the threshold value (1.1), the secondary battery 2 is charged and discharged within the range of SOC use where the contribution of SiO is large, and therefore the deterioration of the metal active material is promoted. The waveform of the alternating current may be a sine wave, although not shown in particular.

In this embodiment, the controller 10 measures the voltage response to the AC current input when charging and discharging of the secondary battery 2 is stopped, and determines the usage range (SOC usage range) of the state of charge (SOC) of the secondary battery 2 based on the difference in responsiveness between the discharge voltage response, which is the voltage response on the discharge side, and the charge voltage response, which is the voltage response on the charge side. Hereinafter, a method of calculating a voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) as a value indicating this difference in responsiveness and determining the SOC usage range based on the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) will be described. FIG. 7 is a flowchart showing the procedure of a control process in a system that determines the SOC usage range. Note that the upper limit value (upper limit SOC) of the SOC usage range is preset, and is set to, for example, 100% or 80%. The upper limit SOC may be changed depending on the specification use and deterioration degree of the secondary battery 2.

In step S1, the controller 10 turns on a switch (not shown) connected between the secondary battery 2 and the power grid to connect the secondary battery 2 to the power grid.

In step S2, the controller 10 calculates the charging voltage amplitude ( _{ΔV_c} ) and the discharging voltage amplitude ( _{ΔV_d} ). FIG. 8 is a flowchart showing a subflow of the control process in step S2. The controller 10 calculates the charging voltage amplitude ( _{ΔV_c} ) and the discharging voltage amplitude ( _{ΔV_d} ) by executing the control process of steps S21 to S26 shown in FIG. 8. The controller 10 detects the voltage and current of the secondary battery 2 at a predetermined cycle using the voltage sensor 11 and the current sensor 12, separately from the control process of steps S21 to S26. The shorter the detection cycle (sample rate), the higher the accuracy, but it is set to, for example, 1 second (1 Hz). The battery temperature may be any temperature, but the lower the battery temperature, the larger the amplitude (ΔV) of the voltage response, and the higher the battery temperature, the smaller the amplitude (ΔV) of the voltage response.

In step S21, the controller 10 prohibits charging and discharging for one minute or more after connecting the secondary battery 2 to the power grid, and places the secondary battery 2 in a hibernation state. If the secondary battery has been in a hibernation state for one minute or more before connecting to the power grid, the hibernation period after connecting to the power grid in step S1 does not need to be provided. Setting the hibernation period to one minute or more is merely an example, and the hibernation period may be, for example, 10 seconds or more. After the hibernation period has elapsed, in step S22, the controller 10 stores the cell voltage V_0 (open circuit voltage) of the secondary battery 2 in the hibernation state in memory.

In step S23, the controller 10 inputs n cycles (n is a positive integer) of AC current to the secondary battery 2. The AC current input here may have the same amplitude and waveform on the discharge side and the charge side. The AC current may have any waveform as long as the waveforms on the charge side and the discharge side are symmetrical, and may be, for example, a triangular wave, a rectangular wave, or a sine wave. The frequency of the AC current may be any value within the band in which the charge and discharge reactions of SiO appear. The measurement cycle (n) may be any integer, or may be the peak value for one cycle. The current value may be any value, but for example, 0.1 C can ensure sufficient accuracy. In step S23, the AC current may be discharged first and then charged, or may be charged first and then discharged.

Next, in step 24, the amplitude of the voltage response on the discharge side for each cycle ( _{V_d} (n)) and the amplitude of the voltage response on the charge side for each cycle ( _{V_c} (n)) are measured, and the voltage amplitudes ( _{V_d} (n), _{V_c} (n)) are stored in memory. At this time, the voltage response amplitude value may be measured for multiple cells or for each cell, but measuring for each cell can improve accuracy.

In step S25, the controller 10 calculates the average value of each of the voltage amplitudes ( _{V_d} (n), _{V_c} (n)) to calculate the average value of the amplitude of the voltage response on the discharge side ( _{V_d} ) and the average value of the amplitude of the voltage response on the charge side ( _{V_c} ). Next, in step 26, the controller 10 calculates the discharge voltage amplitude ( _{ΔV_d} =V_0- _{V_d} ) and the charge voltage amplitude ( _{ΔV_c} = _{V_c} -V_0).

Returning to FIG. 7, after calculating the discharge voltage amplitude ( _{ΔV_d} ) and the charge voltage amplitude ( _{ΔV_c} ) in step S2, the controller 10 calculates the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) in step S3. Then, the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) is compared with the voltage amplitude ratio threshold to determine whether the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) is less than the voltage amplitude ratio threshold. The voltage amplitude ratio threshold is a threshold for setting the lower limit of the SOC usage range, and is set in advance. The voltage amplitude ratio threshold may be set according to an allowable deterioration rate and/or an allowable deterioration degree depending on the use of the secondary battery 2, etc. The voltage amplitude ratio threshold may be set according to the specifications of the negative electrode active material (the ratio of the metal active material, the type of metal used in the metal active material, etc.). The voltage amplitude ratio threshold may also be set according to the detection accuracy of the voltage and/or current of the secondary battery 2, etc. In this embodiment, the voltage amplitude ratio threshold is set to 1.1, for example.

When the calculated voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) is lower than the voltage amplitude ratio threshold value (1.1), the controller 10 executes the control process of steps S4 to S8. On the other hand, when the calculated voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) is equal to or greater than the voltage amplitude ratio threshold value (1.1), the controller 10 executes the control process of steps S9 to S13.

In step S4, the controller 10 discharges the secondary battery 2. When the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) of the secondary battery 2 is lower than the voltage amplitude ratio threshold (1.1), the SOC of the secondary battery 2 is higher than the lower limit of the SOC use range, so the SOC is lowered to determine the lower limit of the SOC use range corresponding to the voltage amplitude ratio threshold (1.1). In step S5, the controller 10 stops discharging. In step S6, the controller 10 recalculates the discharge voltage amplitude ( _{ΔV_d} ) and the charge voltage amplitude ( _{ΔV_c} ). The calculation method of the discharge voltage amplitude ( _{ΔV_d} ) and the charge voltage amplitude ( _{ΔV_c} ) is the same as the calculation method in the control process of step S2.

In step S7, the controller 10 compares the voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) with the voltage amplitude ratio threshold (1.1) and determines whether the voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) is equal to or greater than the voltage amplitude ratio threshold (1.1). If the voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) is lower than the voltage amplitude ratio threshold (1.1), the controller 10 returns the flow of the control process to step S4 and discharges the secondary battery 2. That is, by repeatedly executing the control processes of steps S4 to S7, the voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) is brought closer to the voltage amplitude ratio threshold (1.1) so that the SOC approaches the lower limit of the SOC usage range from the high SOC side.

If the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) is equal to or greater than the voltage amplitude ratio threshold (1.1), in step S8, the controller 10 determines the current SOC as the lower limit (lower limit SOC) of the SOC usage range. In other words, the controller 10 determines the SOC when the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) reaches the voltage amplitude ratio threshold (1.1) as the lower limit SOC of the SOC usage range.

In step S9, the controller 10 charges the secondary battery 2. When the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) of the secondary battery 2 is equal to or greater than the voltage amplitude ratio threshold (1.1), the SOC of the secondary battery 2 is equal to or less than the lower limit of the SOC use range, so the SOC is increased to determine the lower limit of the SOC use range corresponding to the voltage amplitude ratio threshold (1.1). Since the charging in the control process of step S9 is outside the SOC use range, the charging current is reduced so as not to accelerate deterioration of the secondary battery 2. For example, the rate of the charging current may be set to 0.5C or less. In step S10, the controller 10 stops charging the secondary battery 2. Next, in step 11, the controller 10 calculates the discharge voltage amplitude ( _{ΔV_d} ) and the charge voltage amplitude ( _{ΔV_c} ). The calculation method of the discharge voltage amplitude ( _{ΔV_d} ) and the charge voltage amplitude ( _{ΔV_c} ) is the same as the calculation method in the control process of step S2.

In step S12, the controller 10 compares the voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) with the voltage amplitude ratio threshold (1.1) and determines whether the voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) is less than the voltage amplitude ratio threshold (1.1). If the voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) is equal to or greater than the voltage amplitude ratio threshold (1.1), the controller 10 returns the flow of the control process to step S9 and charges the secondary battery 2. That is, by repeatedly executing the control processes of steps S9 to S12, the voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) is brought closer to the voltage amplitude ratio threshold (1.1) so that the SOC approaches the lower limit of the SOC usage range from the low SOC side.

If the voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) is less than the voltage amplitude ratio threshold (1.1), in step S13, the controller 10 determines the current SOC as the lower limit value (lower limit SOC) of the SOC usage range. That is, the controller 10 determines the SOC when the voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) reaches the voltage amplitude ratio threshold (1.1) as the lower limit SOC of the SOC usage range. Then, the controller 10 ends the flow of the control process for determining the SOC usage range. As a result, the controller 10 determines the SOC usage range based on the voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ).

Then, the controller 10 stores in memory the SOC usage range determined by the control processing of steps S8 and S13 in correspondence with the calculated voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ). During normal use of the secondary battery 2, the controller 10 controls the charging and/or discharging of the secondary battery 2 so that the SOC falls within the SOC usage range stored in the memory. Note that the SOC usage range of the secondary battery 2 is a target range for suppressing the deterioration rate of the secondary battery 2, and the secondary battery 2 may be charged or discharged outside the SOC usage range. Furthermore, in order to determine the SOC usage range of the secondary battery 2, charging or discharging may be performed outside the SOC usage range determined previously.

In order to increase the battery capacity of a lithium-ion secondary battery, a mixture of graphite and a metal active material is used in the negative electrode as the negative electrode active material. However, when charging and discharging are performed in the SOC range in which the contribution of the metal active material of the negative electrode is large, deterioration is accelerated. In this embodiment, the SOC range in which the contribution of the metal active material of the negative electrode is large can be detected with high accuracy and in a short time. Then, the SOC usage range is determined while avoiding the SOC range in which the contribution of the metal active material of the negative electrode is large, so that charging and discharging are performed within the SOC range in which the deterioration of the metal active material is small. As a result, the decrease in the capacity retention rate can be suppressed and the cycle characteristics can be improved.

As described above, in this embodiment, the charge and discharge of the secondary battery 2 is controlled, and when the charge and discharge of the secondary battery 2 is stopped, the discharge voltage amplitude ( _{ΔV_d} ) and the charge voltage amplitude ( _{ΔV_c} ) are calculated, and the SOC usage range of the secondary battery 2 is determined based on the difference between the discharge voltage amplitude ( _{ΔV_d} ) and the charge voltage amplitude ( _{ΔV_c} ). The difference between the discharge voltage amplitude ( _{ΔV_d} ) and the charge voltage amplitude ( _{ΔV_c} ) corresponds to the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ). This makes it possible to quickly grasp the range of SOC to which the metal active material of the negative electrode contributes, and to avoid charging and discharging in that range. That is, the SOC usage range of the secondary battery 2 can be determined not only in situations where a long charge and discharge time can be secured. In addition, the SOC usage range is set to avoid the range of SOC in which the metal active material of the negative electrode contributes greatly. During normal use of the secondary battery 2, charging and discharging are performed within the SOC usage range, so that a decrease in capacity of the lithium ion secondary battery due to deterioration of the metallic active material of the negative electrode can be suppressed.

As another method, it is possible to detect the SOC range in which the contribution of the metal active material is large from the difference in the measured values of EIS (Electro-chemical Impedance Spectroscopy) measurements during charging and discharging. In EIS measurements, the analysis is performed without considering the difference in the amplitude of the AC impedance on the charging side and the discharging side, so even if it is measured in a state where charging and discharging are stopped, the AC impedance is uniquely determined, and the presence or absence of the contribution of the metal active material cannot be determined. Therefore, when using EIS measurements, it is necessary to determine the difference in the measurement results during charging and discharging, which requires two measurements. In contrast, in this embodiment, the presence or absence of the contribution of the metal active material can be determined by measuring the responsiveness of the voltage response at least once in a state where charging and discharging are stopped, so that the difference between the discharge voltage amplitude (ΔV_ _d ) and the charge voltage amplitude ( _{ΔV_c} ) can be determined in less than half the time compared to the method using EIS.

Furthermore, in this embodiment, charging and/or discharging of the secondary battery 2 is controlled within an SOC usage range in which a value indicating the difference between the discharge voltage amplitude ( _{ΔV_d} ) and the charge voltage amplitude ( _{ΔV_c} ) (corresponding to the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} )) is less than a voltage amplitude ratio threshold value. This makes it possible to control charging and discharging while avoiding an SOC range in which the contribution of the metal active material of the negative electrode is large. As a result, it is possible to suppress the capacity reduction of the lithium ion secondary battery due to the deterioration of the metal active material of the negative electrode.

In addition, in this embodiment, the metal active material of the negative electrode contains Si. As a result, when Si is used as the metal active material to increase the battery capacity, charging and discharging can be performed while avoiding the SOC range in which the contribution of the metal active material of the negative electrode is large, so that the capacity decrease of the lithium ion secondary battery due to the deterioration of Si can be suppressed.

In addition, in this embodiment, the SOC usage range is determined while the secondary battery 2 is connected to the power grid. As a result, while the secondary battery 2 is connected to the power grid, the SOC range in which the contribution of the negative electrode metal active material is large is detected, and charging and discharging can be performed in as low an SOC range as possible while avoiding the SOC range in which the contribution of the negative electrode metal active material is large. As a result, the SOC usage range can be set to an optimal range while suppressing both the deterioration of the metal active material and the storage deterioration that accelerates with increasing SOC.

In this embodiment, the control process of steps S4 to S7 shown in FIG. 7 is repeatedly executed to move the SOC from the high SOC side toward the lower limit of the SOC usage range. At this time, the shorter the discharge time in the control process of step S4, the more finely the discharge capacity is divided, so the longer it takes to determine the SOC usage range, but the higher the accuracy.

In addition, in this embodiment, the discharge in the control process of step S4 may be divided by SOC instead of time. For example, the discharge is divided for each SOC (1%), and the discharge voltage amplitude ( _{ΔV_d} ) and the charge voltage amplitude ( _{ΔV_c} ) are calculated to determine the SOC use range. That is, the SOC use range is adjusted per unit of SOC (1%). Therefore, even if there is an error in the calculation accuracy of the SOC use range, the error can suppress the SOC range in which the contribution of the metal active material is large to less than SOC (1%).

In addition, in this embodiment, after the control process of step S6, charging may be performed for an amount equivalent to a predetermined SOC, and the SOC after charging may be determined as the lower limit of the SOC usage range. The predetermined SOC may be set to, for example, 5%. In a modified example of this embodiment, the predetermined SOC may be set to 1%. This allows an SOC with a margin equivalent to the predetermined SOC relative to the range of SOCs in which the contribution of the metal active material is large to be determined as the lower limit of the SOC usage range, thereby suppressing the capacity reduction of the lithium-ion secondary battery due to deterioration of the metal active material in the negative electrode.

In this embodiment, the control process of steps S9 to S12 shown in FIG. 7 is repeatedly executed to bring the SOC closer to the lower limit of the SOC usage range from the low SOC side. At this time, the shorter the charging time in the control process of step S9, the more finely the charging capacity is divided, so the longer it takes to determine the SOC usage range, but the higher the accuracy.

In this embodiment, the charging in the control process of step S9 may be divided by SOC instead of time. For example, the charging is divided for each SOC (1%), and the discharge voltage amplitude ( _{ΔV_d} ) and the charge voltage amplitude ( _{ΔV_c} ) are calculated to determine the SOC use range. That is, the SOC use range is adjusted per unit of SOC (1%). Therefore, even if there is an error in the calculation accuracy of the SOC use range, the error can suppress the SOC range in which the contribution of the metal active material is large to less than SOC (1%).

In addition, in this embodiment, after the control process of step S12, charging may be performed for an amount equivalent to a predetermined SOC, and the SOC after charging may be determined as the lower limit of the SOC usage range. The predetermined SOC may be set to, for example, 5%. In a modified example of this embodiment, the predetermined SOC may be set to 1%. This allows an SOC with a margin equivalent to the predetermined SOC relative to the range of SOC in which the contribution of the metal active material is large to be determined as the lower limit of the SOC usage range, thereby suppressing the capacity reduction of the lithium-ion secondary battery due to deterioration of the metal active material in the negative electrode.

Furthermore, as a modified example of this embodiment, in the control process of step S3, if the voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) is equal to or greater than the voltage amplitude ratio threshold value (1.1), the controller 10 may charge the secondary battery 2, and after the voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) becomes less than the voltage amplitude ratio threshold value (1.1), execute the control processes of steps S4 to S8 to determine the SOC usage range.

In this embodiment, the value indicating the difference between the discharge voltage amplitude ( _{ΔV_d} ) and the charge voltage amplitude ( _{ΔV_c} ) is not limited to the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ), but may be the voltage amplitude difference ( _{ΔV_d} - _{ΔV_c} or _{ΔV_c} - _{ΔV_d} ). The controller 10 may calculate the voltage amplitude difference ( _{ΔV_d} - _{ΔV_c} or _{ΔV_c} - _{ΔV_d} ) from the discharge voltage amplitude ( _{ΔV_d} ) and the charge voltage amplitude ( _{ΔV_c} ), and determine the SOC usage range of the secondary battery 2 based on the calculated voltage amplitude difference ( _{ΔV_d} - _{ΔV_c} or _{ΔV_c} - _{ΔV_d} ). Specifically, in the control flow of step S3, the controller 10 calculates the difference between the discharge voltage amplitude ( _{ΔV_d} ) and the charge voltage amplitude ( _{ΔV_c} ) as a voltage amplitude difference ( _{ΔV_d} - _{ΔV_c} or _{ΔV_c} - _{ΔV_d} ). Then, in steps S3, S7, and S12, instead of the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ), the controller 10 compares the calculated voltage amplitude difference ( _{ΔV_d} - _{ΔV_c} or _{ΔV_c} - _{ΔV_d} ) with a voltage amplitude difference threshold. The voltage amplitude threshold is a threshold that is set in advance, similar to the voltage amplitude ratio threshold. Then, in steps S8 and S13, the controller 10 determines the SOC at which the calculated voltage amplitude difference (ΔV _— d − _{ΔV — c} or ΔV _— c − ΔV _{— d} ) reaches the voltage amplitude difference threshold value as the lower limit SOC of the SOC usage range.

Thus, in this embodiment, the voltage amplitude difference (ΔV _d _{-ΔV _c} or ΔV _c -ΔV _{_d} ) between the discharge voltage amplitude (ΔV _{_d} ) and the charge voltage amplitude (ΔV _{_c} ) is calculated as a value indicating the difference between the discharge voltage amplitude (ΔV _{_d )} and the charge voltage amplitude (ΔV _{_c} ), and the SOC usage range of the secondary battery 2 is determined based on the calculated voltage amplitude difference (ΔV _{_d} -ΔV _{_c} or ΔV _{_c -ΔV _d} ). This makes it possible to quickly grasp the range of SOC to which the metal active material of the negative electrode contributes, and to avoid charging and discharging in that range. That is, the SOC usage range of the secondary battery 2 can be determined not only in situations where a long charging and discharging time can be secured. In addition, the SOC usage range is set to avoid the range of SOC in which the metal active material of the negative electrode has a high degree of contribution. During normal use of the secondary battery 2, charging and discharging are performed within the SOC usage range, so that reduction in capacity of the lithium ion secondary battery due to deterioration of the metallic active material of the negative electrode can be suppressed.

In addition, SiO has different charge-side overvoltage and discharge-side overvoltage. This difference in overvoltage during charging and discharging is divided into IR (electrolyte resistance, electric resistance of active material, etc.) and overvoltage other than IR (charge transfer resistance, diffusion resistance, etc.), but since IR can be offset by calculating the voltage amplitude difference (ΔV _{_d} - ΔV _{_c} or ΔV _{_c} - ΔV _{_d} ), the SiO contribution range can be accurately determined even if IR changes occur due to deterioration, etc. This makes it possible to more accurately grasp the SOC usage range to which SiO contributes, and therefore the lower limit of the SOC usage range can be expanded to the vicinity of the SOC usage range to which SiO contributes, and the SOC usage range can be further expanded.

The SOC usage range may be determined by calculating the voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) before degradation and determining the initial value of the SOC usage range from the calculation result. The SiO contribution capacity is calculated from the theoretical capacity (mAh/g) of SiO and the amount of SiO used (g), and the SOC range to which SiO mainly contributes before degradation can be calculated from the calculated SiO contribution capacity and the battery capacity of the secondary battery 2 before degradation. For example, if SiO contribution capacity/battery capacity=15%, the SOC range (0-15%) is the SOC range in which SiO has a large contribution. Then, the range (e.g., 15-100%) that avoids the SOC range (0-15%) is the SOC usage range.

In the above example, the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) corresponding to an SOC of 15% before deterioration is calculated as the voltage amplitude ratio threshold, and after deterioration of the secondary battery 2, the SOC at which the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) becomes the voltage amplitude ratio threshold is set as the lower limit SOC of the SOC usage range. This allows charging and discharging to be performed while avoiding the SOC range in which the contribution of the metal active material of the negative electrode is large, even before and after deterioration of the secondary battery 2.

The battery control system according to this embodiment can be applied in the following cases other than VGI. As an example, in an electric vehicle using a secondary battery 2, an alarm threshold is set to sound a battery remaining capacity alarm for an SOC that is several percent higher than the lower limit of the SOC usage range. Then, if the SOC of the secondary battery 2 falls to the alarm threshold during normal driving, an alarm is output to prompt the user to charge the battery as soon as possible.

As another example, when the SOC of the secondary battery 2 falls to the lower limit SOC of the SOC usage range, the discharge current of the secondary battery 2 may be suppressed, for example by limiting the output torque of the motor. Note that such output limitations do not necessarily need to be applied all the time, and when limited conditions are met, such as when high acceleration performance is required (for example, when there is a torque requirement equivalent to full throttle), the output limitations may be lifted and the secondary battery 2 may be used below the lower limit SOC of the SOC usage range. This reduces the frequency of use within the SOC range where the contribution of the negative electrode metal active material is large, and deterioration of the secondary battery 2 can be suppressed.

As yet another example, the present invention can be applied when the secondary battery 2 is used as a stationary power source, or when a vehicle equipped with the secondary battery 2 is used as an external power source for indoor or outdoor use. In this case, an alarm may be set according to the lower limit SOC of the lower limit SOC usage range, as described above.

Second Embodiment
Next, a battery control system according to a second embodiment will be described. In the second embodiment, the current response of the secondary battery 2 to an AC voltage input is calculated, and the SOC usage range is determined based on the difference in response between the discharge current response, which is the current response on the discharge side, and the charge current response, which is the current response on the charge side.

FIG. 9 is a flowchart showing the procedure of the control process in the system for determining the SOC usage range in the battery control system for the secondary battery according to the second embodiment. FIG. 10 is a flowchart showing a subflow of the control process in step S2' shown in FIG. 9. In this embodiment, the SOC usage range is determined using the value of the current amplitude ratio (ΔI _{_d} /ΔI _{_c} ), which is the ratio between the amplitude (ΔI _{_d} ) of the discharge current response and the amplitude (ΔI _{_c} ) of the charge current response, as the difference in response between the discharge current response and the charge current response. In the flowchart shown in FIG. 9, steps S2, S3, S6, S7, S11, and S12 using the voltage response in the flowchart of FIG. 7 are replaced with steps S2', S3', S6', S7', S11', and S12' using the current response. Hereinafter, steps S2', S3', S6', S7', S11', and S12' will be described in detail. Other than the steps that differ from the judgment system according to the first embodiment, it has the same configuration as the first embodiment, performs the same control as the first embodiment, and operates in the same manner as the first embodiment, and the description of the first embodiment will be used as appropriate.

9, the amplitude (ΔI _d ) of the discharge current response and the amplitude (ΔI _{_c} ) of the charge current response are calculated. In this step S2', the controller 10 calculates the amplitude (ΔI _d ) of the discharge current response and the amplitude (ΔI _{_c} ) of the charge current response by executing the control processing of steps S21' to S24' shown in FIG.

In step S21', the controller 10 prohibits charging and discharging for one minute or more after connecting the secondary battery 2 to the power grid, and places the secondary battery 2 in a hibernation state. Note that if the secondary battery has been in a hibernation state for one minute or more before connecting to the power grid, the hibernation period after connecting to the power grid in step S1 does not need to be provided. Setting the hibernation period to one minute or more is merely an example, and the hibernation period may be, for example, 10 seconds or more.

In step S22', the controller 10 inputs n cycles (n is a positive integer) of AC voltage to the secondary battery 2. The AC voltage input here may have the same amplitude and waveform on the discharge side and the charge side. The AC voltage waveform may be any waveform as long as the waveforms on the charge side and the discharge side are symmetrical, and may be, for example, a triangular wave, a rectangular wave, or a sine wave. The measurement cycle (n) may be any integer, and may be the peak value for one cycle. The frequency of the AC voltage may be any value within the band in which the charge and discharge reaction of SiO appears. The voltage value may be any value, but for example, 0.1C can ensure sufficient accuracy. In step S22', the AC voltage may be discharged first and then charged, or may be charged first and then discharged.

In step S23', the controller 10 measures the amplitude ( _{I_d} (n)) of the current response on the discharge side in each cycle and the amplitude ( _{I_c} (n)) of the current response on the charge side in each cycle, and stores the current amplitudes ( _{I_d} (n), _{I_c} (n)) in memory. At this time, the current amplitude value may be measured for multiple cells or for each cell, but measuring for each cell can improve accuracy.

In step S24', the controller 10 calculates the average value of the amplitude of the current response on the discharge side ( _{I_d} ) and the average value of the amplitude of the current response on the charge side ( _{I_c} ) by calculating the average value of each of the stored current amplitudes ( _{I_d} (n), I_c _(n )). In this embodiment, the average value ( _{I_d} ) is the amplitude of the discharge current response ( _{ΔI_d} ), and the average value ( _{I_c} ) is the amplitude of the charge current response ( _{ΔI_c} ).

Returning to FIG. 9, after calculating the discharge current response amplitude (ΔI _{_d} ) and the discharge current response amplitude (ΔI _{_c} ) in step S2, the controller 10 calculates the current amplitude ratio (ΔI _{_d} /ΔI _{_c} ) in step S3. Then, the current amplitude ratio (ΔI _{_d} /ΔI _{_c} ) is compared with the current amplitude ratio threshold to determine whether the current amplitude ratio (ΔI _{_d} /ΔI _{_c} ) is less than the current amplitude ratio threshold. The current amplitude ratio threshold is a threshold for setting the lower limit of the SOC usage range, and is set in advance. The current amplitude ratio threshold may be set according to an allowable deterioration rate and/or an allowable deterioration degree depending on the use of the secondary battery 2, etc. The current amplitude ratio threshold may be set according to the specifications of the negative electrode active material (the ratio of the metal active material, the type of metal used in the metal active material, etc.). The current amplitude ratio threshold may also be set according to the detection accuracy of the voltage and/or current of the secondary battery 2, etc. In this embodiment, the current amplitude ratio threshold is set to 1.1, for example.

When the calculated current amplitude ratio (ΔI _{_d} /ΔI _{_c} ) is lower than the current amplitude ratio threshold value (1.1), the controller 10 executes the control process of steps S4 to S8. On the other hand, when the calculated current amplitude ratio (ΔI _{_d} /ΔI _{_c} ) is equal to or greater than the current amplitude ratio threshold value (1.1), the controller 10 executes the control process of steps S9 to S13.

Steps S4 and S5 are the same as those in the first embodiment. In step S6', the discharge current response amplitude (ΔI _{_d} ) and the discharge current response amplitude (ΔI _{_c} ) are calculated by the same method as in step S2'. Then, in step S7', the controller 10 compares the current amplitude ratio (ΔI _{_d} /ΔI _{_c} ) with the current amplitude ratio threshold (1.1) and determines whether the current amplitude ratio (ΔI _{_d} /ΔI _{_c} ) is equal to or greater than the current amplitude ratio threshold (1.1). If the current amplitude ratio (ΔI _{_d} /ΔI _{_c} ) is lower than the current amplitude ratio threshold (1.1), the controller 10 returns the flow of the control process to step S4 and discharges the secondary battery 2. That is, by repeatedly executing the control process of steps S4 to S7', the current amplitude ratio (ΔI _{— d} /ΔI _{— c} ) is made to approach the current amplitude ratio threshold value (1.1) so that the SOC approaches the lower limit of the SOC usage range from the high SOC side.

If the current amplitude ratio ( _{ΔI_d} / _{ΔI_c} ) is equal to or greater than the current amplitude ratio threshold (1.1), in step S8, the controller 10 determines the current SOC to be the lower limit (lower limit SOC) of the SOC usage range. In other words, the controller 10 determines the SOC when the current amplitude ratio ( _{ΔI_d} / _{ΔI_c} ) reaches the current amplitude ratio threshold (1.1) as the lower limit SOC of the SOC usage range.

Steps S9 and S10 are the same as those in the first embodiment. In step S11', the discharge current response amplitude (ΔI _{_d} ) and the charge current response amplitude (ΔI _{_c} ) are calculated by the same method as in step S2'. Then, in step S7', the controller 10 compares the current amplitude ratio (ΔI _{_d} /ΔI _{_c} ) with the current amplitude ratio threshold (1.1) and determines whether the current amplitude ratio (ΔI _{_d} /ΔI _{_c} ) is less than the current amplitude ratio threshold (1.1). If the current amplitude ratio (ΔI _{_d} /ΔI _{_c} ) is equal to or greater than the current amplitude ratio threshold (1.1), the controller 10 returns the flow of the control process to step S9 and charges the secondary battery 2. That is, by repeatedly executing the control process of steps S9 to S12', the current amplitude ratio (ΔI _{— d} /ΔI _{— c} ) is made to approach the current amplitude ratio threshold value (1.1) so that the SOC approaches the lower limit of the SOC usage range from the low SOC side.

If the current amplitude ratio ( _{ΔI_d} / _{ΔI_c} ) is less than the current amplitude ratio threshold (1.1), in step S13, the controller 10 determines the current SOC as the lower limit value (lower limit SOC) of the SOC usage range. In other words, the controller 10 determines the SOC when the current amplitude ratio ( _{ΔI_d} / _{ΔI_c} ) reaches the current amplitude ratio threshold (1.1) as the lower limit SOC of the SOC usage range. Then, the controller 10 ends the flow of the control process for determining the SOC usage range. As a result, the controller 10 determines the SOC usage range based on the current amplitude ratio ( _{ΔI_d} / _{ΔI_c} ).

The second embodiment as described above can also provide the same effect as the first embodiment. In the present embodiment, an example has been described in which the value of the current amplitude ratio (ΔI _{_d} /ΔI _{_c} ) is used as a value indicating the difference in responsiveness between the discharge current response and the charge current response, but the present invention is not limited to this. For example, the current amplitude difference (ΔI _{_d} -ΔI _{_c} or ΔI _{_c} -ΔI _{_d} ) may be used as a value indicating the difference. In this case, in steps S3', S6', and S12', the controller 10 calculates the current amplitude difference (ΔI _{_d} -ΔI _{_c} or ΔI _{_c} -ΔI _{_d} ) instead of the current amplitude ratio (ΔI _{_d} /ΔI _{_c} ). Then, the controller 10 compares the calculated current amplitude difference (ΔI _{_d} -ΔI _{_c} or ΔI _{_c} -ΔI _{_d} ) with a current amplitude difference threshold. The current amplitude difference threshold is a threshold that is set in advance, similar to the current amplitude ratio threshold.

Third Embodiment
Next, a battery control system according to a third embodiment will be described. In the third embodiment, a part of the control process for determining the SOC usage range is changed in order to make the time required for determining the SOC usage range even shorter than in the first embodiment. Note that, except for the differences from the determination system according to the first embodiment in the points described below, the third embodiment has the same configuration as the first embodiment, performs the same control as the first embodiment, and operates in the same manner as the first embodiment, and the description of the first embodiment will be used as appropriate.

In this embodiment, the controller 10 determines the SOC use range at a predetermined timing, such as when charging and discharging of the secondary battery 2 is completed. When determining the SOC use range, the controller 10 sets a predetermined SOC range including the lower limit SOC of the SOC use range in order to calculate the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) when the SOC of the secondary battery 2 is close to the lower limit SOC of the SOC use range. The predetermined SOC range (hereinafter referred to as the lower limit SOC determination range) is a range expanded by a predetermined SOC (e.g., ±5%) on both the positive and negative sides with respect to the lower limit SOC of the SOC use range. When the current SOC is outside the lower limit SOC determination range, the controller 10 charges or discharges the secondary battery 2 so that the SOC of the secondary battery 2 is included in the lower limit SOC determination range. Then, after the SOC of the secondary battery 2 falls within the lower limit SOC determination range, the controller 10 calculates the voltage amplitude ratio (ΔV _{— d} /ΔV _{— c} ) and determines the SOC usage range based on the voltage amplitude ratio (ΔV _{— d} /ΔV _{— c} ).

Hereinafter, a control process will be described for calculating a voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) as a value indicating the difference in response between the discharge voltage response and the charge voltage response, and determining the SOC usage range based on the calculated voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ). FIG. 11 is a flowchart showing the procedure of the control process in a system for determining the SOC usage range. In step S31, the controller 10 connects the secondary battery 2 to the power grid. In step S32, the controller 10 estimates the SOC of the secondary battery 2. The SOC may be estimated, for example, by map calculation using a map indicating the correlation between the open circuit voltage (OCV) and the SOC. The map is stored in a memory in the controller 10, and the open circuit voltage (SOC) corresponds to the battery voltage immediately before the secondary battery 2 is connected to the power grid.

In step S33, the controller 10 compares the estimated SOC with a value ( _{SOC_0} +5%) obtained by adding a predetermined SOC (5%) to the lower limit SOC ( _{SOC_0} ) of the previously determined SOC use range, and determines whether the estimated SOC is greater than " _{SOC_0} +5%." _{SOC_0} +5% corresponds to the upper limit of the lower limit SOC determination range.

If the estimated SOC is greater than the upper limit value ( _{SOC_0} +5%) of the lower limit SOC determination range, in step S34, the controller 10 discharges the secondary battery 2 until the SOC reaches the upper limit value ( _{SOC_0} +5%). After the SOC reaches the upper limit value ( _{SOC_0} +5%), in step S35, the controller 10 stops discharging. Next, in step S36, the controller 10 calculates a discharge voltage amplitude ( _{ΔV_d} ) and a charge voltage amplitude ( _{ΔV_c} ). The calculation method of the discharge voltage amplitude ( _{ΔV_d} ) and the charge voltage amplitude ( _{ΔV_c} ) is the same as the calculation method in the control process in step S2 in the first embodiment. In step S37, the controller 10 compares the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) with the voltage amplitude ratio threshold (1.1) and determines whether the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) is equal to or greater than the voltage amplitude ratio threshold (1.1). If the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) is lower than the voltage amplitude ratio threshold (1.1), in step S38, the controller 10 discharges the secondary battery 2. Next, in step S39, the controller 10 stops discharging the secondary battery. Then, the controller 10 executes the control processes of steps S36 and S37 again.

On the other hand, if the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) is equal to or greater than the voltage amplitude ratio threshold value (1.1), in step S40, the controller 10 determines the current SOC to be the lower limit value (lower limit SOC) of the SOC usage range.

If it is determined in the control process of comparison and determination in step S33 that the estimated SOC is equal to or lower than the upper limit value ( _{SOC_0} +5%) of the lower limit SOC determination range, in step S41, the controller 10 compares the estimated SOC with a value ( _{SOC_0} -5%) obtained by subtracting a predetermined SOC (5%) from the lower limit SOC ( _{SOC_0} ) of the previously determined SOC use range, and determines whether the estimated SOC is smaller than " _{SOC_0} -5%". _{SOC_0} -5% corresponds to the lower limit value of the lower limit SOC determination range.

If the estimated SOC is smaller than the lower limit value (SOC _{_0} -5%) of the lower limit SOC determination range, in step S42, the controller 10 charges the secondary battery 2 until the SOC reaches the lower limit value (SOC _{_0} -5%). After the SOC reaches the lower limit value (SOC _{_0} -5%), in step S43, the controller 10 stops charging. Next, in step S44, the discharge voltage amplitude (ΔV_ _d ) and the charge voltage amplitude (ΔV_ _c ) are calculated. The calculation method of the discharge voltage amplitude (ΔV_ _d ) and the charge voltage amplitude (ΔV_ _c ) is the same as the calculation method in the control process in step S2 in the first embodiment. In step S45, the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) is compared with the voltage amplitude ratio threshold (1.1) to determine whether the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) is equal to or less than the voltage amplitude ratio threshold (1.1). If the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) is higher than the voltage amplitude ratio threshold (1.1), in step S46, the controller 10 charges the secondary battery 2. Next, in step S47, charging of the secondary battery is stopped. Then, the controller 10 executes the control processes of steps S44 and S45 again.

On the other hand, if the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) is equal to or smaller than the voltage amplitude ratio threshold value (1.1), in step S48, the controller 10 determines the current SOC to be the lower limit value (lower limit SOC) of the SOC usage range.

If it is determined in the control process of comparison determination in step S41 that the estimated SOC is equal to or greater than the lower limit value (SOC _{_0} -5%) of the lower limit SOC determination range, that is, if the estimated SOC is within the lower limit SOC determination range, then in step S49, the controller 10 calculates the discharge voltage amplitude ( _{ΔV_d} ) and the charge voltage amplitude ( _{ΔV_c} ). The calculation methods for the discharge voltage amplitude ( _{ΔV_d} ) and the charge voltage amplitude ( _{ΔV_c} ) are the same as the calculation methods in the control process of step S2 in the first embodiment.

In step S50, the controller 10 compares the calculated voltage amplitude ratio (ΔV_ _d /ΔV _{_c} ) with the voltage amplitude ratio threshold (1.1) and determines whether the calculated voltage amplitude ratio (ΔV_ _d /ΔV _{_c} ) is less than the voltage amplitude ratio threshold (1.1). If the voltage amplitude ratio (ΔV_ _d /ΔV _{_c} ) is less than the voltage amplitude ratio threshold (1.1), in step S51, the controller 10 discharges the secondary battery 2. Then, the controller 10 executes the control process of steps S52 to S57. The control process of steps S52 to S57 is similar to the control process of steps S35 to S40.

When it is determined in the comparison judgment of step S50 that the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) is equal to or greater than the voltage amplitude ratio threshold value (1.1), the controller 10 charges the secondary battery 2 in step S58. Then, the controller 10 executes the control processing of steps S59 to S64. The control processing of steps S59 to S64 is similar to the control processing of steps S43 to S48.

Then, the controller 10 stores in memory the SOC usage range determined by the control processing of steps S40, S48, S57, and S64 in correspondence with the calculated voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ). During normal use of the secondary battery 2, the controller 10 controls the charging and discharging of the secondary battery 2 so that the SOC falls within the SOC usage range stored in the memory.

12 is a graph for explaining the relationship between the magnitude of the SOC estimated in the control process of step S32 and the charging and discharging of the secondary battery 2. As shown in FIG. 12, the lower limit SOC determination range is set to a width of a predetermined SOC (±5%) centered on the lower limit value of the previously determined SOC use range. When the estimated SOC is larger than the upper limit value ( _{SOC_0} +5%) of the lower limit SOC determination range, the secondary battery 2 is discharged, so the SOC approaches the lower limit SOC determination range from the high SOC side (see arrow H in FIG. 12). On the other hand, when the estimated SOC is smaller than the lower limit value ( _{SOC_0-5} %) of the lower limit SOC determination range, the secondary battery 2 is charged, so the SOC approaches the lower limit SOC determination range from the low SOC side (see arrow L in FIG. 12). Then, in this embodiment, after the SOC falls within the lower limit SOC determination range, the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ) is calculated, and the current SOC usage range is determined based on the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ). This makes it possible to reduce the number of repetitions of the control loops of steps S36 to S39 and steps S53 to S56.

As described above, in this embodiment, the value (voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} )) indicating the difference in responsiveness between the discharge voltage response and the charge voltage response and the determined SOC use range are stored in memory in correspondence with each other, and the charging and/or discharging of the secondary battery 2 is controlled so that the SOC is within a predetermined SOC range including the lower limit value of the previously determined SOC use range. After the SOC falls within the lower limit SOC determination range, the value (voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} )) indicating the difference in responsiveness between the discharge voltage response and the charge voltage response is calculated. Then, the current SOC use range is determined based on the value (voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} )) indicating the difference in responsiveness between the discharge voltage response and the charge voltage response calculated after the SOC falls within the lower limit SOC determination range. As a result, when determining the SOC use range, charging and discharging can be controlled while avoiding as much as possible the SOC range in which the contribution of the negative electrode metal active material is high. As a result, it is possible to suppress the decrease in capacity of the lithium ion secondary battery that is caused by the deterioration of the metal active material in the negative electrode.

In this embodiment, the lower limit SOC determination range is set to have a predetermined SOC width (e.g., ±5%) centered on the lower limit value of the previously determined SOC use range, but the SOC width can be set to any value. If the SOC width is too large, the number of control loops of steps S36 to S39 and steps S53 to S56 may increase. If the SOC width is too small, the calculated voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ) may repeatedly become larger and smaller than the voltage amplitude ratio threshold value, and may not converge to the voltage amplitude ratio threshold value. Therefore, it is preferable to set the SOC width to about twice the SOC estimation error.

In this embodiment, the controller 10 may calculate a voltage amplitude difference (ΔV _{_d} - ΔV _{_c} or ΔV _{_c} - ΔV _{_d} ) from the discharge voltage amplitude (ΔV _{_d} ) and the charge voltage amplitude (ΔV _{_c} ), and determine the SOC usage range of the secondary battery 2 based on the calculated voltage amplitude difference (ΔV _{_d} - ΔV _{_c} or ΔV _{_c} - ΔV _{_d} ). In steps S36, S44, S49, S53, and S60, the controller 10 calculates the voltage amplitude difference (ΔV _{_d} - ΔV _{_c} or ΔV _{_c} - ΔV _{_d} ) instead of the voltage amplitude ratio (ΔV _{_d} /ΔV _{_c} ). In steps S37, S45, S50, S54, and S61, controller 10 compares the calculated voltage amplitude difference (ΔV _{_d} - ΔV _{_c} or ΔV _{_c} - ΔV _{_d} ) with a voltage amplitude difference threshold. The voltage amplitude difference threshold is a threshold that is set in advance, similar to the voltage amplitude ratio threshold. Then, in steps S37, S45, S54, and S61, controller 10 determines the SOC at which the calculated voltage amplitude difference (ΔV _{_d} - ΔV _{_c} or ΔV _{_c} - ΔV _{_d} ) reaches the voltage amplitude difference threshold as the lower limit SOC of the SOC usage range.

Although the embodiments of the present invention have been described above, these embodiments are described to facilitate understanding of the present invention, and are not described to limit the present invention. Therefore, each element disclosed in the above embodiments is intended to include all design modifications and equivalents that fall within the technical scope of the present invention.

The present invention will be described below in more detail with reference to examples, but the present invention is not limited to these examples.

<Creation of prototype (embodiment)>
A negative electrode layer was created by mixing graphite carbon (Gr) and silicon oxide (SiO) in a mass ratio of 95:5 as the negative electrode active material, using styrene butadiene rubber (SBR) as the binder (binding agent), forming a slurry and applying it on copper foil. A positive electrode layer was created by using lithium nickel oxide (NCA) as the positive electrode active material, carbon black as the conductive assistant, and PVDF as the binder (binding agent), forming a slurry and applying it on aluminum foil. A polyethylene (PE) film was used as the separator. A solution in which lithium hexafluorophosphate (LiPF ₆ ) was dissolved in a mixed solvent of ethylene carbonate (EC) / dimethyl carbonate (DMC) / ethyl methyl carbonate (EMC) was used as the electrolyte. A test cell (prototype) was created by sandwiching the negative electrode layer, separator, and positive electrode layer with aluminum laminate, injecting the electrolyte into the inside, and sealing the laminate periphery.

<Evaluation of Examples>
(1) The prototype was charged and discharged as follows to adjust the SOC. First, the prototype was discharged at a discharge current value (0.2 C) until the battery voltage reached 2.5 V. After discharging, the prototype was charged at a charge current value (0.2 C) until the target voltage corresponding to an SOC of 10% was reached. After reaching the target voltage corresponding to an SOC of 10%, the prototype was switched to constant voltage charging, and charging was terminated when the charging current value dropped to 0.01 C.
(2)(1) The voltage change was measured for the prototype after SOC adjustment in the following manner. First, after one minute or more had elapsed since the end of charging, an AC current was input to the prototype for 5 seconds, and the voltage change was recorded. The frequency of this AC current was 2 Hz, the current amplitude was 4 mA, and the waveform was a triangular wave. The voltage change at this time was recorded as a change amount relative to the voltage before the input of the AC current. Then, the peak value _{V_d} (n) of the discharge side voltage and the peak value _{V_c} (n) of the charge side were calculated. Here, n is a positive integer representing the number of periods, and in this embodiment, the value of n is 10. Next, the average value of the calculated 10 points of _{V_d} (n) and _{V_c} (n) was calculated to calculate the discharge voltage amplitude ( _{ΔV_d} ) and the charge voltage amplitude ( _{ΔV_c} ). Finally, the discharge voltage amplitude ( _{ΔV_d} ) was divided by the charge voltage amplitude ( _{ΔV_c} ) to calculate the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ).
(3) After the voltage measurement in (2), the prototype was charged at a charging current value (0.2 C) up to a target voltage corresponding to an SOC of 30%. After the target voltage corresponding to an SOC of 30% was reached, constant voltage charging was performed until the current value decreased to 0.01 C (SOC adjustment).
(4) In a manner similar to that in (2), charge and discharge control was performed by applying an alternating current, the voltage change during the charge and discharge control was recorded, and the voltage amplitude ratio (ΔV_ _d /ΔV_ _c ) was calculated.
(5) After (4), a voltage equivalent to an SOC of 50% was set as a target voltage, and the SOC adjustment of (3) was performed, and the voltage change measurement of (4) (including calculation of the voltage amplitude ratio (ΔV_ _d /ΔV_ _c )) was performed.

<Evaluation Results of Examples>
After adjusting the SOC to 10%, the voltage change measured in (2) was plotted, and the measurement results shown in the graph of FIG. 13(a) were obtained. In FIG. 13(a), graph a represents the change in AC current, and graph b represents the voltage response to AC current. When the calculated voltage amplitudes ( _{ΔV_d} , _{ΔV_c} ) were plotted for each SOC (10%, 30%, 50%) using the evaluation methods (1) to (5), the measurement results shown in the graphs of FIG. 13(b) to FIG. 13(d) were obtained. 13(b) to 13(d), the horizontal axis is common to SOC, and the vertical axis represents the voltage amplitude ( _{ΔV_d} , _{ΔV_c} ), the ratio ( _{ΔV_d} / _{ΔV_c} ) of the voltage amplitudes ( _{ΔV_d} , _{ΔV_c} ), and the difference ( _{ΔV_d} - _{ΔV_c} ) of the voltage amplitudes ( _{ΔV_d} , _{ΔV_c} ). In addition, in FIG. 13(b), graph a represents the change in the discharge voltage amplitude ( _{ΔV_d} ), and graph b represents the change in the charge voltage amplitude ( _{ΔV_c} ).

13(c) and 13(d), it can be confirmed that in the range of SOC of 30% or less, the ratio ( _{ΔV_d} / _{ΔV_c} ) and difference ( _{ΔV_d} - _{ΔV_c} ) of the voltage amplitudes ( _{ΔV_d} , _{ΔV_c} ) are larger than the values in the range of SOC of more than 30%. That is, in the examples, it was confirmed that in the SOC range in which SiO contributes greatly to charging and discharging, the difference in the amplitude of the voltage response during charging and discharging is larger than in the SOC range in which Gr contributes.

Reference Signs List 1... Determination device 2... Secondary battery 10... Controller 11... Voltage sensor 12... Current sensor 13... DCDC converter 21... Positive electrode layer 22... Negative electrode layer 23... Separator 24... Positive electrode tab 25... Negative electrode tab 26... Upper exterior member 27... Lower exterior member

Claims (5)

A method for controlling a lithium ion secondary battery having a negative electrode containing graphite and a metal active material, comprising:
Controlling charging and discharging of the lithium ion secondary battery;
When charging/discharging is stopped, a voltage response to an AC current input or a current response to an AC voltage input is calculated;
A control method for determining a usage range (SOC usage range) of the state of charge (SOC) of the lithium-ion secondary battery based on the difference in responsiveness between a discharge voltage response, which is the voltage response on the discharge side, and a charge voltage response, which is the voltage response on the charge side, or the difference in responsiveness between a discharge current response, which is the current response on the discharge side, and a charge current response, which is the current response on the charge side. 2. The control method according to claim 1,
A _control method for _controlling charging and/or discharging of the lithium ion secondary battery within the SOC usage range in which the value of the voltage amplitude ratio ( _{ΔV_d} / _{ΔV_c} ), which is the ratio of the amplitude of the discharge voltage response ( _{ΔV_d} ) to the amplitude of the charge voltage response ( _{ΔV_c} ), or the value of the current amplitude ratio ( _{ΔI_d} / _{ΔI_c} ), which is the ratio of the amplitude of the discharge current response (ΔI_d) to the amplitude of the charge current response (ΔI_d), is less than a predetermined threshold value. 3. The control method according to claim 1,
a value indicating a difference in responsiveness between the discharge voltage response and the charge voltage response or a difference in responsiveness between the discharge current response and the charge current response is associated with the determined SOC use range and stored in a memory;
Controlling charging and/or discharging of the lithium ion secondary battery so that the SOC falls within a predetermined SOC range including a lower limit value of the previously determined SOC use range;
After the SOC falls within the predetermined SOC range, a value indicating a difference in responsiveness between the discharge voltage response and the charge voltage response or a difference in responsiveness between the discharge current response and the charge current response is calculated;
A control method for determining the current SOC usage range based on a value indicating the difference in responsiveness between the discharge voltage response and the charge voltage response or the difference in responsiveness between the discharge current response and the charge current response, calculated after the SOC falls within the specified SOC range. In the control method according to any one of claims 1 to 3,
The control method, wherein the metal active material contains Si. In the control method according to any one of claims 1 to 4,
A control method for determining the SOC usage range in a state where the lithium ion secondary battery is connected to a power grid.

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