JP2017084552A - Secondary battery system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make an appropriate determination about whether or not an electrolytic solution has frozen regardless of a battery temperature in a secondary battery system including a lithium ion secondary battery.SOLUTION: A secondary battery system 2 comprises: a battery 100 with an electrolytic solution containing lithium ions; a resistance measuring apparatus 104 operable to measure an internal resistance of the battery 100; and an ECU 300 operable to take the internal resistance of the battery 100 from the resistance measuring apparatus 104. The ECU 300 takes and stores an internal resistance RA at each temperature when the temperature of the battery 100 lowers. If a resistance difference ΔR between an internal resistance RB taken when the temperature of the battery 100 rises, and the stored internal resistance RA at the same temperature exceeds a predetermined value, the ECU 300 determines that the electrolytic solution has frozen. If the resistance difference ΔR between the internal resistance RB taken when the temperature of the battery 100 rises, and the stored internal resistance RA at the same temperature is under the predetermined value, the ECU 300 determines that the electrolytic solution has not frozen.SELECTED DRAWING: Figure 8

Description

  The present invention relates to a secondary battery system, and more particularly to a secondary battery system including a lithium ion secondary battery.

  When a battery including a lithium ion secondary battery is placed in a cryogenic environment (for example, in an environment of about −35 ° C.), the electrolytic solution can be frozen (solidified). When the electrolyte is frozen, it is desirable to limit charging and discharging in order to protect the battery. In order to appropriately limit the charging / discharging of the battery, a technique for determining whether or not the electrolytic solution is frozen has been proposed.

  For example, in the freeze detection device disclosed in Japanese Patent Application Laid-Open No. 2006-155916 (Patent Document 1), when the electrolytic solution starts to freeze partially, the concentration of the electrolytic solution in the non-frozen portion increases and the open circuit is opened. A phenomenon in which the voltage (OCV: Open Circuit Voltage) increases is used. The freeze detection device detects the freezing of the electrolyte when the OCV rises despite the battery temperature being lowered.

JP 2006-155916 A

  In Patent Document 1, although it is described that the electrolyte solution has reached a frozen state due to an increase in OCV, the electrolyte solution that has once reached a frozen state returns to a state where it has not been frozen (non-frozen state). No consideration is given to determining whether or not.

  The present invention has been made to solve the above-described problems, and an object of the present invention is a technique capable of appropriately determining whether an electrolyte is in a frozen state or a non-frozen state in a secondary battery system including a lithium ion secondary battery. Is to provide.

  A secondary battery system according to an aspect of the present invention includes a secondary battery that includes lithium ions in an electrolyte, a measurement unit that measures the internal resistance of the secondary battery, and a control that acquires the internal resistance of the secondary battery from the measurement unit. Device. The control device acquires and stores the internal resistance at each temperature when the temperature of the secondary battery decreases. The control device determines that the electrolyte is frozen when the difference in the same temperature between the internal resistance acquired when the temperature of the secondary battery rises and the stored internal resistance exceeds a predetermined value. When the difference in the same temperature between the internal resistance acquired when the battery temperature rises and the stored internal resistance is below a predetermined value, it is determined that the electrolyte is not frozen.

  According to the above configuration, whether the electrolyte is frozen or not is determined based on the internal resistance of the secondary battery. When the temperature of the electrolytic solution is lowered, other solvent molecules that are not involved in nucleation (ice crystal nuclei or freezing nucleation) by some of the solvent molecules of the electrolytic solution can move freely to some extent. That is, since the ratio of solvent molecules that can move freely is relatively high, the internal resistance is relatively low. On the other hand, when the temperature of the electrolytic solution rises, even if the temperature is the same, the solvent molecules inside the solid are restricted and cannot move, and only the solvent molecules present on the solid surface can move freely. That is, since the ratio of solvent molecules that can move freely is relatively low, the internal resistance is relatively high. Therefore, the internal resistance acquired at the time of temperature rise is higher than the internal resistance at the same temperature acquired at the time of temperature decrease. By using this feature, it is possible to determine that the electrolytic solution is in a frozen state, and it is also possible to determine that the electrolytic solution once frozen is in a non-frozen state. That is, it can be appropriately determined whether the electrolytic solution is in a frozen state or a non-frozen state.

  ADVANTAGE OF THE INVENTION According to this invention, in a secondary battery system provided with a lithium ion secondary battery, it can determine appropriately whether electrolyte solution is a frozen state or a non-freezing state.

It is a block diagram which shows roughly the whole structure of the vehicle by which the secondary battery system which concerns on this Embodiment is mounted. It is a figure which shows the structure of a battery in detail. It is a figure for demonstrating in detail the structure of each cell. It is a figure which shows an example of the measurement result of internal resistance at the time of the temperature rise of a battery of an initial state, and a temperature fall. It is a figure for demonstrating the mode of the electrolyte solution at the time of the temperature rise of a battery, and the temperature fall. It is a flowchart which shows the measurement process of internal resistance at the time of the temperature fall of a battery. It is a flowchart for demonstrating the measurement process of internal resistance at the time of the temperature rise of a battery, and the freezing determination process of electrolyte solution. It is a figure which shows an example of the measurement result of internal resistance at the time of the temperature rise of a battery after deterioration, or the temperature fall.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated.

  In the embodiments described below, a configuration in which the secondary battery system according to the present invention is mounted on a hybrid vehicle will be described as an example. However, the use of the secondary battery system according to the present invention is not limited to vehicles.

[Embodiment]
FIG. 1 is a block diagram schematically showing an overall configuration of a vehicle equipped with a secondary battery system according to the present embodiment. Vehicle 1 is a hybrid vehicle, and includes secondary battery system 2, motor generators 10 and 20, power split mechanism 30, drive shaft 40, speed reducer 50, engine 60, drive wheels 70, and power control. A unit (PCU: Power Control Unit) 200.

  The engine 60 is an internal combustion engine such as a gasoline engine or a diesel engine. Engine 60 generates a driving force for vehicle 1 to travel in response to a control signal from ECU 300.

  Each of motor generators 10 and 20 is a three-phase AC rotating electric machine. Motor generator 10 is coupled to the crankshaft of engine 60 through power split mechanism 30. The motor generator 10 rotates the crankshaft of the engine 60 using the electric power of the battery 100 when starting the engine 60. The motor generator 10 can also generate power using the power of the engine 60. The AC power generated by the motor generator 10 is converted into DC power by the PCU 200 and the battery 100 is charged. Further, the AC power generated by the motor generator 10 may be supplied to the motor generator 20.

  Motor generator 20 rotates drive shaft 40 using at least one of the electric power from battery 100 and the electric power generated by motor generator 10. The motor generator 20 can also generate power by regenerative braking. The AC power generated by the motor generator 20 is converted into DC power by the PCU 200 and the battery 100 is charged.

  Power split device 30 is, for example, a planetary gear mechanism, and mechanically connects the three elements of the crank shaft of engine 60, the rotation shaft of motor generator 10, and drive shaft 40. The drive shaft 40 is connected to the drive wheels 70 via the speed reducer 50. Reducer 50 transmits power from power split mechanism 30 or motor generator 20 to drive wheels 70. Further, the reaction force from the road surface received by the drive wheels 70 is transmitted to the motor generator 20 via the speed reducer 50 and the power split mechanism 30.

  Although not shown, PCU 200 includes two inverters and a converter. Each of the two inverters is a general three-phase inverter. During the boosting operation, the converter boosts the voltage supplied from the battery 100 and supplies the boosted voltage to the inverter. During the step-down operation, the converter steps down the voltage supplied from the inverter and charges battery 100.

  The secondary battery system 2 includes a battery 100, a monitoring unit 102, a resistance measuring device 104, a duct 62, a fan 64, and an electronic control unit (ECU: Electronic Control Unit) 300.

  The battery 100 is a power storage device that includes a lithium ion secondary battery and is configured to be rechargeable. The configuration of the battery 100 will be described in detail with reference to FIGS.

  The duct 62 is provided between the battery 100 and the engine 60 and guides air (hot air indicated by an arrow AR) heated by the heat of the engine 60 to the battery 100. The duct 62 can be formed so as to branch off from a hot air path (not shown) for heating the passenger compartment, for example. A fan 64 is provided in the duct 62. When fan 64 is driven in accordance with a control signal from ECU 300, the warm air is guided to battery 100. Thereby, the battery 100 can be warmed up. Note that the warm air is not limited to exhaust air if it is air heated by the heat of the engine 60.

  The monitoring unit 102 detects the voltage VB of the battery 100, the current IB input / output to / from the battery 100, and the temperature TB of the battery 100. Monitoring unit 102 outputs a signal indicating the detection result to ECU 300. It can be approximated that the temperature of the electrolyte solution of the battery 100 is equal to the temperature TB.

  The resistance measuring instrument (measurement unit) 104 measures the internal resistance (impedance) of the battery 100 and outputs a signal indicating the measurement result to the ECU 300. As a method for measuring the internal resistance, various known methods such as an AC impedance measurement method can be used. When the equivalent circuit of the battery 100 is expressed as a combined circuit of a liquid resistance (DC resistance) Rs, a charge transfer resistance Rct, and an electric double layer capacitance Cd, the internal resistance includes a component of the DC resistance Rs. It is desirable to use it. However, the method of measuring the internal resistance is not limited to this, and for example, a value obtained by dividing the difference between the voltage VB and the OCV when the battery 100 is charged / discharged by a pulse current by the current IB may be used as the internal resistance.

  The ECU (control device) 300 includes a CPU (Central Processing Unit) (not shown), a memory 302, an input / output buffer, and the like. ECU 300 controls each device so that vehicle 1 is in a desired state based on a signal received from each sensor and a map and a program stored in memory 302. More specifically, ECU 300 determines allowable discharge power Wout and allowable charge power Win of battery 100 based on voltage VB, current IB, and temperature TB of battery 100. Further, ECU 300 controls charging / discharging of battery 100 so that the discharge power from battery 100 does not exceed allowable discharge power Wout and the charge power to battery 100 does not exceed allowable charge power Win.

  FIG. 2 is a diagram showing the configuration of the battery 100 in more detail. FIG. 3 is a diagram for explaining the configuration of each cell 110 in more detail. It is assumed that the vehicle 1 is in a horizontal state.

  Referring to FIG. 2, battery 100 includes a plurality of cells 110, a pair of end plates 120, a restraining band 130, and a plurality of bus bars 140. In FIG. 2, one end in the stacking direction (x-axis direction) is partially shown in the stacked body formed by stacking the plurality of cells 110. A pair of end plates 120 are arranged so as to face one end of the stacked body and the other end in the stacking direction. The pair of end plates 120 are restrained by the restraining band 130 in a state where all the cells 110 are sandwiched. Each cell 110 has a positive terminal 113 and a negative terminal 114 (see FIG. 3). A positive electrode terminal 113 of a certain cell and a negative electrode terminal 114 of an adjacent cell are fastened and electrically connected by a bus bar 140. Thereby, a plurality of cells 110 are connected in series.

  In FIG. 3, the cell 110 is shown through the inside. The cell 110 has a battery case 111 having a substantially rectangular parallelepiped shape. The short side direction (thickness direction) of the battery case 111 is the x-axis direction, the long side direction (length direction) is the y-axis direction, and the height direction is the z-axis direction. The vertical direction is the negative z-axis direction, and the horizontal direction is the xy plane direction.

  The upper surface of the battery case 111 is sealed with a lid 112. One end of each of the positive terminal 113 and the negative terminal 114 protrudes from the lid 112 to the outside. The other ends of the positive electrode terminal 113 and the negative electrode terminal 114 are respectively connected to an internal positive electrode terminal and an internal negative electrode terminal (both not shown) inside the battery case 111. An electrode body 115 is accommodated in the battery case 111. The electrode body 115 is formed by laminating a positive electrode 116 and a negative electrode 117 via a separator 118 and winding the laminated body. The electrolytic solution is held by the positive electrode 116, the negative electrode 117, the separator 118, and the like.

As the positive electrode 116, the negative electrode 117, the separator 118, and the electrolytic solution, conventionally known configurations and materials can be used as the positive electrode, the negative electrode, the separator, and the electrolytic solution of the lithium ion secondary battery, respectively. As an example, the electrolyte includes an organic solvent (for example, a mixed solvent of DMC (dimethyl carbonate), EMC (ethyl methyl carbonate) and EC (ethylene carbonate)), a lithium salt (for example, LiPF ₆ ), and an additive (for example, LiBOB). (Lithium bis (oxalate) borate) or Li [PF ₂ (C ₂ O ₄ ) ₂ ]).

  In the secondary battery system 2 configured as described above, the electrolyte solution of each cell 110 in the battery 100 can be solidified under a cryogenic environment. In order to protect the battery 100 at an extremely low temperature, it is desirable to limit charging / discharging (particularly charging) of the battery 100.

  As a battery protection method, for example, it may be determined that the electrolyte has solidified when the temperature TB of the battery 100 decreases and reaches the freezing point. However, in general, the temperature at which the electrolytic solution freezes (freezing point) may vary depending on conditions such as the composition of the electrolytic solution (for example, salt concentration or solvent ratio) or SOC (State Of Charge). Further, even in an environment where the temperature of the electrolytic solution is kept substantially constant, the electrolytic solution may be frozen as time passes even if the electrolytic solution does not freeze in a short time. Therefore, if an attempt is made simply to determine whether the electrolytic solution is in a frozen state or a non-frozen state based on the temperature TB of the battery 100, an appropriate determination result may not be obtained.

  Therefore, in the present embodiment, a configuration is adopted in which it is determined whether or not the electrolytic solution is frozen based on the measurement result of the internal resistance by the resistance measuring device 104. More specifically, when the difference (resistance difference) at the same temperature between the internal resistance acquired when the temperature of the battery 100 is increased and the internal resistance acquired when the temperature of the battery 100 is decreased is greater than a predetermined value, electrolysis is performed. The liquid is determined to be frozen. On the other hand, when the resistance difference at the same temperature is not more than the predetermined value, the electrolytic solution is determined to be in a non-frozen state.

  FIG. 4 is a diagram illustrating an example of a measurement result of the internal resistance when the temperature of the battery 100 in the initial state is increased and when the temperature is decreased. In FIG. 4 and FIG. 8 described later, the horizontal axis represents the temperature TB of the battery 100. The difference between the temperatures T1 to T9 (T1 <T2 <T3 <T4 <T5 <T6 <T7 <T8 <T9) is 1 ° C. The vertical axis represents the internal resistance of the battery 100.

  FIG. 4A relates to the battery 100 in an initial state (for example, a state immediately after manufacturing), and acquires the internal resistance RA every time the temperature TB changes by 1 ° C. when the temperature TB of the battery 100 decreases from T9 to T3. Further, after that, when the temperature TB rises from T3 to T9, the result of acquiring the internal resistance RB every time the temperature TB changes by 1 ° C. is shown. In this temperature range (T3 <TB <T9), the electrolyte solution of the battery 100 is not frozen. In this case, the internal resistance RA acquired when the temperature decreases and the internal resistance RB acquired when the temperature increases are substantially equal.

  On the other hand, FIG. 4B shows the same battery 100, and when the temperature TB of the battery 100 decreases from T9 to T1, the internal resistance RA is acquired every time the temperature TB changes by 1 ° C. The result of acquiring the internal resistance RB every time the temperature TB changes by 1 ° C. when increasing from T1 to T9 is shown. At the temperature TB = T2, it is confirmed that the electrolyte solution of the battery 100 is in a frozen state. In this case, it can be seen that the internal resistance RB acquired at the temperature TB = T2 when the temperature rises is larger than the internal resistance RA acquired at the same temperature TB = T2 when the temperature decreases.

  Thereafter, when the temperature TB of the battery 100 further increases, it is observed that, for example, at the temperature TB = T4, the once frozen electrolyte melts and returns to the liquid (non-frozen state). When the electrolytic solution returns to the non-freezing state, the internal resistance RA and the internal resistance RB become substantially equal again.

  As described above, when the electrolytic solution freezes, a resistance difference ΔR (= RB−RA) larger than a predetermined value is generated between the internal resistance RB at the time of temperature rise and the internal resistance RA at the time of temperature fall. In other words, there is hysteresis in the internal resistance measurement. This is thought to be due to the following reasons.

  FIG. 5 is a diagram for explaining the state of the electrolytic solution when the temperature of the battery 100 is increased and when the temperature is decreased. When the liquid electrolyte is frozen as the temperature of the electrolyte decreases, nuclei (so-called ice nuclei or frozen nuclei) are generated by some of the solvent molecules in the electrolyte. Other solvent molecules that are not involved in the nucleation can move freely to some extent, although the intermolecular distance is limited by the intermolecular force. On the other hand, when the solid electrolyte melts as the temperature of the battery 100 rises, even if the temperature is the same, the solvent molecules inside the solid are constrained and hardly move and exist on the solid surface. Only solvent molecules that move can move freely.

  In this way, the proportion of solvent molecules that can move freely when the electrolyte is frozen (ie, the proportion of solvent molecules that do not participate in nucleation when the temperature is lowered) is freely set at the same temperature when the electrolyte is melted. It is higher than the percentage of solvent molecules that can move (ie, the percentage of solvent molecules that are present on the stationary surface when the temperature is increased). For this reason, the internal resistance RA acquired when the temperature is lowered is relatively lower than the internal resistance RB measured at the same temperature when the temperature is increased. Therefore, by acquiring the resistance difference ΔR between the internal resistance RB and the internal resistance RA at the same temperature when the temperature of the battery 100 changes, it can be determined whether the electrolytic solution is frozen or not frozen.

  Next, processing executed in secondary battery system 2 according to the present embodiment will be described in detail using the flowcharts shown in FIGS. 6 and 7. In parallel, the processes of these flowcharts are called from the main routine and executed repeatedly at predetermined control cycles, for example. Each step (hereinafter abbreviated as S) of these flowcharts is basically realized by software processing by the ECU 300, but may be realized by hardware processing using an electronic circuit produced in the ECU 300.

  FIG. 6 is a flowchart showing a process for measuring the internal resistance RA when the temperature of the battery 100 is lowered. In S <b> 10, ECU 300 acquires temperature TB of battery 100 from monitoring unit 102.

  In S20, ECU 300 determines whether or not battery temperature TB has decreased. ECU 300 determines that temperature TB has decreased, for example, when battery temperature TB is 1 ° C. lower than the previous measurement of internal resistance RA (YES in S20), and battery 100 at lower temperature TB is determined. The internal resistance RA is acquired from the resistance measuring device 104 (S30). The acquired internal resistance RA is stored in a nonvolatile manner in the memory 302 of the ECU 300 (S40). 6 is repeatedly executed, the internal resistance RA (see FIG. 4) is acquired every time the temperature TB of the battery 100 decreases by 1 ° C.

  FIG. 7 is a flowchart for explaining the internal resistance RA measurement process and the electrolytic solution freezing determination process when the temperature of the battery 100 rises. In S110, ECU 300 determines whether or not temperature TB of battery 100 is equal to or lower than a predetermined reference temperature Tc. For example, in the example shown in FIG. 4B, the reference temperature Tc is set to a temperature that is higher by a predetermined value α than T1. When battery temperature TB is higher than reference temperature Tc (NO in S110), ECU 300 skips the subsequent processing and returns the processing to the main routine.

  On the other hand, for example, when temperature TB becomes equal to or lower than reference temperature Tc as the outside air temperature decreases (YES in S110), ECU 300 drives engine 60 (or maintains the driving state) and is warmed by the heat generated in engine 60. The air (warm air) is sent to the battery 100 using the fan 64. Thereby, the battery 100 is warmed up (S120). The warming up of battery 100 is continued until temperature TB rises by a predetermined temperature (for example, 1 ° C.) (NO in S130). In addition, the warming-up method of the battery 100 is not specifically limited.

  When temperature TB increases (YES in S130), ECU 300 acquires internal resistance RB at the time of temperature increase (S140). Then, ECU 300 compares a resistance difference ΔR between internal resistance RB at the time of temperature rise and internal resistance RA at the time of temperature drop with a predetermined value.

  When resistance difference ΔR is larger than the predetermined value (YES in S150), ECU 300 determines that the electrolytic solution is frozen (S160). In this case, ECU 300 restricts charging / discharging of battery 100 in order to protect battery 100 (S170). For example, ECU 300 prohibits charging / discharging of battery 100 and prohibits traveling of vehicle 1.

  On the other hand, when resistance difference ΔR is equal to or smaller than the predetermined value (NO in S150), ECU 300 determines that the electrolyte is not frozen (is in an unfrozen state) (S180). In this case, it is not particularly necessary to limit the charging / discharging of the battery 100 in consideration of the freezing of the electrolytic solution, and therefore, within the range of the allowable charging power Win of the battery 100 according to the temperature TB or the SOC, as in normal times. Or discharge within the range of allowable discharge power Wout is performed (S190).

  As described above, according to the present embodiment, whether the electrolyte is frozen or not is determined by the internal resistances RA and RB of battery 100. By using the characteristics of the internal resistances RA and RB described in FIG. 5, it is possible to determine that the electrolytic solution is frozen, and in addition, it is determined that the once frozen electrolytic solution is not frozen. It is also possible to do. That is, it can be appropriately determined whether the electrolytic solution is in a frozen state or a non-frozen state.

[Modification]
Although the measurement result of the internal resistance of the battery 100 in the initial state has been described with reference to FIG. 4, the internal resistance of the battery 100 may change as the battery 100 deteriorates.

  FIG. 8 is a diagram illustrating an example of a measurement result of the internal resistance when the temperature of the battery 100 after deterioration is increased or decreased. In the initial state (see FIG. 4), the electrolyte solution was frozen at a temperature TB = T3, but after the deterioration, the electrolyte solution was at a temperature TB higher than T3 (T4 in the example shown in FIG. 8B). Freezes. Therefore, after the deterioration, when determining whether or not the electrolytic solution is frozen, the temperature TB can be increased after the temperature TB reaches a temperature higher than the T2 by the predetermined value α.

  Even after the battery 100 is deteriorated, the temperature TB may be increased after the temperature TB reaches T1 + α, as in the initial state (see FIG. 4). However, when the temperature drop is suppressed to T2 + α, the electrolyte solution is likely to melt when the battery 100 is subsequently warmed up, compared to the case where the temperature TB drops to T1 + α. Therefore, the time required for the vehicle 1 to return to a state where it can run is reduced. Accordingly, in the processing of S110 shown in FIG. 7, it is preferable to set the reference temperature Tc = T2 + α after the deterioration while the reference temperature Tc = T1 + α is set in the initial state.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is shown not by the above description of the embodiments but by the scope of claims for patent, and is intended to include meanings equivalent to the scope of claims for patent and all modifications within the scope.

  1 vehicle, 2 secondary battery system, 10, 20 motor generator, 30 power split mechanism, 40 drive shaft, 50 reducer, 60 engine, 62 duct, 64 fan, 70 drive wheel, 100 battery, 102 monitoring unit, 104 resistance Measuring instrument, 110 cell, 111 battery case, 112 lid, 113 positive electrode terminal, 114 negative electrode terminal, 115 electrode body, 116 positive electrode, 117 negative electrode, 118 separator, 120 end plate, 130 binding band, 140 bus bar, 200 PCU, 300 ECU, 302 memory.

Claims (1)

A secondary battery containing lithium ions in the electrolyte;
A measuring unit for measuring the internal resistance of the secondary battery;
A control device that acquires the internal resistance of the secondary battery from the measurement unit;
The controller is
Acquire and store the internal resistance at each temperature when the temperature of the secondary battery drops,
When the difference between the internal resistance acquired at the time of temperature rise of the secondary battery and the stored internal resistance at the same temperature exceeds a predetermined value, it is determined that the electrolytic solution is frozen,
A secondary battery that determines that the electrolytic solution is not frozen when a difference in the same temperature between the internal resistance acquired at the time of temperature rise of the secondary battery and the stored internal resistance is lower than the predetermined value; system.

Images