JP2017224522A - Battery system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To exercise performance of nickel metal hydride battery, in a battery system including a nickel metal hydride battery containing a hydrogen-storing alloy as a negative electrode active material.SOLUTION: A battery system 2 includes a battery 100 (nickel metal hydride battery) containing a hydrogen-storing alloy as a negative electrode active material 5, and an ECU300 for controlling a PCU200 so that charge power to the battery 100 is less than a charge power upper limit Win, and discharge power from the battery 100 is less than a discharge power upper limit Wout. The ECU300 calculates a crack index value X related to a crack amount occurring in the negative electrode active material 5 from OCV, a current Ib and a temperature Tb, and increases the charge power upper limit Win and discharge power upper limit Wout as the crack amount indicated by the crack index value X increases.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a battery system, and more particularly to a battery system including a nickel-metal hydride battery containing a hydrogen storage alloy as a negative electrode active material.

  In recent years, electric vehicles such as hybrid vehicles equipped with a battery system have been widely used. In these battery systems, the characteristics (resistance value, capacity value, etc.) of the battery can change due to various factors. Therefore, a technique has been proposed in which presence / absence (or degree) of a change in battery performance is determined and charge / discharge is controlled according to the determination result. For example, Japanese Patent Laying-Open No. 2011-228213 (Patent Document 1) discloses a method for determining a crack of a positive electrode active material in an active material layer formed on a positive electrode plate of a lithium ion battery. According to Patent Document 1, whether or not the positive electrode active material is cracked is determined using a graph in which a set data of the square root of the integrated value of the discharge amount of the lithium ion battery and the battery capacity is plotted.

JP 2011-228213 A

  In many battery systems installed in electric vehicles, etc., in order to protect the battery, a control upper limit value of charge power to the battery (charge power upper limit value) and a control upper limit value of discharge power from the battery (discharge power) Upper limit value). The charging / discharging of the battery is controlled such that the charging power to the battery is less than the charging power upper limit value and the discharging power from the battery is less than the discharging power upper limit value.

  In the nickel-metal hydride battery containing a hydrogen storage alloy as a negative electrode active material, the present inventor determines the mode at the time of occurrence and progress of cracks in the negative electrode active material in the negative electrode active material of another secondary battery (for example, a lithium ion battery). We focused on the differences from the embodiment. Although the details will be described later, the present inventor decided that the internal resistance of the nickel-metal hydride battery (more specifically, the reaction resistance of the negative electrode) decreases as the amount of the negative electrode active material cracks (crack amount) increases. Pay attention.

  When the internal resistance of the nickel metal hydride battery decreases, the amount of voltage change during charge / discharge (for example, the amount of voltage drop) decreases, so that the charge power to the nickel metal hydride battery does not easily reach the charge power upper limit, and the nickel metal hydride battery. It becomes difficult for the discharge power from to reach the discharge power upper limit. When charging / discharging a nickel metal hydride battery without considering the amount of cracks generated in the negative electrode active material, each upper limit value may be set to be smaller than necessary. As a result, charging / discharging of the nickel metal hydride battery is excessively limited, and the performance of the nickel metal hydride battery may not be sufficiently exhibited.

  The present invention has been made in order to solve the above-described problems, and an object thereof is a technique for exerting the performance of a nickel-metal hydride battery in a battery system including a nickel-metal hydride battery containing a hydrogen storage alloy as a negative electrode active material. Is to provide.

  A battery system according to an aspect of the present invention supplies power to an external device. The battery system includes a nickel metal hydride battery containing a hydrogen storage alloy as a negative electrode active material, a power conversion device that converts electric power between the nickel metal hydride battery and an external device, and a charge power to the nickel metal hydride battery is an upper limit of charge power control. And a control device that controls the power converter so that the discharge power from the nickel metal hydride battery is less than the control upper limit value of the discharge power. The control device calculates an index value related to the amount of cracks generated in the negative electrode active material from the current input to and output from the nickel metal hydride battery and the temperature of the nickel metal hydride battery. The control device increases the control upper limit value of the charging power and the control upper limit value of the discharge power as the crack amount indicated by the index value increases.

  As described above, the internal resistance of the nickel-metal hydride battery decreases as the crack amount increases. According to the above configuration, the control upper limit value of charging power and the control upper limit value of discharge power are set larger as the crack amount increases. This makes it difficult for the charge power to reach the control upper limit value of the charge power and also makes it difficult for the discharge power to reach the control upper limit value of the discharge power, thereby preventing the charge / discharge of the nickel metal hydride battery from being excessively limited. The Therefore, the performance of the nickel metal hydride battery can be exhibited.

It is a block diagram which shows roughly the whole structure of the hybrid vehicle by which the battery system which concerns on this Embodiment is mounted. It is a figure which shows the structure of each cell contained in a battery in detail. It is a figure for demonstrating the crack which arises in the negative electrode active material of a secondary battery. It is a figure for demonstrating the calculation process of the crack index value X. FIG. It is a time chart for demonstrating the setting process of the charge power upper limit of a battery, and the discharge power upper limit. It is a flowchart for demonstrating the setting process of the charge power upper limit of a battery, and the discharge power upper limit.

  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.

  Hereinafter, a configuration in which the battery system according to the embodiment of the present invention is mounted on a hybrid vehicle will be described as an example. However, the use of the battery system is not limited to vehicles, and may be stationary.

[This embodiment]
<Battery system configuration>
FIG. 1 is a block diagram schematically showing an overall configuration of a hybrid vehicle equipped with a secondary battery system according to the present embodiment. The vehicle 1 includes a battery system 2, motor generators (MG) 10 and 20, a power split mechanism 30, an engine 40, and drive wheels 50. The battery system 2 includes a battery 100, a system main relay (SMR) 150, a power control unit (PCU) 200, and an electronic control unit (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 40 via power split mechanism 30. The motor generator 10 rotates the crankshaft of the engine 40 using the power of the battery 100 when starting the engine 40. The motor generator 10 can also generate power using the power of the engine 40. 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 the drive shaft 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 40, the rotation shaft of motor generator 10, and the drive shaft. The engine 40 is an internal combustion engine such as a gasoline engine, and generates a driving force for the vehicle 1 to travel in response to a control signal from the ECU 300.

  PCU 200 converts electric power between battery 100 and motor generators 10 and 20. Although neither is shown, an inverter and a converter are included. The inverter 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. SMR 150 is electrically connected to a current path connecting battery 100 and PCU 200. When SMR 150 is closed in response to a control signal from ECU 300, power can be exchanged between battery 100 and PCU 200. Motor generators 10 and 20 correspond to “external equipment” according to the present invention, and PCU 200 corresponds to “power converter” according to the present invention.

  The battery 100 includes a plurality of cells 101 each of which is a nickel metal hydride battery. The configuration of each cell 101 will be described with reference to FIG. The battery 100 is provided with a voltage sensor 110, a current sensor 120, and a temperature sensor 130. Voltage sensor 110 detects voltage Vb of battery 100. Current sensor 120 detects current Ib input / output to / from battery 100. The temperature sensor 130 detects the temperature Tb of the battery 100. Each sensor outputs the detection result to ECU 300. ECU 300 calculates the SOC (State Of Charge) of battery 100 based on the detection results of the sensors.

  The ECU 300 includes a CPU (Central Processing Unit) 301, a memory 302, an input / output buffer (not shown), 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. As main control executed by the ECU 300, charge / discharge control of the battery 100 can be cited. For example, for the purpose of protecting the battery 100, the ECU 300 has a charge power to the battery 100 that is less than the charge power upper limit value (control upper limit value) Win, and a discharge power from the battery 100 is a discharge power upper limit value (control upper limit value). The charging / discharging of the battery 100 is controlled so as to be less than Wout. This charge / discharge control will be described in detail later.

  FIG. 2 is a diagram illustrating a configuration of the cell 101 included in the battery 100. Since the configuration of each cell 101 is common, only one cell 101 is representatively shown in FIG. The cell 101 is, for example, a square sealed cell, and includes a case 102, a safety valve 103 provided in the case 102, an electrode body 104 and an electrolytic solution (not shown) accommodated in the case 102. In FIG. 2, the electrode body 104 is shown through a part of the case 102.

  The case 102 includes a case main body and a lid made of metal, and the lid is hermetically sealed by being welded all around the opening of the case book. When the pressure inside the case 102 exceeds a predetermined value, the safety valve 103 discharges a part of the gas (hydrogen gas or the like) inside the case 102 to the outside. The electrode body 104 includes a positive electrode plate, a negative electrode plate, and a separator. The positive electrode plate is inserted into a bag-shaped separator, and the positive electrode plate and the negative electrode plate inserted into the separator are alternately laminated. The positive electrode plate and the negative electrode plate are electrically connected to a positive electrode terminal and a negative electrode terminal (not shown), respectively.

Various conventionally known materials can be used as the material for the electrode body 104 and the electrolytic solution. In this embodiment, as an example, an electrode plate including a positive electrode active material layer containing nickel hydroxide (Ni (OH) ₂ or NiOOH) and an active material support such as foamed nickel is used for the positive electrode plate. It is done. As the negative electrode plate, an electrode plate containing a hydrogen storage alloy (for example, LaNi ₅ or ReNi ₅ ) as a negative electrode active material is used. For the separator, a nonwoven fabric made of a synthetic fiber that has been subjected to a hydrophilic treatment is used. An alkaline aqueous solution containing potassium hydroxide (KOH) or sodium hydroxide (NaOH) is used for the electrolytic solution.

<Occurrence of cracks in the negative electrode active material>
In a hydrogen storage alloy that is generally used as a negative electrode active material of a nickel metal hydride battery, an unevenness (distribution) of hydrogen concentration may occur in the negative electrode active material as the nickel hydride battery is charged and discharged. When the local hydrogen amount (proton amount) in the negative electrode active material is different, the local volume change amount is also different. As a result, distortion may occur in the crystal lattice and stress may be generated. As described above, the stress generated in the negative electrode active material exceeds the mechanical limit strength of the negative electrode active material, and thus cracks may occur. This crack will be described below in comparison with a crack generated in the negative electrode active material of the lithium ion battery.

FIG. 3 is a diagram for explaining a crack generated in the negative electrode active material of the secondary battery. 3A schematically shows a negative electrode active material 4 (for example, graphite or silicon) of a lithium ion battery, and FIG. 3B schematically shows a negative electrode active material of a nickel metal hydride battery (for example, LaNi ₅ ).

  As shown in FIG. 3A, a coating (SEI (Solid Electrolyte Interphase) film) 42 is formed on the negative electrode active material 4 of the lithium ion battery. When a crack 41 is generated in the negative electrode active material 4 due to the uneven lithium concentration in the negative electrode active material 4 due to charge / discharge of the lithium ion battery and the volume of the negative electrode active material 4, a new surface is exposed. A new coating 42 is formed on the exposed surface by the reaction between the negative electrode active material 4 and the electrolytic solution. As a result, the internal resistance of the lithium ion battery (more specifically, the reaction resistance of the negative electrode) can be increased.

  On the other hand, as shown in FIG. 3B, even if a crack 51 occurs in the negative electrode active material 5 of the nickel metal hydride battery and a new surface is exposed, the influence of the film formation is very small. That is, in the nickel-metal hydride battery, the reaction surface area of the negative electrode active material 5 that can simply contribute to the battery reaction increases, so the internal resistance (more specifically, the reaction resistance of the negative electrode) decreases.

  The amount (crack amount) of the crack 51 generated in the negative electrode active material 5 can be expressed using the crack index value X as follows. The amount of cracks can include both the number of cracks 51 and the depth of cracks 51.

  FIG. 4 is a diagram for explaining a calculation process of the crack index value X. In FIG. 4, the horizontal axis represents elapsed time, and the vertical axis represents stress Fs acting on the negative electrode active material 5. When the stress Fs exceeds the mechanical limit strength (typically tensile limit strength) Flim of the negative electrode active material 5, it is considered that the crack 51 is generated. Therefore, as shown by hatching, the damage progress due to cracks within a predetermined period by performing time integration of an excess amount (in other words, an amount of damage) ΔF that is an amount in which the stress Fs exceeds the limit strength Flim. The degree can be calculated.

More specifically, as shown in the following formula (1), the crack index value X (t2) at time t2 after a period Δt from time t1 is the crack index value (increase amount) ΔX increased during Δt. It is calculated by adding to the crack index value X (t1) at time t1.
X (t2) = X (t1) + ΔX (1)

In the above formula (1), the increase amount ΔX can be calculated by multiplying the crack generation coefficient k per unit time based on the stress excess amount ΔF at time t2 (or time t1) by the period Δt (below). (Refer Formula (2)).
ΔX = k × Δt (2)

  The relationship between the stress excess ΔF and the crack generation coefficient k per unit time is determined in advance by performing an endurance test of the battery 100 under various conditions in which the combinations (Ib, Tb) of the current Ib and the temperature Tb of the battery 100 are different. It can be quantified by performing. Hereinafter, this quantification method will be described in more detail.

  The battery 100 is disassembled after endurance tests under various conditions, and the negative electrode active material 5 is observed using an SEM (Scanning Electron Microscope) or various image processing is performed. The amount of cracks generated according to the combination of conditions (Ib, Tb) can be quantified. On the other hand, during the durability test in each combination (Ib, Tb), the stress Fs generated in the negative electrode active material 5 is obtained by refining the model formula (for example, calculating the hydrogen concentration distribution by the diffusion equation and calculating the stress by the Hooke's law). For example). That is, the limit strength Flim can be defined from the crack generation determination result in the SEM observation or image processing after the above-described durability test, whereby the amount of stress Fs exceeding the limit strength Flim (ΔF) and the unit time The relationship with the crack generation coefficient k can be defined in advance. However, the limit strength Flim may be determined based on theoretical material properties (for example, tensile strength limit). The correlation between the excess amount ΔF defined in advance and the crack generation coefficient k per unit time can be prepared in advance as a map MP1 (or relational expression) and stored in the memory 302.

  In addition, a model formula obtained by refining the above-described material diffusion and stress prediction is mounted on the ECU 300, so that the stress Fs generated under an arbitrary traveling condition on the vehicle 1 is calculated in accordance with the control cycle. Is possible. By sequentially calculating the excess amount ΔF that is the difference between the stress Fs and the limit strength Flim, the crack occurrence coefficient k can be calculated by referring to the map MP1. When the stress Fs is equal to or less than the limit strength Flim, the excess amount ΔF is 0 and the crack generation coefficient k is also 0, so that it is determined that the generation of cracks has not progressed.

<Setting of upper limit value according to crack index value>
When the internal resistance of the battery 100 decreases, the amount of voltage change during charging and discharging decreases, so that the charging power does not easily reach the charging power upper limit value Win, and the discharging power from the nickel metal hydride battery reaches the discharging power upper limit value Wout. It becomes difficult to reach. When charging / discharging the battery 100 without considering the amount of cracks, the upper limit values (Win, Wout) may be set to be smaller than necessary. As a result, charging / discharging of the battery 100 is excessively limited, and the performance of the battery 100 may not be sufficiently exhibited.

  Therefore, in the present embodiment, as described below, the charging power upper limit Win and the discharging power upper limit Wout are set larger as the crack amount indicated by the crack index value X is larger.

  FIG. 5 is a time chart for explaining the setting processing of charging power upper limit value Win and discharging power upper limit value Wout of battery 100 in the present embodiment. In FIG. 5, the horizontal axis indicates the elapsed time. The initial time (0) is, for example, a time immediately after the battery 100 is manufactured. The vertical axis indicates the crack index value X, the internal resistance of the battery 100 (more specifically, the reaction resistance of the negative electrode), the discharge power upper limit value Wout (upward is in the increasing direction), and the charge power upper limit value Win (lower) in order from the top. Indicates an increasing direction).

  As shown in FIG. 5, the internal resistance of the battery 100 decreases as the crack index value X increases. Therefore, in the present embodiment, the discharge power upper limit Wout is increased and the charge power upper limit Win is increased as the crack index value X increases. That is, the larger the crack amount in the negative electrode active material 5, the larger the discharge power upper limit Wout (its absolute value) and the charge power upper limit Win (the absolute value thereof). A correlation as shown in FIG. 5 as an example is stored in the memory 302 of the ECU 300 as a map MP2.

<Charge / discharge control flow>
FIG. 6 is a flowchart for explaining the setting process of the charging power upper limit value Win and the discharging power upper limit value Wout of the battery 100. This flowchart is called from the main routine and executed when a predetermined condition is satisfied or every predetermined time interval. Note that each step (hereinafter abbreviated as “S”) included in these flowcharts is basically realized by software processing by the ECU 300, but realized by dedicated hardware (electric circuit) produced in the ECU 300. May be.

  The crack index value X is sequentially updated by repeatedly executing the processing of the flowchart shown in FIG. This flowchart shows the nth calculation process, and it is assumed that the crack index value X (n−1) until the (n−1) th (previous) process is stored in the memory 302.

  In S10, ECU 300 acquires voltage Vb, current Ib, and temperature Tb from each sensor provided in battery 100. ECU 300 also calculates OCV from voltage Vb. In addition, although the example which contains OCV as a parameter is demonstrated here, OCV is not an essential parameter for stress estimation.

  In S20, ECU 300 calculates stress Fs by using the above-described model formula from the OCV, current Ib, and temperature Tb acquired in S10. Since this calculation method has already been described, detailed description will not be repeated.

  In S30, the ECU 300 calculates the crack generation coefficient k by referring to the map MP1 from the excess amount ΔF of the stress Fs with respect to the limit strength Flim, and uses this to calculate the current crack generation amount ΔX (n). As described in the above equation (2), the current crack generation amount ΔX (n) can be calculated as the product of the excess amount ΔF and the excess period.

In S40, a crack index value X (n) up to the current (n-th) calculation process is calculated. As shown in the following formula (3), the crack index value X (n) is the crack index value X up to the previous ((n-1) th) calculation process by calculating the crack generation amount ΔX (n) in the current calculation process. It can be calculated by adding to (n-1).
X (n) = X (n−1) + ΔX (n) (3)

  In S50, with reference to map MP2 (see FIG. 5), ECU 300 sets charge power upper limit Win and discharge power upper limit Wout in accordance with crack index value X (n) calculated in S40. Thereby, the charging power upper limit Win and the discharge power upper limit Wout are set larger as the crack amount in the negative electrode active material 5 is larger.

  As described above, according to the present embodiment, in the battery system 2 including the battery 100 including the hydrogen storage alloy as the negative electrode active material 5, the amount of cracks (the number or depth of cracks) generated in the negative electrode active material 5. The charging power upper limit value Win and the discharging power upper limit value Wout are set to be larger as the value becomes larger. This makes it difficult for the charging power to reach the charging power upper limit value Win and also makes it difficult for the discharging power to reach the discharging power upper limit value Wout. The discharge is prevented from being excessively limited. Therefore, the performance of the battery 100 can be exhibited. In other words, the battery 100 can be utilized up to the limit of its performance (maximized performance).

  In the above embodiment, the example using the tensile strength has been described. In general, although the parameter used for determining the presence or absence of cracks in the negative electrode active material is considered to be tensile strength, the influence of compressive strength may be considered in addition to (or instead of) tensile strength. By considering the influence of both tensile strength and compressive strength, it is possible to determine the presence or absence of cracks with higher accuracy.

  Further, in FIG. 5, the configuration that limits both the charging power upper limit value Win and the discharging power upper limit value Wout has been described as an example. However, only one of the charging power upper limit value Win and the discharging power upper limit value Wout is limited. May be.

  Furthermore, you may provide the idle period which pauses charging / discharging in the middle of charging / discharging of the battery 100. FIG. During the rest period, the uneven hydrogen concentration in the negative electrode active material 5 is alleviated, and as a result, the distortion of the crystal lattice in the negative electrode active material 5 is alleviated, so that cracks are less likely to occur in the negative electrode active material 5. It is. Alternatively, the battery 100 may be positively charged / discharged in order to promote relaxation of the uneven hydrogen concentration. That is, when the battery 100 is charged and discharged, the generation direction of the hydrogen concentration in the negative electrode active material 5 is opposite. For example, after the battery 100 is discharged for a certain time, the battery 100 is charged in reverse. , The uneven hydrogen concentration can be alleviated. As a result, the occurrence of cracks can be suppressed.

  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, 4, 5 negative electrode active material, 10, 20 motor generator, 30 power split mechanism, 40 engine, 41, 51 crack, 42 coating, 50 driving wheel, 100 battery, 101 cell, 102 case , 103 Safety valve, 104 Electrode body, 110 Voltage sensor, 120 Current sensor, 130 Temperature sensor, 150 SMR, 200 PCU, 300 ECU, 301 CPU, 302 Memory.

Claims (1)

A battery system for supplying power to an external device,
A nickel metal hydride battery containing a hydrogen storage alloy as a negative electrode active material;
A power conversion device that converts power between the nickel metal hydride battery and the external device;
Control for controlling the power converter so that charging power to the nickel-metal hydride battery is less than a control upper limit value of the charging power, and discharge power from the nickel-metal hydride battery is less than a control upper limit value of the discharge power. With the device,
The controller is
From the current input to and output from the nickel metal hydride battery and the temperature of the nickel metal hydride battery, calculate an index value related to the amount of cracks generated in the negative electrode active material,
The battery system, wherein the control upper limit value of the charging power and the control upper limit value of the discharge power are increased as the crack amount indicated by the index value increases.

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