JP2019110043A - Battery system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To estimate a battery internal pressure with high accuracy in consideration of the influence of hydrogen gas generated when an aqueous battery is over-discharged on the battery internal pressure. An ECU determines whether or not a target cell is being over-discharged based on whether or not a target cell is being discharged with a positive electrode potential being lower than a threshold value Th1 (S11). ). Then, when the target cell is over-discharging, the ECU estimates the hydrogen gas generation amount Hm using the integrated amount of electricity flowing during over-discharging of the target cell (S12). Further, the ECU also estimates the oxygen gas generation amount Om (S13). Then, the ECU estimates the cell internal pressure Pc of the target cell using the hydrogen gas generation amount Hm and the oxygen gas generation amount Om (S14). [Selection diagram] Fig. 5

Description

  The present disclosure relates to a battery system, and more particularly to a battery system that estimates the pressure in a case of a secondary battery using a water-based electrolyte.

  A nickel hydrogen battery is known as a secondary battery (hereinafter sometimes referred to as "aqueous battery") using an aqueous electrolyte solution. At the time of overcharging of the nickel hydrogen battery, oxygen gas is generated from the positive electrode. With the generation of such oxygen gas, the pressure in the battery case (hereinafter sometimes referred to as "battery internal pressure") increases. Since changes in the battery internal pressure affect the performance of the battery, it is required to detect the battery internal pressure with high accuracy. JP-A-2017-91790 (Patent Document 1) discloses a battery system that estimates the battery internal pressure using the amount of oxygen gas generation in the positive electrode, the amount of oxygen gas absorbed in the negative electrode, and the hydrogen equilibrium pressure in the negative electrode. It is done.

JP, 2017-91790, A

  In an aqueous battery, hydrogen gas is generated from the positive electrode at the time of overdischarge. In the battery system described in Patent Document 1 above, the influence of hydrogen gas generated at the time of overdischarge of the water-based battery on the battery internal pressure is not considered. Therefore, there is room for further improvement in order to increase the estimation accuracy of the battery internal pressure.

  The present disclosure has been made to solve the above problems, and its object is to estimate the battery internal pressure with high accuracy in consideration of the influence of hydrogen gas generated at the time of overdischarge of the water-based battery on the battery internal pressure. is there.

  The battery system of the present disclosure includes a secondary battery, a gas amount estimation unit, and a pressure estimation unit. The secondary battery has a positive electrode, a negative electrode, and an aqueous electrolyte solution in the case. The gas amount estimation unit is configured to estimate the amount of hydrogen gas generated at the positive electrode during overdischarge of the secondary battery using the integrated amount of electricity flowing during overdischarge of the secondary battery. The pressure estimation unit is configured to estimate the pressure in the case of the secondary battery using the amount of hydrogen gas estimated by the gas amount estimation unit. During the above-described overdischarge, discharge is performed in a state where the potential of the positive electrode is lower than the threshold.

In the above battery system, the gas amount estimation unit uses the integrated amount of electricity (hereinafter sometimes referred to as "integrated amount of electricity ΔC") that flows during overdischarge of the water system battery, and The amount of hydrogen gas generated (hereinafter, may be referred to as “hydrogen gas generation amount H _m ”) is estimated. Since the hydrogen gas generation amount H _m and the accumulated electricity amount ΔC show high correlation, the gas amount estimation unit can estimate the hydrogen gas generation amount H _m with high accuracy by using the accumulated electricity amount ΔC. It will be possible. The pressure estimation unit, by using the hydrogen gas generation amount H _m estimated with high accuracy, it is possible to estimate the battery internal pressure is the pressure in the aqueous battery case with high accuracy.

  As described above, according to the above-described battery system, it is possible to estimate the battery internal pressure with high accuracy in consideration of the influence of hydrogen gas generated at the time of overdischarge of the water-based battery on the battery internal pressure.

In addition, by estimating the battery internal pressure and the hydrogen gas generation amount _Hm with high accuracy as described above, it is possible to suppress, for example, the deterioration of the battery described below.

  If the battery is left in a state where the SOC (State Of Charge) of the battery is close to 100%, or if the battery is used (discharged) in a state where the SOC of the battery is close to 0%, the deterioration of the battery tends to progress. Therefore, the SOC of the battery is controlled to fall within the working SOC range determined by the lower limit SOC and the upper limit SOC. In addition, SOC shows the electrical storage residual amount of a cell, for example, represents the electrical storage amount with respect to a full charge state by 0 to 100%.

  By narrowing the use SOC range, the battery is less likely to deteriorate, and it becomes easier to maintain high performance. However, if the use SOC range is narrow, the above-described high performance can not be sufficiently exhibited in battery control. Specifically, in the control of the battery, the working SOC range corresponds to the substantial discharge capacity of the battery, so the narrower the working SOC range, the smaller the discharge capacity of the battery. For example, when the SOC of the battery reaches the lower limit SOC, the power (the power corresponding to the lower limit SOC) can not be used (discharged) even if the power stored in the battery remains. Therefore, in order to increase the discharge capacity of the battery, it is desirable to widen the use SOC range of the battery.

  The inventor of the present application has confirmed that the deterioration of the positive electrode when the aqueous battery is discharged at a low SOC is suppressed by changing the material and the treatment of the positive electrode of the water-based battery. In a water-based battery (for example, a nickel-hydrogen battery having a high-order Na-treated positive electrode) having the improved positive electrode as described above, the lower limit SOC can be set low to widen the SOC range.

  Further, by providing a valve (hereinafter sometimes referred to as a "pressure regulating valve") in the battery case, it is possible to adjust the battery internal pressure. For example, if hydrogen gas is generated at the positive electrode of the water-based battery during overdischarge, release the generated hydrogen gas to the outside of the battery case by opening the above-mentioned pressure regulating valve to suppress an increase in battery internal pressure Can.

By setting the lower limit SOC low, the frequency at which the battery is discharged when the SOC is low becomes high. Since the potential of the positive electrode is also lowered due to the decrease of the SOC, when the lower limit SOC is set low, the battery is likely to be in the overdischarged state. Further, in the assembled battery, since there is variation in SOC among cells, only a specific cell may be in an overdischarged state. Even if the battery is in an overdischarged state, the hydrogen pressure generated during the overdischarge can be released to the outside of the battery case by opening the above-mentioned pressure regulating valve, so it is possible to suppress an increase in the battery internal pressure . However, in water-based batteries, the source of generation of hydrogen gas is water (H ₂ O) contained in the electrolyte, and if the pressure regulating valve is opened to release hydrogen gas out of the battery case, the water in the battery case decreases. (Thus, water contained in the electrolyte is reduced). When water contained in the electrolyte solution of the aqueous battery is excessively reduced, the deterioration of the aqueous battery proceeds. Hereinafter, excessive reduction of water contained in the electrolyte solution of the water-based battery may be referred to as "electrolyte solution wither."

When the pressure control valve is opened when the hydrogen gas generation amount _Hm is large, a large amount of hydrogen gas is released out of the battery case, and the electrolytic solution is likely to be dead. Therefore, the hydrogen gas generation amount H _m is a predetermined threshold (Th3) or more and the battery internal pressure (Pc) is the output power of the aqueous battery when the predetermined threshold (Th4) or It is preferable to limit the increase in the battery internal pressure without opening the pressure regulating valve. By doing this, it is possible to suppress the deterioration of the battery due to the electrolyte withering. If hydrogen gas generated by the overdischarge is kept in the case for a certain period of time without being released out of the case, the hydrogen gas returns to water (electrolyte solution) through the reaction at the electrode.

  According to the present disclosure, it is possible to estimate the battery internal pressure with high accuracy in consideration of the influence exerted on the battery internal pressure by hydrogen gas generated at the time of overdischarge of the water-based battery.

FIG. 1 is a block diagram schematically showing an entire configuration of a vehicle equipped with a battery system according to an embodiment of the present disclosure. It is a figure which shows the structure of one cell contained in the battery shown in FIG. It is a figure which shows the transition of the state of the cell in an overdischarge test. It is a figure which shows transition of the cell voltage in discharge, and transition of the electric potential of each electrode (positive electrode, negative electrode). It is a flowchart which shows the process procedure of discharge control of the battery performed by the battery system according to embodiment of this indication. Is a flowchart showing the procedure of estimating the amount of hydrogen gas H _m which is executed in the process shown in FIG. It is a flowchart showing the procedure of estimating the oxygen gas generation amount O _m executed in the process shown in FIG. FIG. 7 is a diagram showing an example of the operation of the battery system according to the embodiment of the present disclosure.

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

  Below, the example to which the battery system which concerns on this embodiment is applied to an electric vehicle is demonstrated. However, the application of the battery system is not limited to electric vehicles, and may be hybrid vehicles. Further, the application of the battery system is not limited to vehicles, and may be stationary.

  FIG. 1 is a block diagram schematically showing an entire configuration of a vehicle 1 equipped with a battery system according to the present embodiment.

  Referring to FIG. 1, a vehicle 1 includes a motor generator (hereinafter referred to as "MG (Motor Generator)") 10, a power transmission gear 20, a drive wheel 30, and a power control unit (hereinafter referred to as "PCU (Power)". A control unit (hereinafter referred to as “control unit)” 40, a system main relay (hereinafter referred to as “SMR (system main relay)) 50, and a battery system 2. The battery system 2 includes a battery 100, a voltage sensor 210, a current sensor 220, a temperature sensor 230, and an electronic control unit (hereinafter referred to as "ECU (Electronic Control Unit)") 300.

  MG 10 is, for example, a three-phase alternating current rotating electric machine. The output torque of the MG 10 is transmitted to the drive wheel 30 via a power transmission gear 20 configured by a reduction gear or the like. The MG 10 can also generate electric power by the rotational force of the drive wheel 30 during regenerative braking operation of the vehicle 1. In a hybrid vehicle equipped with an engine (not shown) in addition to the MG 10, the required vehicle driving force is generated by operating the engine and the MG 10 in a coordinated manner. Although FIG. 1 shows a configuration in which only one MG is provided, the number of MGs is not limited to this, and a plurality of (for example, two) MGs may be provided.

  The PCU 40 includes an inverter and a converter (both not shown). When the battery 100 is discharged, the converter boosts the voltage supplied from the battery 100 and supplies it to the inverter. The inverter converts the DC power supplied from the converter into AC power to drive the MG 10. On the other hand, when the battery 100 is charged, the inverter converts the AC power generated by the MG 10 into DC power and supplies the DC power to the converter. The converter steps down the voltage supplied from the inverter and supplies it to battery 100.

  SMR 50 is electrically connected to a current path connecting battery 100 and PCU 40. When SMR 50 is closed in response to a control signal from ECU 300, power can be exchanged between battery 100 and PCU 40.

  The battery 100 is a DC power supply configured to be recharged. The battery 100 is configured to include an assembled battery composed of a plurality of secondary batteries. In this embodiment, the battery pack included in the battery 100 is configured of a plurality of water-based batteries connected in series. In addition, a nickel hydrogen battery is adopted as a water system battery. The nickel-metal hydride battery is a secondary battery having a positive electrode, a negative electrode, and an aqueous electrolyte (for example, an alkaline aqueous solution) in a case. Hereinafter, a secondary battery (a nickel hydrogen battery in this embodiment) constituting the assembled battery is referred to as a "cell". The detailed configuration of each cell 101 included in the battery 100 will be described with reference to FIG.

  Voltage sensor 210 detects voltage Vb for each cell of battery 100. Current sensor 220 detects current Ib input / output to / from battery 100. Temperature sensor 230 detects temperature Tb of each cell of battery 100. Each sensor outputs the detection result to the ECU 300. Each of the voltage sensor 210 and the temperature sensor 230 is provided, for example, one for each cell. However, the present invention is not limited thereto, and each of the voltage sensor 210 and the temperature sensor 230 may be provided one for each of a plurality of cells, or only one may be provided for one assembled battery. .

  The ECU 300 includes a central processing unit (CPU) 301, a memory 302, and an input / output buffer (not shown). The memory 302 includes a read only memory (ROM), a random access memory (RAM), and a rewritable non-volatile memory. The CPU 301 executes a program stored in the memory 302 (for example, a ROM) to execute various controls. ECU 300 controls each device such that vehicle 1 and battery system 2 are in a desired state based on, for example, signals received from each sensor, and a map and a program stored in memory 302. The various controls performed by the ECU 300 are not limited to processing by software, and may be processed by dedicated hardware (electronic circuit).

  The ECU 300 outputs the acquired information (the calculation result of the CPU 301 and the like) to the memory 302 (for example, a rewritable non-volatile memory) and stores the information in the memory 302. The memory 302 may store in advance information used for detection of the battery state, control of the vehicle 1 and the like. For example, the memory 302 may store in advance initial information of the battery 100 (type of cell, capacity, internal resistance, thickness of electrode, weight per unit area, etc.).

  The memory 302 may further store correspondence information (for example, a map for detecting the positive electrode potential and the negative electrode potential, an equation (A) described later, and the like) used for discharge control of the battery. Correspondence information is information indicating the relationship between a plurality of correlated parameters. The correspondence information may be a map, a table, an equation, or a model. Further, the correspondence information may be configured by combining a plurality of maps and the like.

  ECU 300 controls charging / discharging of battery 100 in accordance with a working SOC range determined by the lower limit SOC and the upper limit SOC. When the SOC of battery 100 approaches the upper limit SOC, ECU 300 limits the input power (charging power) of battery 100 so that the SOC of battery 100 does not exceed the upper limit SOC. Further, when the SOC of battery 100 approaches the lower limit SOC, ECU 300 limits the output power (discharged power) of battery 100 so that the SOC of battery 100 does not fall below the lower limit SOC. The lower limit SOC and the upper limit SOC are stored, for example, in the memory 302. The numerical value of each of the lower limit SOC and the upper limit SOC can be changed by the ECU 300. As a method of measuring the SOC, various known methods such as a method by current value integration (coulomb counting) or a method by estimation of an open circuit voltage (OCV: Open Circuit Voltage) can be adopted.

  ECU 300 is configured to control input / output power of battery 100 based on input limit Win indicating the upper limit value of charge power of battery 100 and output limit Wout indicating the upper limit value of discharge power of battery 100. . ECU 300 executes a process of limiting input power to battery 100 so that input power to battery 100 does not exceed input limit Win. Further, the ECU 300 executes a process of limiting the output power from the battery 100 so that the output power from the battery 100 does not exceed the output limit Wout. These restriction processes are performed, for example, by controlling the PCU 40, the SMR 50, and the like. The input limit Win and the output limit Wout are stored, for example, in the memory 302. The numerical values of each of the input limit Win and the output limit Wout can be changed by the ECU 300.

  FIG. 2 is a diagram showing the configuration of the cell 101 included in the battery 100. As shown in FIG. Since the configuration of each cell 101 is common, only one cell 101 is representatively shown in FIG. In FIG. 2, a part of the case 102 of the cell 101 is seen through to show the electrode body 104.

  Referring to FIG. 2, cell 101 is a square sealed cell including case 102 made of metal, for example. In the case 102, an electrode body 104 and an electrolytic solution (not shown) constituting a nickel hydrogen battery are accommodated. A safety valve 103 is provided at the top of the case 102.

  The case 102 is a square case including a case main body and a lid both made of metal, and the lid is sealed by being welded all around the opening of the case main body. The safety valve 103 is opened when the pressure in the case 102 (hereinafter sometimes referred to as “the cell internal pressure”) exceeds a predetermined value (hereinafter sometimes referred to as “the valve opening pressure Px”). Exhaust some of the internal gas (such as hydrogen gas) to the outside. In this embodiment, the cell internal pressure corresponds to the battery internal pressure, and the safety valve 103 corresponds to the pressure regulating valve.

  The electrode body 104 includes a positive electrode plate, a negative electrode plate, and an insulating separator. The positive electrode plate and the negative electrode plate are alternately stacked via a separator. That is, an insulating separator is sandwiched between the positive electrode plate and the negative electrode plate. 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.

As materials of the electrode body 104 and the electrolytic solution constituting the nickel hydrogen battery, materials arbitrarily selected from various materials known as materials of nickel hydrogen batteries can be used. In this embodiment, the positive electrode plate includes a positive electrode active material layer containing a solid solution of nickel hydroxide (Ni (OH) ₂ or NiOOH) and a cobalt compound, and an active material support (such as foamed nickel). An electrode plate is used. The positive electrode plate may be subjected to high-order Na treatment. For the negative electrode plate, an electrode plate containing a hydrogen storage alloy is used. The hydrogen storage alloy is, for example, an alloy of a metal (such as Ti, Zr, Pd, Mg, etc.) excellent in hydrogen storage capacity and a metal (Fe, Co, Ni, etc.) excellent in hydrogen release capacity. For the separator, a non-woven fabric made of synthetic fibers subjected to hydrophilization treatment is used. An alkaline aqueous solution containing potassium hydroxide (KOH) or sodium hydroxide (NaOH) is used as the electrolytic solution.

  By the way, in a water-based battery (for example, a nickel hydrogen battery having a high-order Na-treated positive electrode) having an improved positive electrode, deterioration of the positive electrode (for example, elution of cobalt) occurs when discharge is performed at low SOC. Since the control is suppressed, the lower limit SOC can be set low to widen the use SOC range.

By setting the lower limit SOC low, the frequency at which the battery is discharged when the SOC is low becomes high. Therefore, the battery is likely to be overdischarged. Even if the battery is in the overdischarged state, the hydrogen gas generated during the overdischarge can be released to the outside of the case 102 by opening the safety valve 103, so it is possible to suppress the rise in the cell internal pressure. However, in a water-based battery, the source of hydrogen gas generation is water (H ₂ O) contained in the electrolyte, and if the hydrogen gas is released out of the case 102 by opening the safety valve 103, the water in the case 102 decreases. As a result, the electrolytic solution is likely to die.

  Hereinafter, the deterioration of the cell 101 when the safety valve 103 is opened during overdischarge and the hydrogen gas is continuously released out of the case 102 will be described using FIG. 3. FIG. 3 is a diagram showing transition of the state (voltage, internal pressure) of the cell 101 when the overdischarge test is performed under the following conditions and the safety valve 103 is opened at timing t11 during overdischarge. In FIG. 3, the line k11 indicates the voltage of the cell 101, the line k12 indicates the gas discharge amount (integrated value), and the line k13 indicates the internal pressure of the cell 101. In this test, a pressure sensor for measuring the internal pressure of the cell 101 was added to the cell 101. That is, the internal pressure of the cell 101 indicated by the line k13 is a value measured by the pressure sensor.

[Conditions of overdischarge test]
・ Discharge range: From 0% to -100% of SOC ・ Discharge current: 20A
Discharge time: 21 minutes Environmental temperature: 65 ° C
Referring to FIG. 3, when the discharge is continued even if the SOC of cell 101 is less than 0%, cell 101 is in the overdischarged state. In the above-mentioned over-discharge test, since the discharge is started from the state where the SOC of the cell 101 is 0%, the cell 101 is over-discharged immediately (before the timing t11) as shown by the line k11. In this state, the voltage of the cell 101 drops rapidly (changes to the negative side). Then, hydrogen gas was generated at the positive electrode of the cell 101 by continuing the overdischarge. As indicated by a line k13, the internal pressure of the cell 101 rapidly rises due to the generation of hydrogen gas.

  In the example of FIG. 3, the safety valve 103 is opened at timing t11 when the internal pressure of the cell 101 exceeds 0.7 MPa (a pressure corresponding to the above-described valve opening pressure Px). As indicated by a line k12, the gas in the case 102 is released out of the case 102 by opening the safety valve 103. As indicated by a line k13, the internal pressure of the cell 101 is kept substantially constant by the opening of the safety valve 103.

  As indicated by a line k11, a second sharp drop in the voltage of the cell 101 occurred 10 minutes after the start of the discharge. Further, although not shown in FIG. 3, the temperature of the cell 101 rapidly increased with the decrease of the voltage. The temperature rise of the cell 101 continued until the discharge was stopped.

  Thereafter, the discharge of the cell 101 was stopped at a timing t12 (a timing of 21 minutes after the start of the discharge). By stopping the discharge, hydrogen gas was not generated at the positive electrode of the cell 101, and the internal pressure of the cell 101 decreased with time. In addition, the voltage of the cell 101 rapidly rises (changes to the + side) at timing t12 when the discharge of the cell 101 is stopped, and thereafter, the voltage of the cell 101 gradually increases with time.

  As the gas discharge amount (integrated value) indicated by the line k12 increases, water contained in the electrolyte solution of the cell 101 decreases. In the overdischarge test shown in FIG. 3, a gas of an amount exceeding 10 L was released to the outside of the case 102 before the discharge of the cell 101 was stopped, and the cell 101 became in the electrolytic solution dead state. And deterioration of the cell 101 progressed by electrolyte solution withering.

  In the battery system 2 according to this embodiment, it is possible to suppress the deterioration of the cell 101 by the following configuration.

  The ECU 300 is in the process of overdischarging the cell 101 based on whether or not the cell 101 is being discharged in a state where the positive electrode potential is lower than a threshold (hereinafter, may be referred to as “threshold Th1”). It is configured to determine whether or not

Then, when the cell 101 is in overdischarge, the ECU 300 (in particular, a portion functioning as a “gas amount estimation unit”) is a hydrogen gas that is the amount of hydrogen gas generated at the positive electrode of the cell 101 during the overdischarge. The generation amount H _m is estimated using an integrated amount of electricity ΔC which is an integrated amount of electricity flowing during overdischarge of the cell 101. Since the hydrogen gas generation amount H _m and the integrated amount of electricity ΔC show high correlation, the ECU 300 can estimate the hydrogen gas generation amount H _m with high accuracy by using the integrated amount of electricity ΔC. .

Further, ECU 300 (in particular, the portion that functions as a "pressure estimator"), using the estimated amount of hydrogen gas H _m as described above, configured to estimate a cell internal pressure of the cell 101. ECU300, by using a hydrogen gas generation amount H _m estimated with high accuracy, it is possible to estimate the cell internal pressure of the cell 101 with high accuracy.

As described above, by estimating the cell internal pressure and the hydrogen gas generation amount _Hm with high accuracy, it is possible to suppress the deterioration of the cell 101 by performing, for example, the following processing.

Hydrogen gas generation amount H _m is a predetermined threshold value (at less, it referred to as "threshold value Th3 'is) or more, and the cell internal pressure is a predetermined threshold value (hereinafter," threshold value Th4 " When it is above, the ECU 300 limits the output power of the cell 101 to suppress the rise of the internal pressure of the cell without opening the safety valve 103. By doing so, it is possible to suppress the deterioration of the cell 101 due to the electrolyte withering.

When the hydrogen gas generation amount _Hm is sufficiently low, even if the safety valve 103 is opened, the electrolytic solution wither will not occur, so the output power of the cell 101 may not be limited. In addition, when the cell internal pressure is sufficiently low, it is not necessary to reduce the cell internal pressure, so the output power of the cell 101 may not be limited.

  Next, the reaction at each electrode of the cell 101 will be described. In each of the chemical reaction formulas shown below, a hydrogen storage alloy is represented by "M".

  At the time of discharge of the nickel hydrogen battery, reactions shown in the following formulas (1) and (2) occur on the positive electrode and the negative electrode, respectively. Further, at the time of charging of the nickel hydrogen battery, reactions shown in the following formulas (3) and (4) occur on the positive electrode and the negative electrode, respectively.

(Reaction during discharge)
Positive electrode: NiOOH + H ₂ O + e ⁻ → Ni (OH) ₂ + OH ⁻ (1)
_{^{Negative: MH + OH - → M +}} H 2 O + e - ... (2)
(Reaction on charging)
Positive electrode: Ni (OH) ₂ + OH ⁻ → NiOOH + H ₂ O + e ⁻ (3)
Negative electrode: M + H ₂ O + e ⁻ → MH + OH ⁻ (4)
According to the reactions shown in the above formulas (1) to (4), no gas is generated during discharge or charge. However, in the overdischarged state in which the discharge is performed excessively and the overcharged state in which the charge is performed excessively, a subsidiary as shown in the following formulas (5) to (8) Reaction occurs and gas is generated.

(Reaction at overdischarge)
Positive electrode: 2H ₂ O + 2e ⁻ → H ₂ + 2OH ⁻ (5)
Negative electrode: 2 M + H ₂ → 2 MH (6)
(Reaction during overcharge)
Positive electrode: 4OH ⁻ → O ₂ + 2H ₂ O + 4e ⁻ (7)
Negative electrode: 4 MH + O ₂ → 4 M + 2 H ₂ O (8)
As shown in the above equation (5), hydrogen gas (H ₂ ) is generated at the positive electrode during overdischarge. However, when the generated hydrogen gas is retained in the case 102 for a certain period without being released to the outside of the case 102, the hydrogen gas is stored in the hydrogen storage alloy (M) by the reaction of formula (6). It returns to water (electrolyte solution) by reaction of Formula (2).

As shown in the above equation (7), oxygen gas (O ₂ ) is generated at the positive electrode during overcharge. Further, as shown in the above equation (8), at the time of overcharge, hydrogen and oxygen gas stored in the hydrogen storage alloy (M) at the negative electrode react with each other to become water. Although the details will be described later, it has been confirmed by the experiments of the inventor of the present invention that the reactions shown in the above formulas (7) and (8) can occur even at the time of overdischarge.

  The reaction of each electrode during discharge of the cell 101 will be described below with reference to FIG. FIG. 4 is a diagram showing the transition of the battery voltage (cell voltage) and the transition of the potential of each electrode (positive electrode, negative electrode) during discharge of the cell 101. In FIG. 4, the line k21 shows the transition of the positive electrode potential, the line k22 shows the transition of the negative electrode potential, and the line k23 shows the transition of the cell voltage.

  Referring to FIG. 4, as indicated by line k23, the cell voltage is kept substantially constant at the beginning of the discharge. When the discharge is continued until around the timing t21, as shown by the line k21, the positive electrode potential starts to gradually drop, and the positive electrode potential falls sharply at the timing t21. It is considered that this rapid decrease in the positive electrode potential is due to the SOC of the cell 101 falling below the capacity of the positive electrode. Due to this rapid drop, the positive electrode potential drops to about -1.0 V, and the positive electrode potential becomes lower than the negative electrode potential shown by the line k22.

In the period of timing t21 to t22, the positive electrode potential becomes substantially the same as the negative electrode potential, and the reaction shown in the above-mentioned equation (5) occurs. That is, hydrogen gas (H ₂ ) is generated at the positive electrode. The amount of electricity (current) flowing between the electrodes increases as the reaction shown in equation (5) progresses during overdischarge, so the correlation between the above-mentioned accumulated amount of electricity ΔC and the hydrogen gas generation amount H _m is high. .

When the discharge is continued until around the timing t22, as shown by the line k22, the negative electrode potential starts to rise gradually, and at the timing t22, the negative electrode potential rapidly rises. It is considered that this rapid rise in the negative electrode potential is due to the SOC of the cell 101 falling below the capacity of the negative electrode. Due to this rapid rise, the polarity (positive / negative) of the potential is reversed with respect to the potential of each electrode at the beginning of discharge, the negative electrode potential becomes positive, and the positive electrode potential becomes negative. Hereinafter, such a case where the positive electrode and the negative electrode are at the positive / negative opposite potential may be referred to as “bipolar polarization”. By the occurrence of the polarity inversion, the reaction shown in the above-mentioned equation (7) will occur. That is, in the period after timing t22, oxygen gas (O ₂ ) is generated at the positive electrode.

In this embodiment, ECU 300, in addition to the hydrogen gas generation amount H _m described above, in the over-discharge (especially, the period after the timing t22 shown in FIG. 4) the amount of oxygen gas generated at the positive electrode of the cell 101 ( Hereinafter, the cell internal pressure of the cell 101 (hereinafter, may be referred to as “cell internal pressure Pc”) is estimated using “oxygen gas generation amount O _m ”. Then, ECU 300 is hydrogen gas generation amount _{H m} is the threshold Th3 or more, and the estimated cell internal pressure Pc as described above if the threshold value Th4 above, perform the output restriction of the battery 100 Control the rise in internal pressure of the cell. Hereinafter, the discharge control of the battery performed by the ECU 300 will be described in detail with reference to FIGS. 5 to 8.

  FIG. 5 is a flowchart showing a procedure of discharge control of the battery which is executed by the ECU 300. The process shown in this flowchart is repeatedly called from the main routine and executed repeatedly at predetermined time intervals while the battery 100 is being discharged.

  In the process shown in FIG. 5, whether the ECU 300 estimates the internal pressure (cell internal pressure Pc) of a specific cell 101 included in the battery 100 (step S14), and limits the output of the battery 100 using the cell internal pressure Pc It is determined whether or not it is (step S15). The cell 101 which is the target of this estimation is hereinafter referred to as a "target cell".

  In this embodiment, one cell 101 is a target cell. However, the present invention is not limited to this, and a plurality of cells 101 may be set as target cells. When a plurality of cells 101 are set as target cells, it is determined whether or not to limit the output of the battery 100 (step S15), representative values (average value, median value or maximum value) of a plurality of estimated values (cell internal pressure) A value or the like) can be used as the cell internal pressure Pc of the battery 100.

  Referring to FIG. 5, ECU 300 determines whether or not the positive electrode potential of the target cell has become equal to or lower than threshold value Th1 during discharging of battery 100 (step S11).

  The positive electrode potential of the target cell can be measured using the reference electrode. For example, a standard hydrogen electrode (SHE) can be used as a reference electrode. The ECU 300 can measure the positive electrode potential of the target cell based on the potential difference between the positive electrode of the target cell and the reference electrode. However, the method of measuring the positive electrode potential of the target cell is not limited to such a method and is arbitrary. For example, the ECU 300 may estimate the positive electrode potential of the target cell from the SOC of the target cell by referring to correspondence information (such as a map) in the memory 302.

  The threshold value Th1 can be detected in step S11 so as to be able to detect a sharp drop in the positive electrode potential (for example, a sharp drop in the positive electrode potential as occurs at timing t21 in FIG. 4) due to overdischarge of the target cell. It is set. For example, the transition of the positive electrode potential during discharge of battery 100 is obtained in advance by experiment or the like, and threshold value Th1 appropriate for the transition of the positive electrode potential is stored in memory 302. Specifically, the positive electrode potential of the target cell becomes higher than the threshold Th1 before the sharp drop of the positive electrode potential caused by the overdischarge of the target cell occurs, and the rapid drop described above occurs to cause the positive electrode of the target cell to The threshold Th1 is set such that the potential is lower than the threshold Th1. The threshold value Th1 may be a fixed value or may be variable according to the state of the target cell (for example, the cell temperature).

If the positive electrode potential of the target cell is higher than the threshold Th1 during discharge of the battery 100 (NO in step S11), it is determined that the target cell is not in the overdischarge state, and the process is returned to the main routine. On the other hand, when the positive electrode potential of the target cell becomes less than or equal to threshold value Th1 during discharge of battery 100 (YES in step S11), it is determined that the target cell is overdischarged, and ECU 300 generates hydrogen gas H _m is estimated (step S12). Hereinafter, with reference to FIG. 6, illustrating a method for estimating the amount of hydrogen gas generated H _m. Figure 6 is a flowchart illustrating a processing procedure of estimating the amount of hydrogen gas H _m which is executed by the ECU 300.

  Referring to FIG. 6, ECU 300 calculates integrated amount of electricity ΔC, and stores the obtained integrated amount of electricity ΔC in memory 302 (step S21). If the integrated amount of electricity ΔC is already stored in the memory 302, the integrated amount of electricity ΔC may be updated.

  When the reaction represented by the above-mentioned equation (5) occurs, a current flows from the positive electrode to the negative electrode of the target cell. The ECU 300 detects the direction and amount (amount of electricity) of the current flowing between the electrodes of the target cell using the detection value of the current sensor 220 to thereby determine the amount (amount of electricity) of current flowing from the positive electrode to the negative electrode of the target cell. You can ask for Then, the ECU 300 can calculate the integrated amount of electricity ΔC by integrating the amount (amount of electricity) of the current flowing from the positive electrode to the negative electrode of the target cell. The processes of FIG. 5 and FIG. 6 are repeatedly performed during overdischarge of the target cell, so that an integrated amount of electricity ΔC, which is an integrated value of the amount of electricity flowing during overdischarge of the target cell, is calculated in step S21.

Then, ECU 300 uses the accumulated electrical quantity ΔC calculated in step S21, estimates the amount of hydrogen gas generated H _m is the amount of hydrogen gas generated in the positive electrode during over-discharge of the target cell (step S22).

ECU300, taking into account the temperature of the target cell in addition to the cumulative amount of electricity [Delta] C, may be estimated amount of hydrogen gas generated H _m. For example, even if information indicating the relationship between the integrated amount of electricity ΔC, the temperature of the cell, and the hydrogen gas generation amount H _m (hereinafter referred to as “H _m estimation information”) is obtained in advance by experiment etc. Good. The ECU 300 can use the detection value (temperature Tb) of the temperature sensor 230 to obtain the temperature of the target cell. ECU300 refers to the H _m estimation information from the temperature of the integrated electricity quantity ΔC and cells can be obtained a hydrogen gas generation amount H _m.

After the estimation of the hydrogen gas generation amount H _m , the process is returned to step S13 of FIG. Referring to FIG. 5, ECU 300 estimates oxygen gas generation amount O _m (step S13). Hereinafter, a method of estimating the oxygen gas generation amount O _m will be described using FIG. 7. FIG. 7 is a flow chart showing a processing procedure of estimation of the oxygen gas generation amount O _m executed by the ECU 300.

  Referring to FIG. 7, ECU 300 determines whether the negative electrode potential of the target cell has become equal to or higher than threshold value Th2 (step S31).

  The negative electrode potential of the target cell can be measured using a reference electrode. For example, a standard hydrogen electrode (SHE) can be used as a reference electrode. The ECU 300 can measure the negative electrode potential of the target cell based on the potential difference between the negative electrode of the target cell and the reference electrode. However, the method of measuring the negative electrode potential of the target cell is not limited to such a method and is arbitrary. For example, the ECU 300 may estimate the negative electrode potential of the target cell from the SOC of the target cell by referring to correspondence information (such as a map) in the memory 302.

  The threshold value Th2 can be detected in step S31 so as to be able to detect a sharp rise in the negative electrode potential (for example, a sharp rise in the negative electrode potential as occurs at timing t22 in FIG. 4) due to overdischarge of the target cell. It is set. For example, the transition of the negative electrode potential during discharge of battery 100 is determined in advance by experiment or the like, and threshold value Th2 appropriate for the transition of the negative electrode potential is stored in memory 302. More specifically, the negative electrode potential of the target cell becomes lower than the threshold Th2 before the rapid increase of the negative electrode potential caused by the overdischarge of the target cell, and the above-mentioned rapid rise (as a result, bipolar polarity) The threshold value Th2 is set such that the negative electrode potential of the target cell becomes higher than the threshold value Th2 due to the generation. The threshold value Th2 may be a fixed value, or may be variable according to the state of the target cell (for example, the cell temperature).

If the negative electrode potential of the target cell is lower than threshold value Th2 during discharge of battery 100 (NO in step S31), it is determined that no bipolar polarity inversion has occurred, and the process returns to step S14 in FIG. Be On the other hand, when the negative electrode potential of the target cell becomes equal to or higher than threshold value Th2 during the discharge of battery 100 (YES in step S31), it is determined that bipolar polarization has occurred, and ECU 300 generates oxygen gas generation amount O _m . It estimates (step S32).

The ECU 300 can calculate the oxygen gas generation amount O _m which is the amount of oxygen gas generated at the positive electrode by the reaction of the equation (7) described above, for example, using the following equation (A).

Oe (t) = ∫Oe (V ₀ , Tb) dt (A)
In Formula (A), “V ₀ ” represents the open circuit voltage (OCV), “Tb” represents the temperature of the target cell, and “t” represents time. The reaction of the formula (7) proceeds as the negative electrode potential of the target cell becomes higher than the oxygen generation potential. The higher the temperature of the target cell, the lower the oxygen evolution potential tends to be.

The method of estimating the oxygen gas generation amount O _m is not limited to the above and can be arbitrarily changed. For example, the ECU 300 may estimate the oxygen gas generation amount O _m using the integrated value of the amount (the amount of electricity) of the current flowing from the negative electrode of the target cell to the positive electrode.

After the calculation of the oxygen gas generation amount O _m , the process is returned to step S14 of FIG. Referring to FIG. 5, ECU 300 uses the hydrogen gas generation amount H _m which is estimated in step S12, an oxygen gas generation amount O _m estimated in step S13, estimates the cell internal pressure Pc of the target cell (Step S14). For example, information indicating the relationship between hydrogen gas generation amount H _m and an oxygen gas generation amount O _m and the cell internal pressure Pc (hereinafter, referred to as "Pc estimation information") to be stored in the memory 302 obtained in advance by experiment or the like It is also good. ECU300 refers to the Pc estimation information, it can be determined cell internal pressure Pc from a hydrogen gas generation amount _{H m} and the oxygen gas generation amount _{O m.}

ECU300, in the estimation of the cell internal pressure Pc, in addition to the hydrogen gas generation amount H _m and the oxygen gas generation amount O _m, may be other parameters. For example, in addition to the hydrogen gas generation amount H _m and the oxygen gas generation amount O _m described above, the ECU 300 takes into consideration the oxygen gas absorption amount at the negative electrode (hydrogen storage alloy) and the hydrogen equilibrium pressure, You may make it estimate. The amount of absorbed oxygen gas is the amount of oxygen gas absorbed by hydrogen adsorbed on the surface layer of the negative electrode (for example, Ni-Co layer) and the amount of oxygen gas absorbed by hydrogen absorbed inside the negative electrode. It corresponds to sum. The hydrogen equilibrium pressure corresponds to the hydrogen gas pressure (hydrogen partial pressure) when the amount of hydrogen stored in the negative electrode and the amount of hydrogen released from the negative electrode are balanced. The hydrogen equilibrium pressure can vary with cell temperature.

ECU300 is hydrogen gas generation amount _{H m} is the threshold Th3 or more, the cell internal pressure Pc is equal to or threshold value Th4 or more (step S15). The threshold Th3, the lowest amount of the hydrogen gas generation amount H _m that may cause withering electrolytic solution is obtained and set in advance by experiment or the like. If the threshold value Th3 is too high, it is impossible to prevent the electrolyte withering. If the threshold value Th3 is too low, unnecessary output limitation may be performed in step S16 described later, and the use of the power stored in the battery 100 may be limited. For example, a pressure lower than the valve opening pressure Px is set as the threshold value Th4. The threshold values Th3 and Th4 may be independently fixed values, or may be variable according to the state of the target cell (for example, the cell temperature).

In the case of hydrogen gas generation amount H _m it is less than the threshold value Th3 (NO at step S15), and also open the safety valve 103 is determined not to wither electrolyte, output limit of the battery 100 is not performed, Processing is returned to the main routine.

  If the cell internal pressure Pc is less than the threshold Th4 (NO in step S15), it is determined that the pressure in the case 102 of the target cell need not be reduced, and the output restriction of the battery 100 is not performed. Is returned to the main routine.

On the other hand, the hydrogen gas generation amount _{H m} is the threshold Th3 or more, and, when the cell internal pressure Pc is the threshold value Th4 or more (YES in step S15) is, ECU 300 is output limit of the battery 100 (Wout Restriction) to suppress the rise of the internal pressure of the cell without opening the safety valve 103 (step S16). Thereafter, the process is returned to the main routine.

  ECU 300 can limit the discharged power of battery 100 by reducing the value of output limit Wout indicating the upper limit value of the discharged power of battery 100. When the discharge power of the battery 100 is limited, the reactions of the above equations (5) and (7) are less likely to proceed, and the generation of gas in the case 102 of the target cell (thus, the increase in cell internal pressure) is suppressed. Ru.

  FIG. 8 is a diagram showing an example of the operation of the battery system 2 according to this embodiment. As the ECU 300 repeatedly performs the process of FIG. 5, the positive electrode potential of the target cell, the integrated amount of electricity ΔC, the cell internal pressure Pc, and the output limit Wout are, for example, line k31, line k32, line k33, line k34 in FIG. Transition as shown in. In FIG. 8, time “0” on the horizontal axis indicates the discharge start time.

Referring to FIG. 8, it is determined that the positive electrode potential of the target cell has become equal to or lower than threshold value Th1 at timing t31 (YES in step S11 of FIG. 5), calculation of integrated amount of electricity ΔC in step S21 of FIG. Integration of the amount of electricity flowing during over-discharge of the cell starts. In the period of timing t31 to t32, hydrogen gas is generated by the reaction represented by the above-mentioned equation (5). Then, before the timing t32, the hydrogen gas generation amount H _m (not shown in FIG. 8) becomes equal to or more than the threshold value Th3.

Thereafter, the hydrogen gas generation amount _{H m} at timing t32 and the threshold Th3 or more, and, (YES in step S15 of FIG. 5) the cell internal pressure Pc is determined to be the threshold value Th4 above, ECU 300 by the battery 100 Output restriction (Wout restriction) is performed (step S16 in FIG. 5). The ECU 300 gradually reduces the value of the output limit Wout and finally sets the output limit Wout to zero. When the output limit Wout becomes 0 at timing t33, the discharge of the battery 100 is prohibited. As shown by the line k33, the output restriction of the battery 100 is performed to suppress the increase in the internal cell pressure. The cell internal pressure is controlled so as not to exceed the valve opening pressure Px. By performing such control, the deterioration of the cell 101 due to the electrolyte withering is suppressed.

In each of steps S11, S31, and S15, it is arbitrary whether it is determined as YES or NO when each parameter (positive electrode potential, negative electrode potential, hydrogen gas generation amount _Hm , oxygen gas generation amount _Om ) matches the threshold value. And may be changed to be different from the examples shown in FIGS. 5 and 7.

When the battery system 2 is used in an application where the battery 100 is discharged under the condition that oxygen gas is not generated, step S13 of FIG. 5 (thus, a series of processes shown in FIG. 7) is omitted. There may be estimated cell internal pressure Pc by using only hydrogen gas generation amount H _m.

  In the above embodiment, an example is shown in which the battery system 2 estimates the current battery internal pressure taking into consideration only the change in the battery internal pressure at the time of discharge. However, the present invention is not limited to this, and the battery system may estimate the current battery internal pressure in consideration of the change in the battery internal pressure during charge in addition to the change in the battery internal pressure during discharge.

  According to the above-described battery system 2, it is possible to estimate the cell internal pressure with high accuracy. The ECU 300 may perform estimation of the cell internal pressure and output of the result thereof, and the subsequent processing may be left to the user.

  The configuration of the vehicle 1 to which the above battery system 2 is applied can be changed as appropriate. Also, the configuration of the battery 100 can be changed as appropriate. For example, a single battery may be employed instead of the assembled battery.

  It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is shown not by the above description of the embodiments but by the scope of claims, and is intended to include all modifications within the meaning and scope equivalent to the scope of claims.

  1 vehicle, 2 battery system, 10 MG, 20 power transmission gear, 30 drive wheel, 40 PCU, 100 battery, 101 cell, 102 case, 103 safety valve, 104 electrode body, 210 voltage sensor, 220 current sensor, 230 temperature sensor, 300 ECU, 301 CPU, 302 memory.

Claims (1)

A secondary battery having a positive electrode, a negative electrode, and an aqueous electrolyte solution in a case;
A gas amount estimation unit that estimates the amount of hydrogen gas generated at the positive electrode during overdischarge of the secondary battery using the integrated amount of electricity flowing during the overdischarge of the secondary battery;
A pressure estimation unit that estimates the pressure in the case of the secondary battery using the amount of hydrogen gas estimated by the gas amount estimation unit;
Equipped with
A battery system in which discharge is performed with the potential of the positive electrode being lower than a threshold during the overdischarge.

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