Battery system and temperature control method

Abstract

The present disclosure improves reliability of a battery system. A battery system (1) of the present disclosure is provided with: a storage battery (11) having a period in which an internal resistance decreases from a start of use to a certain time point in a life cycle; a temperature measuring unit (T1, 121) that measures a temperature of the storage battery (11); and a heating unit (13) for heating the storage battery (11). A control target lower limit temperature at the start of use of the storage battery (11) is set higher than a control target lower limit temperature that is changed in the period.

Description

Technical Field

[0001] This disclosure relates to battery systems and temperature control methods. Background Technology

[0002] Nickel-metal hydride (NiMH) batteries are alkaline batteries that utilize a hydrogen-coated alloy at the negative electrode and nickel hydroxide at the positive electrode. Generally, the hydrogen-coated alloy requires initial activation, which is achieved through repeated high-temperature holding for a given time and a given number of charge-discharge cycles. Initial activation improves initial output, but at the cost of accelerating battery degradation, thus shortening the battery's actual service life / total lifespan. In particular, when the battery is used at extremely low temperatures ranging from -30°C to 0°C, the degradation caused by initial activation has a greater impact on the battery's total service life / lifespan characteristics, sometimes significantly reducing it.

[0003] Patent Document 1 discloses an output unit that outputs a signal to heat the battery, a control unit that determines the degree of battery degradation, and a battery charging control device that controls the heating signal based on the degree of degradation. The heating signal is controlled so that the charging speed of a battery with high degradation becomes the same as that of a battery with low degradation.

[0004] Patent Document 2 discloses a battery control device comprising: a battery temperature detection unit; a unit for calculating the charge and discharge capacity of the battery; and a warm-up operation judgment unit. In the event of battery deterioration, a target temperature higher than that in the case of non-deterioration is set.

[0005] Both documents describe heating the battery with a heater to compensate for the battery's performance at the end of its lifespan (charging speed in Patent Document 1 and discharging performance in Patent Document 2), rather than heating it at the beginning of its lifespan.

[0006] Prior art literature

[0007] Patent documents

[0008] Patent Document 1: International Publication No. 14 / 064750

[0009] Patent Document 2: JP 2003-23704 Summary of the Invention

[0010] The problem that the invention aims to solve

[0011] This disclosure is made in view of the following circumstances, and its purpose is to provide a battery system with superior reliability.

[0012] Methods for solving problems

[0013] To address the aforementioned issues, a battery system according to a certain aspect of this disclosure includes: a battery having a period during its lifespan from the start of use until a specific point in time where its internal resistance decreases; a temperature measuring unit for measuring the temperature of the battery; and a heating unit for heating the battery. The control target lower limit temperature at the start of use is set higher than the control target lower limit temperature that changes during the period.

[0014] Furthermore, any combination of the above-mentioned constituent elements, or any scheme that transforms the manifestation of this disclosure among apparatus, systems, methods, computer programs, etc., is also valid as a form of this disclosure.

[0015] Invention Effects

[0016] According to this disclosure, the reliability of the battery system can be improved. Attached Figure Description

[0017] Figure 1 This is a diagram used to illustrate the battery system involved in the implementation method.

[0018] Figure 2 (a) is a graph that illustrates an example of the output shift during the life cycle of a nickel-metal hydride battery cell under certain conditions. Figure 2 (b) is a figure illustrating an example of the shift in the lower limit temperature of the control target that should be controlled by the heater during the life cycle of a nickel-metal hydride battery cell.

[0019] Figure 3 This is a flowchart illustrating an example of the control target lower limit temperature determination process for a battery module in a battery system according to the embodiment.

[0020] Figure 4 This is a flowchart illustrating an example of temperature control processing in a battery system according to an embodiment.

[0021] Figure 5 (a) is a diagram representing the first example of heating control performed by the heater. Figure 5 (b) is a diagram representing the second example of heating control performed by the heater. Figure 5 (c) is a diagram representing the first example of heating control performed by the heater. Detailed Implementation

[0022] Figure 1This diagram illustrates the battery system 1 according to the embodiment. The battery system 1 according to the embodiment is configured as a chargeable and dischargeable battery pack including a battery module 11, a battery management device 12, a heater 13, and a heater drive unit 14. The load is normally powered by an external power source, but in an emergency, power can be supplied to the load 2 from the battery system 1. In this embodiment, the load 2 is envisioned as an emergency light, a mobile phone base station, or a disaster relief wireless device used outdoors in cold regions. For the battery system 1 used as a power source in cold regions, it is desirable to balance low-temperature discharge performance and long lifespan. Furthermore, the battery system 1 of this disclosure can be used not only as a backup power source but also as a main power source. Additionally, the term "low temperature" here refers to, for example, below 0°C, and can be 0°C to -30°C. However, it is not limited to this temperature range.

[0023] Charger 4 is connected to commercial power system 3, converting the AC power input from commercial power system 3 into DC power of a given voltage or current and supplying it to battery system 1. Alternatively, charger 4 can be a structure built into the battery pack.

[0024] The battery module 11 comprises multiple cells E1-En connected in series. The number of cells connected in series is determined according to the specifications of the load 2. In this embodiment, a nickel-metal hydride battery cell is envisioned as an example, in which the active material of the positive electrode is a nickel compound, the active material of the negative electrode is a hydrogen-encapsulated alloy, and an alkaline aqueous solution is used as the electrolyte. Alternatively, in order to increase the capacity, multiple cells can be connected in parallel within the series stage of each cell.

[0025] A switch SW1, which switches the connection between the battery module 11 and the load 2 or charger 4, is inserted into the power line connecting them. The switch SW1 can be a semiconductor switch or a relay.

[0026] The battery management device 12 includes a measurement unit 121 and a control unit 122. The measurement unit 121 is composed of an AFE (Analog Front End) IC or an ASIC (Application Specific Integrated Circuit). The control unit 122 is composed of a microcontroller.

[0027] The measuring unit 121 measures the voltage of the battery module 11 by dividing the voltage of the battery module 11 using a voltage divider circuit (voltage divider resistor R1 and voltage divider resistor R2) connected to the two ends of the battery module 11. In addition, when the number of cells in the battery module 11 connected in series is small, the measuring unit 121 can also directly measure the voltage across the two ends of the battery module 11.

[0028] The measurement unit 121 includes an A / D converter. The A / D converter converts the measured analog voltage of the battery module 11 into a digital value. The measurement unit 121 transmits the converted digital voltage value of the battery module 11 to the control unit 122 via a serial communication interface.

[0029] The measuring unit 121 measures the current flowing through the battery module 11. A shunt resistor Rs is connected to the power line connecting the battery module 11 and the load 2 or charger 4. A differential amplifier (not shown) amplifies the voltage across the shunt resistor Rs and outputs it to an A / D converter within the measuring unit 121. The A / D converter converts the analog voltage representing the current flowing through the battery module 11, which is input from the differential amplifier, into a digital value. The measuring unit 121 then sends the converted digital current value to the control unit 122 via a serial communication interface.

[0030] A temperature sensor T1 (e.g., a thermistor) is installed on the surface of the battery module 11. The voltage divided by the temperature sensor T1 and a voltage divider resistor (not shown) is input to the measuring unit 121. An A / D converter in the measuring unit 121 converts the input analog voltage representing the temperature into a digital value. The measuring unit 121 sends the converted temperature value to the control unit 122 via a serial communication interface.

[0031] The control unit 122 manages the state of the battery module 11 based on the voltage, current, and temperature values ​​of the battery module 11 received from the measurement unit 121. If the control unit 122 detects any of the following: overcharge, over-discharge, overcurrent, high temperature abnormality, or low temperature abnormality, it sends a cut-off signal to the measurement unit 121 to turn off the switch SW1.

[0032] Heater 13 heats battery module 11. Heater 13 is, for example, a heating element with built-in heating wire. The heating element is attached to the surface of battery module 11. This structure of attaching a heating element to the surface of battery module 11 to heat it is one example of the structure of a heating section for heating battery module 11. For example, the heating section may also include a flow path for a heat medium (e.g., heated water) located near battery module 11, a heater for heating the heat medium, and a pump for circulating the heat medium.

[0033] The heater drive unit 14 controls the on / off state of the heater 13 or the heating amount. For example, the heater drive unit 14 controls the on / off state of the current supplied to the heating element. In addition, the heater drive unit 14 controls the amount of current supplied to the heating element to adjust the output of the heater 13.

[0034] The control unit 122 executes firmware and other programs within the microcontroller to achieve the following functions. The control unit 122 combines the OCV (Open Circuit Voltage) method and the current accumulation method to estimate the SOC (State of Charge). The OCV method estimates the SOC based on the measured OCV of a single cell and the SOC-OCV curve of that cell. The SOC-OCV curve of the single cell is pre-generated based on characteristic tests conducted by the battery manufacturer and is registered in the control unit 122 at the time of shipment.

[0035] The current accumulation method estimates the state of charge (SOC) based on the initial charge-discharge velocity (OCV) of a single cell and the cumulative value of the measured current. However, in this method, the measurement error accumulates as the charging / discharging time increases. Therefore, it is preferable to use a weighted average of the SOC estimated by the current accumulation method and the SOC estimated by the OCV method.

[0036] The control unit 122 refers to a pre-prepared table or function that describes the relationship between the life cycle progress of the battery module 11 and the lower limit temperature of the control target, to control the heater 13. Specifically, the control unit 122 refers to the table or function to determine the lower limit temperature of the control target corresponding to the measured temperature, and sets the heater 13 on / off command or the output level of the heater 13 to the heater drive unit 14 corresponding to the determined lower limit temperature of the control target.

[0037] Figure 2 (a) is a graph that illustrates an example of the output shift during the life cycle of a nickel-metal hydride battery cell under certain conditions. Figure 2 Figure (b) is a graph showing an example of the shift in the lower limit temperature of the control target that should be controlled by heater 13 during the life cycle of a nickel-metal hydride battery cell. Furthermore, in this measurement of the shift, the ambient temperature of the environment in which the nickel-metal hydride battery cell was placed was -5°C.

[0038] The lifespan of a battery is defined by the number of charge-discharge cycles and the elapsed time. Figure 2 (a) and Figure 2 The lifecycle progression of the horizontal axis in (b) is defined by 0 as the initial value corresponding to the degradation state at the start of use, and by 10 as the final value corresponding to the degradation state that should be set to the end of use.

[0039] Storage degradation of a battery is a process that progresses over time, corresponding to the temperature and state of charge (SOC) at various points in time. It occurs regardless of whether the battery is being charged or discharged. In nickel-metal hydride (NiMH) batteries, generally, the rate of storage degradation increases when the SOC is too low and the temperature is high at various points in time. Cycle degradation of a battery progresses with the number of charge-discharge cycles. Generally, the wider the SOC range used, the higher the temperature, and the higher the current rate, the faster the cycle degradation rate. Furthermore, degradation due to overcharging is accelerated at high or low temperatures.

[0040] In this embodiment, the lifecycle progression C is defined as follows (Equation 1).

[0041] C = A·α + B·β … (Equation 1)

[0042] A is the cumulative charging capacity [Ah], B is the elapsed time since the start of use [h], α is the cycle degradation constant, and β is the retention degradation constant.

[0043] Alternatively, A can replace cumulative charging capacity [Ah] and be used as cumulative charging power [Wh], cumulative discharging capacity [Wh], and cumulative discharging power [Wh].

[0044] The cycle degradation constant α and the retention degradation constant β are determined based on the final value of the lifecycle progression C corresponding to the degradation state that should be set as the end of use, and the cumulative charge capacity A, elapsed time B, and the progression of the degradation state obtained through experiments or simulations. The battery's degradation state can be estimated, for example, using the DC internal resistance under certain conditions. Alternatively, the battery's degradation state can be estimated using the ratio of the current full charge capacity to the initial full charge capacity (SOH). The final value of the lifecycle progression C corresponding to the degradation state that should be set as the end of use is set according to the intended use of battery system 1. The values ​​of the DC internal resistance and SOH that should be set as the end of use for the intended use of battery system 1 differ. Furthermore, in applications where the rest time is relatively longer compared to the charge / discharge time, the contribution of retention degradation increases; in applications where the rest time is relatively shorter compared to the charge / discharge time, the contribution of cycle degradation increases.

[0045] like Figure 2 As shown in (a), the output voltage (discharge characteristics) of a nickel-metal hydride battery cell increases at low temperatures from the start of use (P0) as the lifespan progresses (C), and then gradually decreases after reaching a certain point in time (P1). That is, a nickel-metal hydride battery cell exhibits the characteristic of decreasing DC internal resistance at low temperatures from the start of use and gradually increasing DC internal resistance from a specific point in its lifespan (P1).

[0046] In the past, the common method was to package and ship nickel-metal hydride battery cells that had reached their maximum discharge characteristics at a specific time point P1 after initial activation treatment. That is, the battery pack's lifecycle progression from the state corresponding to the specific time point P1 during its use. Figure 2 (a) is the state of 2 / 10. That is, the product life of the battery pack is shortened by the amount during the period P0 to P1.

[0047] In contrast, the purpose of this embodiment is to package nickel-metal hydride battery cells that have not reached their maximum discharge characteristics at a specific time point P1 and use them while heating them, thereby maximizing the product life of the battery pack.

[0048] like Figure 2 As shown in (b), a target lower limit temperature for the NiMH battery cell based on heater 13 is set to assist in the discharge of the NiMH battery cell. That is, because the NiMH battery cell has a high DC internal resistance at the initial low temperature during use, its low-temperature discharge performance is low; therefore, the initial value of the target lower limit temperature is set high. As the lifespan progression C increases, the target lower limit temperature decreases, and if the discharge characteristics reach their maximum at a specific time point P1, it is gradually increased. Because the discharge performance of the NiMH battery cell gradually decreases towards the end of its lifespan, the target lower limit temperature is gradually increased towards the end of its lifespan to compensate for this decrease in discharge performance.

[0049] exist Figure 2 In the example shown in (b), the initial value of the target lower limit temperature is set to 15°C, and the time point P1 at which the discharge characteristics reach their maximum is set to -5°C. The end-of-use time point of the nickel-metal hydride battery cell is set to 25°C.

[0050] The designer determines the target lower limit temperature for each lifespan stage C based on data obtained through experiments or simulations of the output voltage, temperature, and lifespan progression C of the nickel-metal hydride battery cells. This ensures that the output voltage of the nickel-metal hydride battery cells is above the minimum output voltage set by the manufacturer as a benchmark. The designer generates a table that correlates the lifespan progression C with the target lower limit temperature. This generated table is registered in the control unit 122 at the time of shipment. Alternatively, the designer can generate a function (approximate curve) that approximates the relationship between the lifespan progression C and the target lower limit temperature using multiple plotted points. This generated function is described in the source code within the firmware of the control unit 122.

[0051] exist Figure 2In (b), the target lower limit temperature is a continuously varying value, but the lifecycle progression C can also be divided into multiple intervals, with the target lower limit temperature set as a constant for each interval. By setting the target lower limit temperature discretely, the size of the table can be reduced.

[0052] Figure 3 This is a flowchart illustrating an example of the control target lower limit temperature determination process for the battery module 11 in the battery system 1 according to the embodiment. The control unit 122 sets an initial value for the temperature control range of the battery module 11 based on the heater 13 (S10). The temperature control range is defined by a control target lower limit temperature and a control target upper limit temperature, and the control target upper limit temperature is set as a value obtained by adding a given amplitude to the control target lower limit temperature. The given amplitude depends on the performance of the heater 13, the heat capacity of the battery module 11, etc. The higher the performance of the heater 13, or the smaller the heat capacity of the battery module 11, the narrower the given amplitude can be set.

[0053] The control unit 122 acquires the measured values ​​of voltage, current, and temperature of the battery module 11 from the measurement unit 121 (S11). The control unit 122 updates the cumulative charging capacity A at regular intervals based on the voltage and current of the battery module 11 (S12). The control unit 122 updates the elapsed time B since the start of use of the battery system 1 based on the time of the internal clock (not shown) (S13).

[0054] The control unit 122 calculates the lifecycle progress C based on the accumulated charging capacity A and the elapsed time B (Equation 1) (S14). Referring to the table above, the control unit 122 determines the lower limit temperature of the control target corresponding to the calculated lifecycle progress C (S15). The control unit 122 updates the temperature control range to the determined lower limit temperature of the control target and the upper limit temperature of the control target obtained by adding a given magnitude to the lower limit temperature of the control target (S16).

[0055] If the lifecycle progress C has not reached the preset final value (S17 "No"), proceed to step S11 and repeat the process of steps S11-S16. If the lifecycle progress C has reached the preset final value (S17 "Yes"), end the use of battery system 1.

[0056] In case of concerns about interruption of power supply from external power source to load 2, the battery module 11 is appropriately heated for temperature control so that the battery module 11 can discharge. Figure 4This is a flowchart illustrating an example of temperature control processing in the battery system 1 according to the embodiment. When it is determined that power supply from the battery module 11 is needed (S20 "Yes"), the control unit 122 obtains a measured value of the temperature of the battery module 11 from the measurement unit 121 (S21). If the measured temperature is below the control target lower limit temperature (S22 "Yes"), the control unit 122 turns on the heater 13 (S23). Proceed to step S24. In step S22, if the measured temperature is above the control target lower limit temperature (S22 "No"), proceed to step S20.

[0057] If it is determined that power supply from the battery module 11 is needed while the heater 13 is on (S24 "Yes"), the control unit 122 obtains a measurement value of the temperature of the battery module 11 from the measurement unit 121 (S25). If the measured temperature is above the control target upper limit temperature (S26 "Yes"), the control unit 122 disconnects the heater 13 (S27). Proceed to step S20. If, in step S26, the measured temperature is below the control target upper limit temperature (S26 "No"), proceed to step S24.

[0058] If it is determined that no power supply is needed from the battery module 11 (S20 "No", S24 "No"), the temperature control process ends.

[0059] Figure 5 Figure (a) is a diagram illustrating a first example of heating control based on heater 13. The first example is an illustration of on / off control of heater 13. Figure 4 The temperature control process shown in the flowchart employs heating control. If the temperature of the battery module 11 reaches the upper limit of the control target temperature, the control unit 122 disconnects the heater 13; if the temperature of the battery module 11 reaches the lower limit of the control target temperature, the control unit 122 turns the heater 13 on.

[0060] Figure 5 (b) is a diagram illustrating a second example of heating control based on heater 13. The second example involves controlling the heater 13 to be turned off after the temperature of the battery module 11 reaches the lower limit of the control target temperature, and then periodically turned on in a pulsed manner thereafter.

[0061] Figure 5 (c) is a diagram illustrating the third example of heating control based on heater 13. It shows that heater 13 operates at maximum output and, after the temperature of battery module 11 reaches the lower limit of the control target temperature, the output of heater 13 is reduced to a given level. Figure 5 (c) is controlled at 25%.

[0062] As explained above, according to this embodiment, by setting the lower limit temperature of the heater 13 to a high level at the initial stage of use of the battery system 1 to assist in the discharge of the battery, even in the case of a battery with insufficient activation treatment that cannot perform sufficient low-temperature discharge, the battery system 1 can be used as a battery system with sufficient discharge performance in low-temperature environments. In the past, in batteries such as nickel-metal hydride batteries that use hydrogen-encapsulated alloys with low initial activity, the amount of service life sacrificed due to activation treatment can be used as a product, thus extending the actual lifespan of batteries using materials with low initial activity.

[0063] The present disclosure has been described above with reference to the embodiments. The embodiments are illustrative, and those skilled in the art should understand that various modifications can be made to the combination of these constituent elements and processing techniques, and such modifications are also within the scope of the present disclosure.

[0064] In the case of a battery system 1 requiring high output, the aforementioned battery module 11 can be constructed by connecting multiple battery blocks, each containing multiple individual cells, in series. In this case, a heater 13, a heater drive unit 14, and a temperature sensor T1 are provided for each battery block, and the control unit 122 performs the aforementioned temperature control processing on a per-battery-block basis.

[0065] In the above embodiments, a nickel-metal hydride (NiMH) battery was used as an example of a battery whose internal resistance decreases from the start of use and increases from a specific point in its lifespan. In this respect, the temperature control method disclosed herein can also be applied to batteries other than NiMH batteries, such as NiCd batteries which use a paste-type cadmium negative electrode and whose internal resistance decreases from the start of use and increases from a specific point in their lifespan.

[0066] In addition, the implementation method can be determined by the following items.

[0067] [Project 1]

[0068] A battery system (1) comprising:

[0069] The battery (11) has a period of reduced internal resistance from the start of use to a specific point in time during its lifespan;

[0070] Temperature measuring unit (T1, 121) measures the temperature of the battery (11); and

[0071] Heating unit (13) is used to heat the storage battery (11).

[0072] The initial control target lower limit temperature is set higher than the control target lower limit temperature that changes during the period.

[0073] Therefore, the reliability of the battery system (1) can be improved.

[0074] [Project 2]

[0075] In the battery system (1) described in Project 1,

[0076] It also includes: a control unit (122) that controls the heating unit (13) by referring to a pre-prepared table or function that describes the relationship between the life cycle progress of the battery (11) and the lower limit temperature of the control target.

[0077] Therefore, optimal temperature control can be performed in accordance with the life cycle progression of the battery (11).

[0078] [Project 3]

[0079] In the battery system (1) described in Project 2,

[0080] The control unit (122) calculates the life cycle progress based on at least one of the cumulative charge or discharge amount of the battery (11) and the elapsed time since the start of use of the battery (11).

[0081] Based on this, the life cycle progress of the storage battery (11) can be accurately grasped.

[0082] [Project 4]

[0083] In the battery system (1) described in Project 2,

[0084] The control unit (122) activates the heating unit (13) when the measured temperature is below the lower limit temperature of the control target, and stops the heating unit (13) when the measured temperature is above the upper limit temperature of the control target, which is a given value higher than the lower limit temperature of the control target.

[0085] Therefore, the heating element (13) can be easily constructed, and the cost can be reduced.

[0086] [Project 5]

[0087] In any of the battery systems described in items 1 to 4 (1),

[0088] The battery (11) is a nickel-metal hydride battery (11).

[0089] Accordingly, nickel-metal hydride batteries (11) can be used as products without undergoing activation treatment of the hydrogen-encapsulated alloy of the negative electrode.

[0090] [Project 6]

[0091] In the battery system (1) described in Project 1,

[0092] The battery (11) has a period during its lifespan where its internal resistance increases after the specific time point.

[0093] The lower limit temperature of the control target set at the beginning of the period of internal resistance rise is lower than the lower limit temperature of the control target that changes during the period of internal resistance rise.

[0094] Therefore, the lifespan of the storage battery (11) can be extended.

[0095] [Project 7]

[0096] A temperature control method is a method for controlling the temperature of a battery (11) that has a period of internal resistance reduction from the start of use to a specific point in time during its life cycle.

[0097] The control target lower limit temperature at the start of use of the battery (11) is set higher than the control target lower limit temperature that changes during the period.

[0098] This can improve the reliability of the battery system.

[0099] Explanation of reference numerals in the attached figures

[0100] 1 Battery System

[0101] 2. Load

[0102] 3. Commercial Power Systems

[0103] 4 Chargers

[0104] 11 Battery Module

[0105] 12 Battery Management Device

[0106] 121 Measurement Department

[0107] 122 Control Department

[0108] 13 Heaters

[0109] 14 Heater drive unit

[0110] E1-En monomer

[0111] Voltage divider resistors R1 and R2

[0112] Rs Shunt resistor

[0113] SW1 switch

[0114] T1 Temperature sensor.

Claims

1. A battery system comprising: A storage battery has a period during its lifespan from the start of use until a specific point in time when its internal resistance decreases. Temperature measuring unit, for measuring the temperature of the battery; and The heating element is used to heat the battery. The initial control target lower limit temperature is set higher than the control target lower limit temperature that changes during the period.

2. The battery system according to claim 1, wherein, The battery system further includes a control unit that controls the heating unit by referring to a pre-prepared table or function that describes the relationship between the battery's life cycle progression and the target lower limit temperature.

3. The battery system according to claim 2, wherein, The control unit calculates the life cycle progress based on at least one of the cumulative charge or discharge amount of the battery and the elapsed time since the start of the battery's use.

4. The battery system according to claim 2, wherein, The control unit activates the heating unit when the measured temperature is below the lower limit of the control target temperature, and stops the heating unit when the measured temperature is above the upper limit of the control target temperature, which is a given value higher than the lower limit of the control target temperature.

5. The battery system according to any one of claims 1 to 4, wherein, The battery in question is a nickel-metal hydride battery.

6. The battery system according to claim 1, wherein, The battery has a period during its lifespan where its internal resistance increases after the specific time point. The lower limit temperature of the control target set at the beginning of the period of internal resistance rise is lower than the lower limit temperature of the control target that changes during the period of internal resistance rise.

7. A method for temperature control of a storage battery, specifically a method for temperature control of a storage battery during its lifespan, specifically a period from the start of use to a specific point in time where the internal resistance decreases. The initial control target lower limit temperature of the battery is set higher than the control target lower limit temperature that changes during the period.

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