Battery system and temperature control method

Abstract

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

Description

Battery system and temperature control method

[0001] The present disclosure relates to a battery system and a temperature control method.

[0002] Nickel-metal hydride batteries are alkaline storage batteries that use a hydrogen storage alloy for the negative electrode and nickel hydroxide for the positive electrode. Generally, hydrogen storage alloys require initial activation, which is achieved by maintaining them at high temperatures for a specified period of time or by repeatedly charging and discharging them a specified number of times. While improving initial output, the initial activation process accelerates battery degradation, shortening the battery's effective usage period and total lifespan. In particular, when using batteries at extremely low temperatures between −30°C and 0°C, the degradation caused by the initial activation process significantly impacts the battery's total usage period and lifespan characteristics, potentially significantly shortening the battery's total usage period and lifespan.

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

[0004] Patent Document 2 discloses a battery control device that includes a battery temperature detection unit, a battery charge / discharge capacity calculation unit, and a warm-up operation determination unit. When the battery is deteriorated, a higher target temperature is set than when the battery is not deteriorated.

[0005] In both documents, the heater heats the battery to compensate for battery performance at the end of the battery's life (charge rate in Patent Document 1, discharge performance in Patent Document 2), but not in the early stages of the battery life cycle.

[0006] International Publication No. 14 / 064750 Japanese Patent Application Laid-Open No. 2003-23704

[0007] The present disclosure has been made in view of these circumstances, and its purpose is to provide a battery system with excellent reliability.

[0008] In order to solve the above problem, a battery system according to an embodiment of the present disclosure includes a storage battery having a period in which its internal resistance decreases from the start of use to a specific point in its life cycle, a temperature measurement unit that measures the temperature of the storage battery, and a heating unit that heats the storage battery, wherein a control target lower limit temperature at the start of use is set higher than a control target lower limit temperature that is changed during the period.

[0009] Any combination of the above components, and conversion of the present disclosure into an apparatus, system, method, computer program, etc., are also valid aspects of the present disclosure.

[0010] According to the present disclosure, the reliability of the battery system can be improved.

[0011] 2A is a diagram for explaining a battery system according to an embodiment; FIG. 2A is a diagram showing an example of output transition in the life cycle of a nickel-metal hydride battery cell under certain conditions; FIG. 2B is a diagram showing an example of transition in the control target lower limit temperature to be controlled by a heater in the life cycle of a nickel-metal hydride battery cell; FIG. 2B is a flowchart showing an example of a process for determining the control target lower limit temperature of a storage battery module in a battery system according to an embodiment; FIG. 5A is a diagram showing a first example of heating control by a heater; FIG. 5B is a diagram showing a second example of heating control by a heater; and FIG. 5C is a diagram showing a first example of heating control by a heater.

[0012] FIG. 1 is a diagram illustrating a battery system 1 according to an embodiment. The battery system 1 according to the embodiment is configured as a chargeable and dischargeable battery pack including a storage battery module 11, a battery management device 12, a heater 13, and a heater driver 14. A load is normally supplied with power from an external power source, but in an emergency, power can be supplied from the battery system 1 to the load 2. In this embodiment, the load 2 is assumed to be an emergency light used outdoors in cold regions, a mobile phone base station, a disaster prevention radio, or the like. A battery system 1 used as a power source installed in cold regions is required to achieve both low-temperature discharge performance and a long life. The battery system 1 according to the present disclosure may be used not only as a backup power source but also as a main power source. Here, low temperatures refer to temperatures below 0°C, and may be between 0°C and −30°C, for example. However, the low temperatures are not limited to this temperature range.

[0013] The charger 4 is connected to the commercial power grid 3, converts AC power input from the commercial power grid 3 into DC power of a predetermined voltage or current, and supplies the DC power to the battery system 1. The charger 4 may be built into the battery pack.

[0014] The storage battery module 11 includes a plurality of cells E1-En connected in series. The number of cells connected in series is determined by the specifications of the load 2. In this embodiment, an example is assumed in which nickel-metal hydride battery cells are used, using a nickel compound as the active material for the positive electrode, a hydrogen storage alloy as the active material for the negative electrode, and an alkaline aqueous solution as the electrolyte. Note that, in each series stage of cells, a plurality of cells may be connected in parallel to increase capacity.

[0015] A switch SW1 that switches between electrical continuity with the load 2 or charger 4 is inserted in a power line connecting the storage battery module 11 and the load 2 or charger 4. A semiconductor switch or a relay can be used as the switch SW1.

[0016] The battery management device 12 includes a measurement unit 121 and a control unit 122. The measurement unit 121 is configured by an AFE (Analog Front End) IC or an ASIC (Application Specific Integrated Circuit), and the control unit 122 is configured by a microcontroller.

[0017] The measurement unit 121 measures the voltage of the storage battery module 11 by dividing the voltage of the storage battery module 11 using a resistive voltage divider circuit (voltage dividing resistors R1 and R2) connected to both ends of the storage battery module 11. Note that if the number of cells connected in series in the storage battery module 11 is small, the measurement unit 121 may directly measure the voltage across the storage battery module 11.

[0018] The measurement unit 121 includes an A / D converter. The A / D converter converts the measured analog voltage of the storage battery module 11 into a digital value. The measurement unit 121 transmits the converted digital voltage value of the storage battery module 11 to the control unit 122 via the serial communication interface.

[0019] The measurement unit 121 measures the current flowing through the storage battery module 11. A shunt resistor Rs is connected to a power line connecting the storage battery module 11 to the load 2 or the charger 4. A differential amplifier (not shown) amplifies the voltage across the shunt resistor Rs and outputs it to an A / D converter in the measurement unit 121. The A / D converter converts the analog voltage indicating the current flowing through the storage battery module 11, which is input from the differential amplifier, into a digital value. The measurement unit 121 transmits the current value converted into a digital value to the control unit 122 via a serial communication interface.

[0020] A temperature sensor T1 (e.g., a thermistor) is installed on the surface of the storage battery module 11. A divided voltage between the temperature sensor T1 and a voltage dividing resistor (not shown) is input to the measurement unit 121. An A / D converter in the measurement unit 121 converts the input analog voltage indicating the temperature into a digital value. The measurement unit 121 transmits the converted digital temperature value to the control unit 122 via a serial communication interface.

[0021] The control unit 122 manages the state of the storage battery module 11 based on the voltage value, current value, and temperature value of the storage battery module 11 received from the measurement unit 121. When the control unit 122 detects overcharge, overdischarge, overcurrent, abnormally high temperature, or abnormally low temperature, it sends a shutoff signal for the switch SW1 to the measurement unit 121 to turn off the switch SW1.

[0022] The heater 13 heats the storage battery module 11. The heater 13 is configured, for example, as an electric heating sheet with an electric heating wire built in. The electric heating sheet is attached to the surface of the storage battery module 11. Note that the configuration in which the electric heating sheet is attached to the surface of the storage battery module 11 to heat the storage battery module 11 is an example of the configuration of a heating unit that heats the storage battery module 11. For example, the heating unit may be configured to include a flow path for a heat medium (e.g., heated water) installed near the storage battery module 11, a heater for heating the heat medium, and a pump for circulating the heat medium.

[0023] The heater driving unit 14 controls the on / off or heating amount of the heater 13. For example, the heater driving unit 14 controls the on / off of the current supplied to the electric heating sheet. The heater driving unit 14 also controls the amount of current supplied to the electric heating sheet to adjust the output of the heater 13.

[0024] The control unit 122 executes programs such as firmware within the microcontroller to achieve the following functions. The control unit 122 estimates the SOC (State Of Charge) by combining the OCV (Open Circuit Voltage) method and the current integration method. The OCV method is a method for estimating the SOC based on the measured cell OCV and the cell's SOC-OCV curve. The cell's SOC-OCV curve is created in advance by the battery manufacturer based on characteristic tests and is registered in the control unit 122 at the time of shipment.

[0025] The current integration method is a method for estimating the SOC based on the OCV at the start of cell charging and discharging and the integrated value of the measured current. With the current integration method, current measurement errors accumulate as the charging and discharging time increases. Therefore, it is preferable to use a weighted average of the SOC estimated by the current integration method and the SOC estimated by the OCV method.

[0026] The control unit 122 controls the heater 13 by referring to a table or function prepared in advance that describes the relationship between the life cycle progress and the control target lower limit temperature of the storage battery module 11. Specifically, the control unit 122 refers to the table or function to identify the control target lower limit temperature that corresponds to the measured temperature, and sets an on / off command for the heater 13 or an output level for the heater 13 in the heater driving unit 14 according to the identified control target lower limit temperature.

[0027] 2(a) is a diagram showing an example of output transition over the life cycle of a nickel-metal hydride battery cell under certain conditions. FIG. 2(b) is a diagram showing an example of transition over the life cycle of the nickel-metal hydride battery cell in terms of the target lower limit temperature to be controlled by the heater 13. Note that when measuring this transition, the temperature of the environment in which the nickel-metal hydride battery cell was used was −5°C.

[0028] The life cycle of a storage battery is defined by the number of charge / discharge cycles and the elapsed time. The horizontal axis of Figures 2(a) and 2(b) shows the life cycle progress, with a start value of 0 corresponding to the deterioration state at the start of use and an end value of 10 corresponding to the deterioration state at which use should be terminated.

[0029] Storage degradation of a battery is degradation that progresses over time depending on the temperature and SOC of the battery at each point in time. It progresses over time regardless of whether the battery is being charged or discharged. In nickel-metal hydride batteries, the rate of storage degradation generally increases when the SOC at each point in time is too low or the temperature at each point in time is high. Cycle degradation of a battery is degradation that progresses as the number of charge / discharge cycles increases. Generally, the wider the SOC range used, the higher the temperature, and the higher the current rate, the faster the rate of cycle degradation. Furthermore, the rate of degradation due to overcharging is accelerated at high or low temperatures.

[0030] In this embodiment, the life cycle progress C is defined as shown in the following (Equation 1).

[0031] C=A·α+B·β (Formula 1) A is the cumulative charge capacity [Ah], B is the elapsed time from the start of use [h], α is the cycle deterioration constant, and β is the storage deterioration constant.

[0032] It should be noted that A may use the cumulative charge amount [Wh], cumulative discharge amount [Wh], or cumulative discharge amount [Wh] instead of the cumulative charge capacity [Ah].

[0033] The cycle degradation constant α and the storage degradation constant β are determined based on the end value of the life cycle progress C corresponding to the degradation state at which use should be terminated, the cumulative charge capacity A, the elapsed time B, and the progression of the degradation state obtained through experiments or simulations. The degradation state of the storage battery is estimated, for example, by the DC internal resistance under certain conditions. Alternatively, the degradation state of the storage battery may be estimated by the ratio of the current full charge capacity to the initial full charge capacity (SOH). The end value of the life cycle progress C corresponding to the degradation state at which use should be terminated is set according to the application in which the battery system 1 is used. The DC internal resistance value and SOH value at which use should be terminated differ depending on the application in which the battery system 1 is used. Furthermore, the contribution of storage degradation is greater in applications in which the downtime is relatively longer than the charge / discharge time, while the contribution of cycle degradation is greater in applications in which the downtime is relatively shorter than the charge / discharge time.

[0034] 2A, as the life cycle progress C of a nickel-metal hydride battery cell increases from the start of use P0, the output voltage (discharge characteristics) at low temperatures initially rises, and then gradually decreases after reaching a specific point P1. That is, a nickel-metal hydride battery cell has a characteristic in which the DC internal resistance at low temperatures decreases from the start of use, and then gradually increases from the specific point P1 in the life cycle.

[0035] In the past, nickel-metal hydride battery cells were typically shipped after reaching the maximum discharge characteristics at a specific time point P1 through initial activation processing. This meant that the battery pack's product lifespan began at the life cycle progress level corresponding to the specific time point P1 (2 / 10 in Figure 2(a)). This meant that the battery pack's product lifespan was shortened by the time between P0 and P1.

[0036] In contrast, the present embodiment aims to maximize the product life of the battery pack by packing nickel-metal hydride battery cells that have not yet reached the discharge characteristics at specific time point P1, where the discharge characteristics are at their maximum, and using them while heating them.

[0037] As shown in Figure 2(b), the control target lower limit temperature of the nickel-metal hydride battery cell by the heater 13 is set to assist the discharge of the nickel-metal hydride battery cell. That is, nickel-metal hydride battery cells have high DC internal resistance at low temperatures in the early stages of use and low low-temperature discharge performance, so the initial value of the control target lower limit temperature is set high. As the life cycle progress C increases, the control target lower limit temperature is lowered, and when it reaches a specific point P1 at which the discharge characteristics are at their maximum, it is gradually raised. As the discharge performance of nickel-metal hydride battery cells gradually deteriorates toward the end of their life, the control target lower limit temperature is gradually raised toward the end of their life to compensate for this deterioration in discharge performance.

[0038] In the example shown in Figure 2(b), the initial value of the control target lower limit temperature is set to 15°C, and when it reaches a specific point P1 where the discharge characteristics are at their maximum, it is set to -5°C. When the nickel-metal hydride battery cell reaches the end of its life, it is set to 25°C.

[0039] From data on the output voltage, temperature, and life cycle progress C of the nickel-metal hydride battery cell obtained through experiments or simulations, the designer determines the control target lower limit temperature at each life cycle progress C so that the output voltage of the nickel-metal hydride battery cell is equal to or higher than the minimum output voltage set as a standard by the manufacturer. The designer generates a table linking the life cycle progress C with the control target lower limit temperature. The generated table is registered in the control unit 122 at the time of shipment. The designer may also generate a function (approximation curve) that approximates multiple plots showing the relationship between the life cycle progress C and the control target lower limit temperature. The generated function is written in the source code in the firmware of the control unit 122.

[0040] 2B, the control target lower limit temperature is a value that changes continuously, but the life cycle progress C may be divided into multiple sections, and the control target lower limit temperature may be set as a constant for each section. When the control target lower limit temperature is set discretely, the size of the table can be reduced.

[0041] 3 is a flowchart showing an example of a process for determining a control target lower-limit temperature of the storage battery module 11 in the battery system 1 according to the embodiment. The control unit 122 sets an initial value of the temperature control range of the storage battery module 11 by 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 to a value obtained by adding a predetermined width to the control target lower-limit temperature. The predetermined width is set depending on the performance of the heater 13, the thermal capacity of the storage battery module 11, and the like. The higher the performance of the heater 13 or the smaller the thermal capacity of the storage battery module 11, the narrower the predetermined width can be set.

[0042] The control unit 122 acquires measured values ​​of the voltage, current, and temperature of the storage battery module 11 from the measurement unit 121 (S11). The control unit 122 updates the accumulated charge capacity A at regular intervals based on the voltage and current of the storage 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 an internal clock (not shown) (S13).

[0043] The control unit 122 calculates the life cycle progress C by calculating the above formula (1) based on the cumulative charge capacity A and the elapsed time B (S14). The control unit 122 references the above table and specifies a control target lower limit temperature according to the calculated life cycle progress C (S15). The control unit 122 updates the temperature control range to the specified control target lower limit temperature and a control target upper limit temperature obtained by adding a predetermined width to the control target lower limit temperature (S16).

[0044] If the life cycle progress C has not reached the preset end value (N in S17), the process proceeds to step S11, and the processes of steps S11 to S16 are repeated. If the life cycle progress C has reached the preset end value (Y in S17), the use of the battery system 1 is terminated.

[0045] When there is a concern that the power supply from the external power source to the load 2 may be interrupted, the storage battery module 11 is appropriately heated to enable discharge, thereby performing temperature control. Fig. 4 is a flowchart showing an example of a temperature control process in the battery system 1 according to the embodiment. When it is determined that power supply from the storage battery module 11 is necessary (Y in S20), the control unit 122 acquires a measured value of the temperature of the storage battery module 11 from the measurement unit 121 (S21). When the measured value of the temperature is equal to or lower than the control target lower limit temperature (Y in S22), the control unit 122 turns on the heater 13 (S23). The process proceeds to step S24. When the measured value of the temperature is higher than the control target lower limit temperature (N in S22) in step S22, the process proceeds to step S20.

[0046] If it is determined that power supply from the storage battery module 11 is necessary while the heater 13 is on (Y in S24), the control unit 122 acquires the measured temperature value of the storage battery module 11 from the measurement unit 121 (S25). If the measured temperature value is equal to or higher than the control target upper limit temperature (Y in S26), the control unit 122 turns off the heater 13 (S27). The process proceeds to step S20. If the measured temperature value is lower than the control target upper limit temperature in step S26 (N in S26), the process proceeds to step S24.

[0047] If it is determined that power supply from the storage battery module 11 is not required (N in S20, N in S24), the temperature control process ends.

[0048] Fig. 5A is a diagram showing a first example of heating control by the heater 13. The first example is an example of on / off control of the heater 13, and is the heating control adopted in the temperature control process shown in the flowchart of Fig. 4. The control unit 122 turns off the heater 13 when the temperature of the storage battery module 11 reaches a control target upper limit temperature, and turns on the heater 13 when the temperature of the storage battery module 11 reaches a control target lower limit temperature.

[0049] 5B is a diagram showing a second example of heating control by the heater 13. In the second example, the heater 13 is turned off after the temperature of the storage battery module 11 reaches the control target lower limit temperature, and then periodically turned on in a pulsed manner.

[0050] 5C is a diagram showing a third example of heating control by the heater 13. In this control, the heater 13 is operated at maximum output, and after the temperature of the storage battery module 11 reaches the control target lower limit temperature, the output of the heater 13 is reduced to a predetermined level (25% in FIG. 5C).

[0051] As described above, according to this embodiment, by assisting the discharge of the storage battery by setting the control target lower limit temperature of the heater 13 higher in the early stages of use of the battery system 1, even if a storage battery that has not been sufficiently activated and is therefore unable to perform sufficient low-temperature discharge is used, the battery system 1 can be used as a product with sufficient discharge performance in low-temperature environments. Conventionally, in storage batteries such as nickel-metal hydride batteries that use hydrogen storage alloys with low initial activity, the period of use that would have been sacrificed due to activation treatment can now be used as a product, thereby extending the effective life of batteries that use materials with low initial activity.

[0052] The present disclosure has been described above based on the embodiments. The embodiments are merely examples, and it will be understood by those skilled in the art that various modifications are possible in the combination of the respective components and processing steps, and that such modifications are also within the scope of the present disclosure.

[0053] In the case of a battery system 1 requiring high output, the above-described storage battery module 11 may be configured by connecting multiple storage battery blocks, each including multiple cells, in series. In this case, a heater 13, a heater driving unit 14, and a temperature sensor T1 are provided for each storage battery block, and the control unit 122 executes the above-described temperature control process for each storage battery block.

[0054] In the above-described embodiment, a nickel-metal hydride battery has been described as an example of a storage battery whose internal resistance decreases from the start of use and increases from a specific point in its life cycle. In this regard, the temperature control method according to the present disclosure can be applied to storage batteries other than nickel-metal hydride batteries, such as nickel-cadmium batteries that use a paste-type cadmium negative electrode as the negative electrode, as long as the internal resistance decreases from the start of use and increases from a specific point in its life cycle.

[0055] The embodiment may be specified by the following items.

[0056] [Item 1] A battery system (1) comprising: a storage battery (11) having a period in which internal resistance decreases from the start of use to a specific point in time during its life cycle; a temperature measurement unit (T1, 121) that measures the temperature of the storage battery (11); and a heating unit (13) that heats the storage battery (11), wherein a control target lower limit temperature at the start of use is set higher than a control target lower limit temperature that is changed during the period.

[0057] This can improve the reliability of the battery system (1).

[0058] [Item 2] The battery system (1) according to Item 1, further comprising a control unit (122) that controls the heating unit (13) by referring to a table or function that is prepared in advance and that describes a relationship between a life cycle progress of the storage battery (11) and a control target lower limit temperature.

[0059] This allows optimal temperature control depending on the progress of the life cycle of the storage battery (11).

[0060] [Item 3] The battery system (1) according to Item 2, wherein the control unit (122) calculates the life cycle progress level based on at least one of an accumulated charge amount or an accumulated discharge amount of the storage battery (11) and an elapsed time since the start of use of the storage battery (11).

[0061] This allows the progress of the life cycle of the storage battery (11) to be accurately grasped.

[0062] [Item 4] The battery system (1) according to Item 2, wherein the control unit (122) operates the heating unit (13) when the measured temperature is equal to or lower than the control target lower limit temperature, and stops the heating unit (13) when the measured temperature is equal to or higher than a control target upper limit temperature that is higher than the control target lower limit temperature by a predetermined value.

[0063] This allows the heating section (13) to be simply configured, thereby reducing costs.

[0064] [Item 5] The battery system (1) according to any one of Items 1 to 4, wherein the storage battery (11) is a nickel-metal hydride storage battery (11).

[0065] This allows the nickel-metal hydride storage battery (11) to be used as a product without undergoing activation treatment of the hydrogen storage alloy of the negative electrode.

[0066] [Item 6] The battery system (1) according to Item 1, wherein the storage battery (11) has a period in which internal resistance increases after the specific point in time in its life cycle, and the control target lower limit temperature set at the start of the period in which the internal resistance increases is set lower than the control target lower limit temperature changed during the period in which the internal resistance increases.

[0067] This allows the life of the storage battery (11) to be extended.

[0068] [Item 7] A temperature control method for a storage battery (11) that has a period in its life cycle in which its internal resistance decreases from the start of use to a specific point in time, the temperature control method comprising: setting a control target lower limit temperature at the start of use of the storage battery (11) higher than a control target lower limit temperature that is changed during the period.

[0069] This can improve the reliability of the battery system.

[0070] REFERENCE SIGNS LIST 1 Battery system 2 Load 3 Commercial power system 4 Charger 11 Storage battery module 12 Battery management device 121 Measurement unit 122 Control unit 13 Heater 14 Heater driving unit E1-En Cell R1, R2 Voltage dividing resistor Rs Shunt resistor SW1 Switch T1 Temperature sensor

Claims

1. A battery system comprising: a storage battery that has a period in its life cycle during which its internal resistance decreases from the start of use to a specific point in time; a temperature measurement unit that measures the temperature of said storage battery; and a heating unit that heats said storage battery, wherein the control target lower limit temperature at the start of use is set higher than the control target lower limit temperature that is changed during said period.

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

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

4. The battery system of claim 2, wherein the control unit operates the heating unit when the measured temperature is equal to or lower than the control target lower limit temperature, and stops the heating unit when the measured temperature is equal to or higher than the control target upper limit temperature that is higher than the control target lower limit temperature by a predetermined value.

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

6. The battery system according to claim 1, wherein the storage battery has a period in which the internal resistance increases after the specific point in time during its life cycle, and the control target lower limit temperature set at the start of the period in which the internal resistance increases is set lower than the control target lower limit temperature to which the temperature is changed during the period in which the internal resistance increases.

7. A temperature control method for a storage battery that has a period in its life cycle during which its internal resistance decreases from the start of use to a specific point in time, wherein the control target lower limit temperature at the start of use of the storage battery is set higher than the control target lower limit temperature that is changed during the period.

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