JP6493762B2 - Battery system - Google Patents

Description

  The present invention relates to a battery system.

  A lithium ion secondary battery that is lightweight and has a high energy density is preferably used as a vehicle-mounted power source. In this type of lithium ion secondary battery, charge and discharge are performed by transferring lithium ions between a positive electrode including a positive electrode active material and a negative electrode including a negative electrode active material. That is, during charging, lithium (charge carrier) is extracted from the positive electrode active material and released as lithium ions into the electrolytic solution (electrolyte). At the time of charging, the lithium ions enter the structure of the negative electrode active material provided on the negative electrode side. Here, the electrons that have passed through the external circuit from the positive electrode active material are obtained and occluded. As the negative electrode active material, graphite having a layered structure is preferably used. During charging, lithium ions enter the graphite layer. Patent document 1 is mentioned as a prior art regarding this kind of lithium ion secondary battery.

JP 2014-107032 A

  By the way, in order to effectively use this type of lithium ion secondary battery, it is desired to accurately grasp the remaining capacity (SOC: State Of Charge) remaining in the lithium ion secondary battery. Conventionally, the SOC is estimated from the correspondence between the battery voltage and the SOC by utilizing the fact that the battery voltage has a slope with respect to the fluctuation of the SOC. For example, Patent Document 1 describes that in a lithium ion secondary battery using a two-layer coexisting positive electrode active material, a map is divided according to the reaction region of the positive electrode to estimate the SOC. However, even such a technique is insufficient to sufficiently satisfy the recent requirement level regarding the estimation accuracy of the SOC, and there is still room for improvement.

  This invention is made | formed in view of said situation, The main objective is to provide the battery system which can improve the estimation precision of SOC.

The battery system proposed here includes a lithium ion secondary battery using graphite as a negative electrode active material, a temperature sensor that detects the temperature of the lithium ion secondary battery, and a current value that enters and exits the lithium ion secondary battery. A voltage sensor for detecting a closed circuit voltage value of the lithium ion secondary battery, and an SOC estimation means for estimating the SOC of the lithium ion secondary battery. The SOC estimation means includes the following processes A to D:
(Process A) Estimating the internal resistance value corresponding to the temperature detected by the temperature sensor using the first map showing the correspondence between the temperature of the lithium ion secondary battery and the internal resistance value;
(Process B) The overvoltage of the lithium ion secondary battery is calculated by multiplying the estimated internal resistance value and the current value detected by the current sensor;
(Process C) The open circuit voltage value of the lithium ion secondary battery is calculated by subtracting the calculated overvoltage from the closed circuit voltage value detected by the voltage sensor;
(Process D) Estimating the SOC corresponding to the calculated open circuit voltage value using the second map showing the correspondence between the open circuit voltage value and the SOC;
Is configured to run. Further, the SOC estimation means includes a plurality of the first maps having different characteristics and a graphite stage determination unit that determines an intercalation stage of the graphite based on the SOC estimated in the processing D. The first map used for the processing A is switched based on an intercalation stage of graphite. According to this configuration, it is possible to improve the SOC estimation accuracy.

It is a block diagram which shows the structure of the battery system which concerns on this embodiment. It is a figure which shows the processing flow of the battery system which concerns on this embodiment. It is a figure which shows the relationship between SOC, the resistance A, and the resistance B. It is a figure which shows the relationship between SOC, the resistance A, and the resistance B. It is a figure which shows the relationship between SOC, the resistance A, and the resistance B. It is a figure which shows the relationship between SOC, the resistance A, and the resistance B.

  Embodiments according to the present invention will be described below with reference to the drawings. In the following drawings, members / parts having the same action are described with the same reference numerals. Note that the dimensional relationship (length, width, thickness, etc.) in each drawing does not reflect the actual dimensional relationship. In addition, matters other than the matters specifically mentioned in the present specification and matters necessary for the implementation of the present invention (for example, the construction and manufacturing method of the positive electrode and the negative electrode, and the general construction of the lithium ion secondary battery and other batteries) Technical technology etc.) can be understood as a design matter of those skilled in the art based on the prior art in the field.

  FIG. 1 is a block diagram showing a configuration of a battery system 1 according to the present embodiment. This battery system 1 is suitably used for a vehicle (typically, an automobile including an electric motor such as an automobile, particularly a hybrid automobile, an electric automobile, or a fuel cell automobile).

  The battery system 1 includes a lithium ion secondary battery (hereinafter sometimes simply referred to as “secondary battery”) 10, a load 20 connected thereto, and an electronic control unit (ECU) 30. . The ECU 30 is configured to control the operation of the lithium ion secondary battery 10 connected to the load 20, and drives and controls the load 20 based on predetermined information. The load 20 connected to the lithium ion secondary battery 10 may include a power consumer (for example, a motor) that consumes the power stored in the battery 10. The load 20 may include a power supply device (charger) that supplies power that can charge the battery 10.

  The lithium ion secondary battery 10 includes a positive electrode and a negative electrode facing each other, and an electrolyte containing lithium ions supplied between the positive and negative electrodes. The positive electrode and the negative electrode contain an active material capable of inserting and extracting lithium ions. In the lithium ion secondary battery 10 disclosed herein, graphite is used as the negative electrode active material. When the secondary battery 10 is charged, lithium ions are released from the positive electrode active material, and the lithium ions enter and occlude between graphite (negative electrode active material) provided on the negative electrode side through the electrolyte. At that time, as the amount of lithium held between the graphite layers increases, the graphite intercalation stage sequentially shifts to the fourth stage, the third stage, the second stage, and the first stage. Here, the third stage is a stage in which lithium is inserted between every three layers of graphite (typically a stage in which the amount of lithium held between graphite layers is one lithium atom per 18 carbon atoms). The second stage is a stage in which lithium is inserted between every two layers of graphite (typically a stage in which the amount of lithium held between graphite layers is one lithium atom per 12 carbon atoms) The first stage is a stage in which lithium is inserted between all the graphite layers (typically a stage in which the amount of lithium held between the graphite layers is one lithium atom per six carbon atoms). is there. On the contrary, when the secondary battery 10 is discharged, lithium ions stored between the layers of graphite (negative electrode active material) are released, and the lithium ions are again stored in the positive electrode active material through the electrolyte. As the lithium ions move between the positive electrode active material and the negative electrode active material, electrons flow from the active material to the external terminal. Thereby, the load 20 is discharged.

  The battery system 1 includes a temperature sensor 40, a current sensor 50, and a voltage sensor 60. The temperature sensor 40 is configured to detect the temperature of the lithium ion secondary battery 10 and output the detection result to the ECU 30. The current sensor 50 is configured to detect a current value that enters and exits the lithium ion secondary battery 10 and outputs the detection result to the ECU 30. The voltage sensor 60 is configured to detect a closed circuit voltage (CCV) of the lithium ion secondary battery 10 and output the detection result to the ECU 30.

In the battery system 1 disclosed herein, the ECU 30 performs the following processes A to D:
(Process A) Estimating the internal resistance value corresponding to the temperature detected by the temperature sensor using the first map showing the correspondence between the temperature of the lithium ion secondary battery and the internal resistance value;
(Process B) The overvoltage of the lithium ion secondary battery is calculated by multiplying the estimated internal resistance value and the current value detected by the current sensor;
(Process C) The open circuit voltage value of the lithium ion secondary battery is calculated by subtracting the calculated overvoltage from the closed circuit voltage value detected by the voltage sensor;
(Process D) Estimating the SOC corresponding to the calculated open circuit voltage value using the second map showing the correspondence between the open circuit voltage value and the SOC;
Is configured to run.
Further, the ECU 30 includes a plurality of first maps having different characteristics and a graphite stage determination unit that determines an intercalation stage of the graphite based on the SOC estimated in the processing D. And ECU30 is comprised so that the said 1st map used for the said process A may be switched based on the determined intercalation stage of graphite.

  The typical configuration of the ECU 30 stores at least a ROM (Read Only Memory) storing a program for performing such control, a CPU (Central Processing Unit) capable of executing the program, and temporarily stores data. A random access memory (RAM) and an input / output port (not shown) are included. Various signals from the temperature sensor 40, the current sensor 50, and the voltage sensor 60 are input to the ECU 30 through the input port. The ROM stores a plurality of preset first maps, second maps, and third maps. In this embodiment, the ECU 30 has a timer (not shown). The timer is used to measure the energization time of the lithium ion secondary battery. The ECU 30 constitutes the SOC estimation means according to this embodiment.

  The operation of the battery system 1 configured as described above will be described. FIG. 2 is a flowchart showing an example of an SOC estimation processing routine executed by the ECU 30 of the battery system 1 according to this embodiment. This processing routine is repeatedly executed every predetermined time when the secondary battery 10 is in an energized state (a charged state or a discharged state), and is not executed when the secondary battery 10 is in a stopped state (a state where charging / discharging is stopped).

  When the SOC estimation processing routine shown in FIG. 2 is executed, ECU 30 first detects the temperature of lithium ion secondary battery 10 based on the output from temperature sensor 40 in step S10. Further, the closed circuit voltage value (CCV) of the lithium ion secondary battery 10 is detected based on the output from the voltage sensor 60. Further, the current value flowing through the lithium ion secondary battery 10 is detected based on the output from the current sensor 50, and the energization time during which the current value does not change is measured using a timer.

  In step S20, the ECU 30 estimates an overvoltage. Here, the overvoltage corresponds to the difference between the closed circuit voltage value (CCV) and the open circuit voltage value (OCV) of the secondary battery 10. That is, when the secondary battery 10 is in an energized state (a charged state or a discharged state), the closed circuit voltage value (CCV) of the secondary battery 10 detected by the voltage sensor 60 due to the occurrence of polarization is the secondary battery 10. It will deviate from the open circuit voltage value (OCV). The difference of the deviation corresponds to the overvoltage here.

  Specifically, in step S22, the ECU 30 estimates the internal resistance value corresponding to the temperature detected by the temperature sensor 40 and the current value detected by the current sensor 50 using the first map (processing A). . In this embodiment, the first map is a map in which the temperature of the secondary battery 10, the current value flowing through the secondary battery 10, and the internal resistance value of the secondary battery 10 corresponding to the temperature and current value are associated with each other. is there. Further, when the overvoltage estimation mode is a normal mode described later, the first map includes a map used when the current energized state is a charged state and a map used when the current energized state is a discharged state. And can be used properly. That is, when two first maps having different characteristics belonging to the normal mode, which will be described later, are stored in the ROM, and the current energized state is the charged state, the first map corresponding to the charged state is used, and the current energized state is used. When the state is the discharge state, the first map corresponding to the discharge state is used. The current energized state (charged state or discharged state) can be determined by confirming the current value (typically, the sign of the current value) detected by the current sensor 50. Each of the first maps may be created by conducting a preliminary experiment or the like in advance.

  In the example described above, the case where the first map in which the current value and temperature are associated with the internal resistance value is used as an example. However, the internal resistance value may change depending on the energization time. For this reason, as the first map, a map in which the current value, the temperature, the energization time, and the internal resistance value are associated may be used. By using this map, the internal resistance value can be specified with higher accuracy based on the current value, temperature, and energization time. Further, the internal resistance value may be specified (estimated) based on at least one parameter (for example, temperature) among the current value, temperature, and energization time parameters.

  In step S24, the ECU 30 calculates the overvoltage of the secondary battery 10 by multiplying the internal resistance value estimated in step S20 by the current value detected by the current sensor 50 (process B). In this way, the overvoltage can be estimated from the temperature detected by the temperature sensor 40, the current value detected by the current sensor 50, and the energization time measured by the timer as necessary.

  In step S30, the ECU 30 estimates the SOC of the secondary battery 10 from the overvoltage estimated in step S20. Specifically, in step S32, the ECU 30 subtracts the overvoltage estimated in step S20 from the closed circuit voltage value (CCV) detected by the voltage sensor 60, so that the open circuit voltage value ( OCV) is calculated (process C).

  In step S34, the ECU 30 estimates the SOC corresponding to the open circuit voltage value (OCV) calculated in step S40 using the second map indicating the correspondence relationship between the open circuit voltage value and the SOC (process D). . Here, since the SOC and the OCV have a 1: 1 correspondence relationship, data indicating the correspondence relationship is stored in the ROM in the form of the second map, and the OCV is supported by referring to the second map. The SOC can be specified. The second map can be created by conducting a preliminary experiment or the like in advance.

  In step S40, the ECU 30 estimates a graphite intercalation stage based on the SOC estimated in step S30. That is, when graphite is used as the negative electrode active material, when the secondary battery 10 is charged, lithium ions are released from the positive electrode active material, and the lithium ions enter and intercalate between the graphite layers provided on the negative electrode side through the electrolyte. The At this time, as the amount of lithium held between the graphite layers increases, the graphite intercalation stage changes to the fourth stage, the third stage, the second stage, and the first stage. This graphite intercalation stage has a 1: 1 correspondence with the SOC of the secondary battery 10. Therefore, the data indicating the correspondence can be stored in the ROM in the form of a third map, and the graphite intercalation stage corresponding to the SOC can be estimated by referring to the third map. The third map can be created by conducting a preliminary experiment or the like in advance.

  Here, according to the knowledge of the present inventor, when the graphite intercalation stage is the second stage, or when it is the first stage and the current energized state is the charged state, the secondary battery 10 is charged. The internal resistance value of the secondary battery 10 may change according to the past history (previous energization state) when it is discharged. Therefore, the internal resistance value of the secondary battery 10 and the secondary battery 10 can be determined using only the first map (first map in the normal mode to be described later) corresponding to the current energization state (charge state or discharge state) of the secondary battery 10 described above. If the overvoltage is estimated, an accurate internal resistance value and overvoltage may not be obtained.

  Therefore, in step S50, the ECU 30 determines whether or not the graphite intercalation stage (estimated stage) estimated in step S40 is the second stage. If the estimation stage is the second stage (in the case of YES), the ECU 30 executes step S70. If the estimated stage is not the second stage (NO), in step S60, the ECU 30 determines whether the estimated stage is the first stage and the current energized state is the charged state. When the estimation stage is the first stage and the current energized state is the charged state (in the case of YES), the ECU 30 executes Step S70. On the other hand, when the estimated stage is the third stage, the fourth stage, or the first stage, but the current energized state is the discharged state (in the case of NO), the ECU 30 executes Step S80. The current energized state (charged state or discharged state) can be determined by confirming whether the current value detected by the current sensor 50 is positive or negative.

  In step S80, the ECU 30 sets the normal mode as the overvoltage estimation mode. The normal mode is a mode for estimating the overvoltage of the secondary battery 10 based on the current energized state (charged state or discharged state) of the secondary battery 10 as described in step S20 described above. Specifically, in the normal mode, the internal resistance value of the secondary battery 10 is specified from the temperature and current value of the secondary battery 10 acquired in step S10 by using the first map corresponding to the current energization state. Then, the overvoltage of the secondary battery 10 is calculated by multiplying the internal resistance value and the current value. When the overvoltage estimation mode is set to the normal mode, the first map used in the process A of step S22 is switched to the first map in the normal mode (that is, the first map corresponding to the current energization state). When the overvoltage estimation mode is already the normal mode in step S20 described above, the setting of the normal mode can be omitted.

  On the other hand, in step S70, the ECU 30 sets the history mode as an overvoltage estimation method. The history mode is a mode for estimating the overvoltage of the secondary battery 10 by a method different from the normal mode. Specifically, in the history mode, the temperature of the secondary battery 10 acquired in step S10 is obtained by using the first map according to the energized state (charged state or discharged state) of the secondary battery 10 at the present time and immediately before. The overvoltage of the secondary battery 10 is calculated by specifying the internal resistance value of the secondary battery 10 from the current value and multiplying the internal resistance value and the current value. Here, the immediately preceding energized state is a past energized state, and means an immediately energized state with respect to the present. For example, when the process of step S10 is performed in a predetermined cycle, the immediately preceding energized state is an energized state specified in the immediately preceding cycle with respect to the current time. In addition, if the current energization state is an energization state immediately after charging / discharging of the secondary battery 10, the energization state immediately before becomes the energization state immediately before stopping charging / discharging of the secondary battery 10. When the overvoltage estimation mode is set to the history mode, the first map used in the process A of step S22 is switched to the first map of the history mode (that is, the first map corresponding to the current and immediately energized state). When the overvoltage estimation mode is already the history mode in step S20 described above, the setting of the history mode can be omitted.

  The first map in the history mode includes a map used when the immediately preceding energized state is the charged state and the current energized state is the charged state, and the immediately preceding energized state is the charged state and the current energized state is A map used when the battery is in a discharged state, a map used when the previous energized state is a discharged state and the current energized state is a charged state, and a map used when the immediately previous energized state is a discharged state and the current energized The map used when the state is the discharge state is properly used. That is, four first maps having different characteristics belonging to the history mode are stored in the ROM, and the first map corresponding to the current and immediately preceding energized states is used as appropriate. Each of the first maps in the history mode is preferably created by conducting a preliminary experiment or the like in advance.

  In step S90, the ECU 30 determines the final SOC. That is, when the first map used in the process A in step S20 is the first map in the normal mode and the normal mode is maintained in step S80, the SOC estimated in step S30 described above is determined as the final SOC. . In addition, when the first map used in the process A of step S20 is the first map in the normal mode and the normal mode is changed to the history mode in step S70, the overvoltage is again set using the first map in the history mode. The SOC is estimated from this overvoltage, and this is set as the final SOC. The estimation of the SOC can be performed in the same procedure as the above-described steps S10 to S30.

  On the other hand, when the first map used in process A of step S20 is the first map in the history mode and the history mode is maintained in step S70, the SOC estimated in step S30 described above is determined as the final SOC. . In addition, when the first map used in the process A in step S20 is the first map in the history mode and the history mode is changed to the normal mode in step S80, the overvoltage is again detected using the first map in the normal mode. The SOC is estimated from this overvoltage, and this is set as the final SOC. The estimation of the SOC can be performed by the same procedure as that in steps S10 to S30 described above.

  According to the above embodiment, when the graphite intercalation stage is the second stage or the first stage and the current energized state is the charged state (that is, the past when the secondary battery was charged / discharged). When the internal resistance value of the secondary battery 10 changes according to the history), the overvoltage and the final SOC are estimated based on the first map of the history mode according to the current and immediately preceding energized state, while the intercalation of graphite When the stage is the third stage or the fourth stage, or when the stage is the first stage and the current energization state is the discharge state (that is, the internal resistance value of the secondary battery 10 changes according to the current energization state) Case), since the overvoltage and the final SOC are estimated based on the first map in the normal mode according to the current energization state, the SOC estimation accuracy is conventionally improved. Compared can be improved.

  Hereinafter, although the test example regarding this invention is demonstrated, it is not intending to limit this invention to what is shown to the following test examples.

<Test Example 1>
As the positive electrode active material, a spinel structure lithium transition metal complex oxide (LiNi _0.5 Mn _1.5 O ₄ ) was used. This positive electrode active material is obtained by dissolving a precursor metal sulfate other than Li in a predetermined amount of water, obtaining a precursor while neutralizing with NaOH, mixing this with a predetermined amount of lithium carbonate, and firing at 900 ° C. for 15 hours. , Obtained by pulverization. Such a positive electrode active material, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder are N-methyl such that the mass ratio of these materials is 90: 5: 5. A slurry-like composition for forming a positive electrode active material layer was prepared by mixing with -2-pyrrolidone (NMP). This composition was applied to an aluminum foil and dried. After drying, roll pressing was performed to produce a positive electrode in which a positive electrode active material layer was provided on the positive electrode current collector.

  Natural graphite powder (average particle size 20 μm) was used as the negative electrode active material. Using such negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a dispersion material, water is used so that the mass ratio of these materials becomes 98: 1: 1. A slurry-like composition for forming a negative electrode active material layer was prepared by mixing. This composition was applied to a copper foil and dried. After drying, roll pressing was performed to prepare a negative electrode in which a negative electrode active material layer was provided on the negative electrode current collector.

The prepared positive electrode and negative electrode were laminated so that the active material layers of both electrodes faced each other with a separator interposed therebetween to prepare an electrode body. Subsequently, this electrode body was accommodated in a laminated bag-shaped battery container together with a non-aqueous electrolyte and sealed to construct a battery assembly.
As the non-aqueous electrolyte, a mixed solvent containing monofluoroethylene carbonate (MFEC) and fluoromethyldifluoromethyl carbonate (F-DMC) at a volume ratio of 1: 1, and LiPF ₆ as a supporting salt is about 1 mol / liter. What was contained in the density | concentration of was used.

  The battery assembly constructed above was charged at a constant current of 0.2 C under a temperature environment of 25 ° C. until the terminal voltage between the positive and negative electrodes reached 4.9 V (upper limit voltage), and then the current value was 0.02 C. The battery was charged at a constant voltage until it became a fully charged state. Thereafter, the battery was discharged at a constant current of 0.2 C until the terminal voltage between the positive and negative electrodes reached 3.5 V (lower limit voltage) (initial charge / discharge process), and the discharge capacity at this time was defined as the initial capacity. In this way, a lithium ion secondary battery was obtained.

  The lithium ion secondary battery was charged with a constant current of 0.2 C for 15 minutes, and the resistance A was determined from the voltage change and the current value after 1 second from the end of charging. Moreover, the resistance B was calculated | required from the voltage change after 5 second progress from 1 second after charge rest. The resistance A and the resistance B were measured on a lithium ion secondary battery adjusted every 5% between SOC 0% and 100%. The results are shown in FIG. FIG. 3 is a graph showing the relationship between the SOC and the resistors A and B.

  In addition, the lithium ion secondary battery was discharged at a constant current of 0.2 C for 15 minutes, and the resistance A was determined from the voltage change and the current value after 1 second from the end of the discharge. Moreover, the resistance B was calculated | required from the voltage change after 5 second progress from 1 second after discharge rest. The resistance A and the resistance B were measured on a lithium ion secondary battery adjusted every 5% between SOC 0% and 100%. The results are shown in FIG. FIG. 4 is a graph showing the relationship between the SOC and the resistors A and B.

  As shown in FIGS. 3 and 4, it can be seen that the resistance B is greater than the resistance A when the energized state is the charged state or the discharged state. Here, the resistance A is a voltage drop (overvoltage) due to the resistance in the battery, whereas the resistance B is considered to be a resistance caused by diffusion in the electrode. In the region where the resistance B is significantly larger than the resistance A, the resistance due to diffusion in the electrode tends to become obvious, and the past history when the secondary battery is charged / discharged (typically the energized state immediately before) is typical. It suggests that it is easily affected. In this test result, the resistance B is significantly larger than the resistance A when the graphite is in the second stage and the energized state is in the charged and discharged state, and the graphite is in the first stage and the energized state is in the charged state. At the time. That is, when the graphite is in the second stage and the energized state is the charged state and the discharged state, and when the graphite is the first stage and the energized state is the charged state, the past history when the secondary battery is charged and discharged. The estimation accuracy can be improved by specifying the internal resistance value according to the above and using it for the estimation of the SOC.

<Test Example 2>
In this example, a layered structure lithium transition metal composite oxide (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) was used as the positive electrode active material instead of the spinel structure lithium transition metal composite oxide, electrolyte solution Ethylene carbonate (EC) instead of MFEC in the inside, dimethyl carbonate (DMC) instead of F-DMC was used, and the upper limit voltage in the initial charge / discharge process was changed to 4.2 V, and the lower limit voltage was changed to 3.0 V A lithium ion secondary battery was constructed in the same manner as in Test Example 1 except that. Then, the resistance A and the resistance B were measured in the same procedure as in Test Example 1. The results are shown in FIG. 5 (charged state) and FIG. 6 (discharged state).

  As shown in FIG. 5 and FIG. 6, even when the layered structure lithium transition metal composite oxide is used as the positive electrode active material, the region where the resistance B is remarkably larger than the resistance A is the second stage in which graphite is energized. The state was a charged state and a discharged state, and the graphite was in the first stage and the energized state was a charged state. That is, it is estimated that the magnitude tendency of the resistance A and the resistance B contributes to the graphite stage structure regardless of the type of the positive electrode active material.

  As mentioned above, although the specific example of this invention was demonstrated in detail, these are only illustrations and do not limit a claim. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

DESCRIPTION OF SYMBOLS 1 Battery system 10 Lithium ion secondary battery 20 Load 30 ECU
40 temperature sensor 50 current sensor 60 voltage sensor

Claims (2)

A lithium ion secondary battery using graphite as a negative electrode active material;
A temperature sensor for detecting a temperature of the lithium ion secondary battery;
A current sensor for detecting a current value entering and exiting the lithium ion secondary battery;
A voltage sensor for detecting a closed circuit voltage value of the lithium ion secondary battery;
SOC estimation means for estimating the SOC of the lithium ion secondary battery,
The SOC estimation means performs the following processes A to D:
(Process A) Estimating the internal resistance value corresponding to the temperature detected by the temperature sensor using the first map showing the correspondence between the temperature of the lithium ion secondary battery and the internal resistance value;
(Process B) The overvoltage of the lithium ion secondary battery is calculated by multiplying the estimated internal resistance value and the current value detected by the current sensor;
(Process C) The open circuit voltage value of the lithium ion secondary battery is calculated by subtracting the calculated overvoltage from the closed circuit voltage value detected by the voltage sensor;
(Process D) Estimating the SOC corresponding to the calculated open circuit voltage value using the second map showing the correspondence between the open circuit voltage value and the SOC;
A battery system configured to perform
The SOC estimation means includes a first map in the normal mode, which is the first map according to the current energization state, and an immediately preceding energization state specified in the cycle of the processes A to D immediately before the present. A plurality of the first maps including a first map in a history mode which is the first map according to an energized state and a current energized state ;
A graphite stage determination unit that determines an intercalation stage of the graphite based on the SOC estimated in the processing D;
Based on the intercalation stage of the determined graphite, the first map used for the processing A, and switches between a first map of the first map and the history mode of the normal mode , Battery system. In the SOC estimation means, the intercalation stage is a first stage in which lithium is inserted between all layers of graphite, and the current energization state is a charged state, and the intercalation stage is graphite. 2. The battery system according to claim 1, wherein, in the second stage in which lithium is inserted between every two layers, the first map is switched to the first map in the history mode.

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