JP2012194053A - Method for determining state of secondary battery and secondary battery system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique for improving accuracy of determining a battery state.SOLUTION: A state determination device 10 determines the state of a secondary battery 12 that is charged in the charging manner switched from constant current charging to constant voltage charging. For state determination device 10, a first current value Is smaller than a reference current value Ik during constant current charging is set while a second current value Ie smaller than the first current value Is is set. The state of secondary battery 12 is determined based on the determination time during which the current flowing through the secondary battery 12 during constant voltage charging is decreased from the first current value Is to the second current value Ie. The clocked determination time shows a significant difference depending on the battery state, so that the battery state can be determined with accuracy.

Description

  The present invention relates to a technique for determining a state of a battery such as deterioration.

  Conventionally, secondary batteries that can be used repeatedly have been used. Secondary batteries can be used many times by repeating charging and discharging, are more environmentally friendly than non-chargeable batteries, and are currently expanding their fields of use such as electric vehicles.

  In such a battery, the internal resistance increases due to deterioration as the number of uses increases. When the internal resistance increases, the performance required for the battery such as maximum voltage and maximum power cannot be realized. Therefore, in order to grasp whether or not the battery is in a state where the required performance can be realized, conventionally, a technique for determining the state of the battery such as deterioration is known (for example, Patent Document 1 and Patent Document 2). . This technology determines the state of the battery based on the charging time of the battery. Specifically, in a constant current-constant voltage charging method in which constant voltage charging is performed after the battery is charged with constant current, a constant voltage is used. The state of the battery is determined based on the charging time of charging. According to this technique, the state of the battery can be determined using the charging time measured when the battery is charged, and the state of the battery can be accurately determined.

JP 2001-289924 A JP 2007-78506 A

  In order to improve the determination accuracy of the state of the battery, the present inventors have scrutinized various behaviors such as charging voltage and charging current to the battery during the charging period of constant voltage charging. It has been found that there is still room for improvement in determination accuracy in the state determination.

  An object of the present invention is to provide a technique for improving the accuracy of battery state determination.

  A detailed examination of the behavior of the charging current during the charging period of constant voltage charging shows that, in the initial stage of constant voltage charging, the change over time of the charging current is almost constant regardless of the difference in the state of the battery, It was found that the difference in the change in the charging current with the passage of time due to the difference in the state of the battery became remarkable in the latter half of the constant voltage charging. Based on this finding, the present inventors have created the following state determination method and the like.

  The present invention relates to a state determination method for a secondary battery that is charged by switching from constant current charging to constant voltage charging, the first current value being smaller than the current value during constant current charging, and the first current value. A smaller second current value is determined, and the state of the secondary battery is determined based on a time period during which the first current value decreases to the second current value during constant voltage charging.

  In the above invention, it is preferable that the first current value is not more than half of the current value during the constant current charging. That is, it is preferable to start measuring the time for determining the state of the secondary battery after the current value at the time of constant voltage charging becomes half or less of the current value at the time of constant current charging.

  In this state determination method, when measuring the determination time for determining the state of the battery, the measurement start time is set when the current value that is smaller than the current value at the time of constant voltage charging is reached. Do not measure the charging time in the initial stage. By setting a determination time for determining the state of the battery in a region where a change with time of the charging current becomes significant due to a difference in the state of the battery, the state of the battery can be accurately determined. Furthermore, the current value at the start of measurement of the determination time is set to half or less of the current value at the time of constant current charging. That is, the battery state can be determined with higher accuracy by starting measurement of the determination time after the current value of constant voltage charging becomes half or less of the current value during constant current charging.

  The present invention is a secondary battery system including a secondary battery that is charged by switching from constant current charging to constant voltage charging, the first current value being smaller than the current value at the time of constant current charging, and the first A second current value smaller than the first current value is determined, and the state of the secondary battery is determined based on a time period during which the first current value decreases to the second current value during constant voltage charging It is embodied also in the secondary battery system which has.

  According to the present invention, the battery state determination accuracy can be improved.

The block diagram of the state determination apparatus of this embodiment The flowchart which shows the state determination process of this embodiment The flowchart which shows the state determination process of this embodiment The graph which shows the change of the electric current which flows into the secondary battery in state judgment processing A graph showing the relationship between the capacity retention rate and the determination time T

<Embodiment>
Hereinafter, embodiments of the present invention will be described with reference to FIGS. 1 to 5.
1. Configuration of State Determination Device FIG. 1 is a diagram illustrating a configuration of a battery system 10 in the present embodiment. The battery system 10 has a function of charging the secondary battery 12 by being connected to the secondary battery 12 and determining a state such as deterioration of the secondary battery 12. In the present embodiment, an example in which an iron-based lithium ion battery using lithium phosphate as a positive electrode is used as the secondary battery 12 will be described.

As shown in FIG. 1, the battery system 10 includes a battery management device (hereinafter referred to as “BMS”) 20, a charging unit 26, and a charging wiring 28, and the BMS 20 includes an ammeter 22 and a voltmeter 24. .
The charging unit 26 is connected to the external power supply 14, and supplies power supplied from the external power supply 14 to the secondary battery 12 through the charging wiring 28 and the connection terminal 16. In the present embodiment, the charging unit 26 is provided in the battery system 10, but the charging unit 26 may be provided outside the battery system 10 as a charger. The ammeter 22 measures the current flowing through the secondary battery 12 via the charging wiring 28 every predetermined period. The voltmeter 24 is connected to the charging wiring 28 and measures the voltage applied from the charging unit 26 to the secondary battery 12 every predetermined period.

  The BMS 20 includes a central processing unit (hereinafter referred to as “CPU”) 30, a memory 32 such as ROM and RAM, an analog-digital converter (hereinafter referred to as “ADC”) 34, and a bus 36 that connects these components to each other. .

  Various programs for controlling the operation of the battery system 10 are stored in the memory 32, and the CPU 30 functions as a timer unit 40, a control unit 42, a determination unit 44, etc. according to the program read from the memory 32. Control each part.

  The ADC 34 is connected to the ammeter 22 and the voltmeter 24. The ADC 34 converts the current value I and the voltage value V, which are analog data transmitted from the ammeter 22 and the voltmeter 24, into digital data, and converts the converted current value. I and the voltage value V are temporarily stored in the memory 32 via the bus 36. The CPU 30 functioning as the control unit 42 and the determination unit 44 controls the charging unit 26 and the like using the current value and the voltage value stored in the memory 32.

2. State Determination Processing State determination processing such as deterioration of the secondary battery 12 in the state determination device 10 will be described with reference to FIGS. 2 and 3 are flowcharts of the state determination process executed by the CPU 30. FIG. FIG. 4 shows changes in the current value when the secondary batteries 12 having different capacity deficiencies are charged. In FIG. 4, the region where the current value is constant is the constant current charging region, and the region where the current value decays with time is the constant voltage charging region. The charging method of constant current-constant voltage charging is initially charged by constant current charging, switched to constant voltage charging when the battery voltage of the secondary battery reaches a predetermined value, and thereafter the battery voltage is held at the predetermined value. This is a charging method in which the battery is charged by constant voltage charging.

  When the secondary battery 12 is connected by the user and a charging instruction is input via the operation unit or the like, the CPU 30 executes processing according to a program stored in the memory 32. When starting the process, the CPU 30 first starts constant current charging for the secondary battery 12 (S2). The memory 32 stores a reference current value Ik in advance, and the CPU 30 controls the charging unit 26 so that the current flowing through the secondary battery 12 in the constant current charging becomes the reference current value Ik. Here, when the current value for discharging the battery capacity from the charging depth 100% to the charging depth 0% of the secondary battery 12 in 1 hour is Ic, the reference current value Ik is 0.7Ic ≦ Ik ≦ 1.5Ic. It is preferably set.

  CPU30 monitors the voltage applied to the secondary battery 12 in constant current charge (S4: NO). A reference voltage value Vk is stored in the memory 32 in advance. When the voltage applied to the secondary battery 12 in constant current charging reaches the reference voltage value Vk (S4: YES), the CPU 30 operates as the control unit 42. Functions, ends constant current charging (S6), and starts constant voltage charging for the secondary battery 12 (S8).

  The CPU 30 controls the charging unit 26 so that the voltage applied to the secondary battery 12 in the constant voltage charging becomes the reference voltage value Vk. As a result, the current flowing through the secondary battery 12 that was at the reference current value Ik immediately after the start of constant voltage charging decreases as the charging proceeds as shown in FIG.

  Moreover, CPU30 monitors the electric current which flows into the secondary battery 12 in constant voltage charge (S10: NO). In the memory 32, a first current value Is and a second current value Ie are set in advance by a user or the like. The first current value Is is set smaller than the reference current value Ik, and the second current value Ie is set smaller than the first current value Is and larger than “0” A. ing. When the current flowing through the secondary battery 12 in the constant voltage charging becomes the first current value Is (S10: YES), the CPU 30 functions as the control unit 42 and starts measuring the determination time T (S12). At this time, the CPU 30 also functions as the timer unit 40.

  After starting measurement of the determination time T, the CPU 30 monitors the current flowing through the secondary battery 12 again (S14: NO), and when the current flowing through the secondary battery 12 becomes the second current value Ie (S14: YES), it functions as the control unit 42 and ends the measurement of the determination time T (S16).

  CPU30 monitors the electric current which flows into the secondary battery 12 again after completion | finish of the measurement of the determination time T (S18: NO). The memory 32 stores a termination current value Iz in advance. When the total time of the constant current charging time and the constant voltage charging time reaches a certain time, or when the current flowing through the secondary battery 12 reaches the termination current value Iz (S18: YES), the charging is terminated. (S20).

  Next, the CPU 30 determines the state of the secondary battery 12 based on the measured determination time T. At this time, the CPU 30 functions as the determination unit 44. The memory 32 stores the verification time Th in advance, and the CPU 30 compares the measured determination time T with the verification time Th (S22). When the determination time T is shorter than the collation time Th (S22: YES), the CPU 30 determines that the deterioration of the secondary battery 12 is small (S24) and ends the process. On the other hand, when the determination time T is equal to or longer than the collation time Th (S22: NO), it is determined that the secondary battery 12 is largely deteriorated (S26), and the user is notified via the display unit (S28). finish.

3. Effect of this embodiment In a 45 ° C. cycle test using a secondary battery having a battery capacity of 10 Ah, the accuracy of the battery state determination when the first current value Is and the second current value Ie are different was verified. Before the cycle test of the 45 ° C cycle test (number of cycles 0), 50 cycles, 150 cycles, 300 cycles, 500 cycles, 750 cycles, and 1000 cycles, the secondary battery was cooled to 25 ° C and then charged. I did it. Charging is performed at a constant current of 3.45 V at a current value of 10 A (corresponding to the above reference current value Ik) at 25 ° C. and then constant voltage charging at 3.45 V. The charging was terminated when the total time of constant current charging and constant voltage charging reached 3 hours. After the above charging was completed, the battery was discharged to 2.0 V at a current value of 10 A at 25 ° C. to determine the discharge capacity. When the ratio of the discharge capacity after each cycle to the discharge capacity before the cycle test is defined as the capacity retention rate, the capacity retention rate at the time when 50 cycles, 150 cycles, 300 cycles, 500 cycles, 750 cycles and 1000 cycles have elapsed, 96.1, 93.7, 92.0, 89.9, 87.9 and 85.7.

  Table 1 shows the first current value Is, the second current value Ie, and the R-square value of each condition set in this verification. FIG. 5 shows the relationship between the determination time T measured from the first current value Is and the second current value Ie and the capacity retention after each cycle. In FIG. 5, the determination time T measured with the same first current value Is and second current value Ie after the elapse of each cycle is connected by an approximate straight line. The R square value in Table 1 is a parameter indicating the amount of deviation between the approximate expression of the above approximate line and the determination time T, and the closer the R square value is to 1, the smaller the amount of deviation.

  The R square values of the third condition, the fourth condition and the fifth condition in Table 1 were 0.95 or more, and the coincidence between the measured determination time T and the approximate line was good. Since the coincidence between the determination time T and the approximate line is good, a highly accurate collation time Th can be set in advance, and the accuracy when the determination time T is collated with the collation time Th, that is, the determination time. The accuracy of the collation is improved whether T is shorter or longer than the collation time Th. On the other hand, in the first condition and the second condition, the amount of deviation between the judgment time T and the approximate line is large, and the result of collating the judgment time T with the collation time Th due to the large amount of deviation may be an erroneous judgment. Increases nature.

  As described above, by making the region not including the initial stage of constant current charging the target of the determination time T, the difference in the determination time T due to the difference in the battery state becomes significant, and the battery state can be determined more accurately. Can do. Furthermore, the first current value Is is set to a current value that is less than or equal to half of the reference current value Ik, that is, the measurement of the determination time T is started after the constant voltage charging current value reaches 0.5 Ik. Thus, the battery state determination accuracy can be further improved.

10: State determination device, 12: Secondary battery, 20: BMS, 22: Ammeter, 24: Voltmeter, 26: Charging unit, 30: CPU, 32: Memory, 34: ADS, 40: Timekeeping unit, 42: Control unit, 44: determination unit, Ik: reference current value, Vk: reference voltage value

Claims (3)

A method for determining the state of a secondary battery that is charged by switching from constant current charging to constant voltage charging,
A first current value smaller than a current value during constant current charging and a second current value smaller than the first current value are determined, and the second current value is determined from the first current value during constant voltage charging. A secondary battery state determination method for determining the state of the secondary battery based on a time period during which the current value decreases. A state determination method for a secondary battery according to claim 1,
The state determination method of a secondary battery, wherein the first current value is smaller than a half value of a current value during the constant current charging. A secondary battery system including a secondary battery that is charged by switching from constant current charging to constant voltage charging,
A first current value smaller than a current value during constant current charging and a second current value smaller than the first current value are determined, and the second current value is determined from the first current value during constant voltage charging. The secondary battery system which has a function which determines the state of the said secondary battery based on the time to reduce to an electric current value.

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