Battery management device, battery system, and vehicle power supply system for managing battery state of charge level when in non-use state

1. A battery management device comprising:

a state estimator that estimates, based on at least one of a voltage, a current, and a temperature of a secondary battery, a state of the secondary battery including a state of charge (SOC) of the secondary battery; and

a discharge controller that discharges, when a state of non-use of the secondary battery lasts for a predetermined period of time, the secondary battery such that an indicator indicating easiness of storage deterioration of the secondary battery to progress decreases in stages, wherein

the indicator includes a parameter that depends on the SOC of the secondary battery, and

a value of the indicator and a duration of stay in each stage are set such that a predetermined relationship holds between products in a plurality of the stages, each of the products being derived from multiplying the value of the indicator and the duration of stay in each stage.

2. The battery management device according to claim 1 , wherein a decrement of the indicator in a transition between the stages is configured to be constant or to be progressively smaller through the stages.

3. The battery management device according to claim 1 , wherein the value of the indicator and the duration of stay in each stage are set such that the product is progressively larger through the stages.

4. The battery management device according to claim 1 , wherein the value of the indicator and the duration of stay in each stage are set such that the products in the plurality of the stages are substantially equal to each other.

5. The battery management device according to claim 1 , wherein a proportion between the products in the plurality of the stages is set based on a probability density function of discharged capacity discharged after long-term non-use, the probability density function being created by sampling a plurality of items of data for capacity discharged by using the secondary battery after the state of non-use lasts for the predetermined period of time.

6. The battery management device according to claim 1 , wherein the value of the indicator in each stage is set based on a cumulative distribution function of discharged capacity discharged after long-term non-use such that the cumulative distribution function has substantially equally spaced intervals, the cumulative distribution function being created by sampling a plurality of items of data for capacity discharged by using the secondary battery after the state of non-use lasts for the predetermined period of time.

7. The battery management device according to claim 1 , wherein the duration of stay in each stage is configured to be progressively shorter through the stages.

8. The battery management device according to claim 1 , wherein the value of the indicator and the duration of stay in each stage are set such that the product is progressively larger through the stages.

9. The battery management device according to claim 1 , wherein a lower limit value of the indicator in a discharging process initiated by the discharge controller when the state of non-use of the secondary battery lasts for the predetermined period of time is set based on a usage record of the secondary battery discharged after the state of non-use lasts for the predetermined period of time.

10. The battery management device according to claim 1 , wherein a lower limit value of the indicator in a discharging process initiated by the discharge controller when the state of non-use of the secondary battery lasts for the predetermined period of time is adjusted in accordance with a parameter that depends on an environment and/or a parameter that depends on a degree of deterioration of the secondary battery.

11. The battery management device according to claim 1 , wherein the indicator includes a parameter that depends on an environment and/or a parameter that depends on a degree of deterioration of the secondary battery, in addition to the parameter that depends on the SOC.

12. A battery system comprising:

a secondary battery; and

the battery management device according to claim 1 that manages the secondary battery.

13. The battery system according to claim 12 , wherein

the secondary battery includes a plurality of cells connected in series,

the battery system further comprises a plurality of discharge resistances for equalization respectively connected in parallel with the plurality of cells, and

the discharge controller discharges the secondary battery when the state of non-use of the secondary battery lasts for the predetermined period of time, by discharging a capacity of the secondary battery to the plurality of discharge resistances.

14. The battery system according to claim 12 , wherein

the battery system further comprises a fan connected in parallel with the secondary battery to cool the secondary battery, and

the discharge controller discharges the secondary battery when the state of non-use lasts for the predetermined period of time, by discharging a capacity of the secondary battery to the fan.

15. The battery system according to claim 12 , wherein

the battery system further comprises a battery for buffering connected in parallel with the secondary battery, and

the discharge controller discharges the secondary battery when the state of non-use lasts for the predetermined period of time, by discharging a capacity of the secondary battery to the battery for buffering.

16. A power supply system for vehicles, comprising:

the battery system according to claim 12 ;

an inverter that, during powering, converts a DC power supplied from the secondary battery into an AC power and supplies the AC power to a motor in the vehicle, and, during regeneration, converts an AC power supplied from the motor into a DC power and supplies the DC power to the secondary battery; and

a switch connected between the battery system and the inverter.