Method of estimating deteriorated state of secondary battery and secondary battery system
A method of estimating a deteriorated state of a battery mounted on a vehicle includes first to third steps. The first step is a step of obtaining a voltage value and a current value of the battery a plurality of times during a data acquisition period and storing the values in a memory. The second step is a step of subjecting the voltage value and the current value during the data acquisition period stored in the memory to Fourier transform and calculating an impedance component for each frequency band based on the voltage value and the current value subjected to Fourier transform. The third step is a step of estimating high-rate deterioration of the battery by comparing a ratio E between a medium-frequency impedance ZM and a low-frequency impedance ZL 2 with a reference value K.
1. A method of estimating a deteriorated state of a secondary battery performed by a controller mounted on a vehicle, the method comprising:
obtaining a plurality of times, a voltage, a current, and a temperature of the secondary battery during a prescribed period and storing the voltage, the current, and the temperature in a memory;
calculating an amount of change in current of the secondary battery, an amount of change in temperature of the secondary battery, and an amount of change in SOC of the secondary battery during the prescribed period;
obtaining from the memory, an allowable amount of change in current, an allowable amount of change in temperature, and an allowable amount of change in SOC, which are determined in advance for each temperature, each current, or each SOC of the secondary battery during the prescribed period and stored in the memory, based on the temperature, the current, or the SOC of the secondary battery, the allowable amount of change in current representing an allowable upper limit of the amount of change in current, the allowable amount of change in temperature representing an allowable upper limit of the amount of change in temperature, the allowable amount of change in SOC representing an allowable upper limit of the amount of change in SOC;
carrying out frequency conversion of the voltage and the current during the prescribed period of the secondary battery stored in the memory and calculating an impedance for each frequency band of the secondary battery based on the frequency-converted voltage and current when all of a current condition, a temperature condition, and an SOC condition are satisfied, the current condition being a condition that the amount of change in current is smaller than the allowable amount of change in current, the temperature condition being a condition that the amount of change in temperature is smaller than the allowable amount of change in temperature, the SOC condition being a condition that the amount of change in SOC is smaller than the allowable amount of change in SOC; and
estimating whether the secondary battery is in a high-rate deterioration state by using a ratio between an impedance corresponding to a prescribed frequency band and an impedance corresponding to another frequency band higher than the prescribed frequency band and a reference value,
the prescribed frequency band being a frequency band in which at least a direct-current (DC) resistance and a reaction resistance mainly contribute.
2. The method according to claim 1 , wherein
the prescribed frequency band is a frequency band in which the DC resistance, the reaction resistance, and a diffusion resistance mainly contribute, and
the another frequency band is a frequency band in which the DC resistance and the reaction resistance mainly contribute and the diffusion resistance does not mainly contribute.
3. The method according to claim 2 , wherein
the prescribed frequency band includes a first frequency band and a second frequency band lower than the first frequency band,
the second frequency band exhibits such a characteristic that an impedance in the high-rate deterioration state of the secondary battery is lower than an impedance in an initial state of the secondary battery, and
when a ratio of the impedance corresponding to the another frequency band to an impedance corresponding to the second frequency band is higher than the reference value, the secondary battery is estimated to be in the high-rate deterioration state.
4. The method according to claim 3 , further comprising restricting input to and output from the secondary battery or prohibiting use of the secondary battery, when the secondary battery is estimated to be in the high-rate deterioration state.
5. A secondary battery system mounted on a vehicle, the secondary battery system comprising:
a secondary battery; and
a controller including a memory and configured to estimate a deteriorated state of the secondary battery,
the controller being configured to
obtain a plurality of times, a voltage, a current, and a temperature of the secondary battery during a prescribed period and store the voltage, the current, and the temperature in the memory,
calculate an amount of change in current of the secondary battery, an amount of change in temperature of the secondary battery, and an amount of change in SOC of the secondary battery during the prescribed period,
obtain from the memory, an allowable amount of change in current, an allowable amount of change in temperature, and an allowable amount of change in SOC, which are determined in advance for each temperature, each current, or each SOC of the secondary battery during the prescribed period and stored in the memory, based on the temperature, the current, or the SOC of the secondary battery, the allowable amount of change in current representing an allowable upper limit of the amount of change in current, the allowable amount of change in temperature representing an allowable upper limit of the amount of change in temperature, the allowable amount of change in SOC representing an allowable upper limit of the amount of change in SOC,
carrying out frequency conversion of the voltage and the current during the prescribed period of the secondary battery stored in the memory and calculating an impedance for each frequency band of the secondary battery based on the frequency-converted voltage and current when all of a current condition, a temperature condition, and an SOC condition are satisfied, the current condition being a condition that the amount of change in current is smaller than the allowable amount of change in current, the temperature condition being a condition that the amount of change in temperature is smaller than the allowable amount of change in temperature, the SOC condition being a condition that the amount of change in SOC is smaller than the allowable amount of change in SOC, and
estimate whether the secondary battery is in a high-rate deterioration state by using a ratio between an impedance corresponding to a prescribed frequency band and an impedance corresponding to another frequency band higher than the prescribed frequency band and a reference value,
the prescribed frequency band being a frequency band in which at least a DC resistance and a reaction resistance mainly contribute.