Method and system for estimating state of charge of battery

1. A method for estimating a state of charge (SOC) of a secondary battery using a full charge capacity (FCC) of the secondary battery, the method comprising:

measuring a voltage and current of the secondary battery, including a voltage change and a current integral of the secondary battery, during a period of time in which secondary battery is discharged after being fully charged, wherein the measuring the voltage and current is performed in each of four state areas into which states of the secondary battery are divided based upon a specific voltage and a specific temperature;

calculating, by a battery management system (BMS), the FCC using the voltage change and the current integral of the secondary battery during the period of time then resetting the FCC when the secondary battery is fully charged, when the secondary battery has reached a lowest discharge level, or when the secondary battery has reached a lowest correction point;

estimating a time remaining until the battery is fully discharged using the calculated FCC and the measured voltage and current to calculate a self-discharge rate according to the following expression:

self-discharge rate=1−( Y 1* t _ y 1+ Y 2* t _ y 2+ Y 3* t _ y 3+ Y 4* t _ y 4)*(number of parallel cells/design capacity),

where Y1, Y2, Y3 and Y4 denote respective self-discharge rate values of the respective four state areas, t_y1, t_y2, t_y3 and t_y4 denote respective discharge times in the four state areas, and Y1, Y2, Y3 and Y4 are set with reference to the specific voltage and the specific temperature;

calculating, by the BMS, a capacity deterioration rate of each charge cycle of the secondary battery according to the following expression:

capacity deterioration rate=1−( C 1*cycle 1+ C 2*cycle 2+ C 3*cycle 3+ C 4*cycle 4)*(number of parallel cells/design capacity),

where C1, C2, C3 and C4 denote respective capacity deterioration rate values of four state areas, into which states of the secondary battery are divided based upon a specific period and a specific temperature, cycle 1, cycle 2, cycle 3 and cycle 4 denote the respective numbers of charge cycles in the four state areas, and C1, C2, C3 and C4 are set with reference to the specific period and the specific temperature;

correcting, by the BMS, the FCC to be the product of the calculated FCC, the self-discharge rate, and the capacity deterioration rate; and

controlling, by the BMS, charging and discharging of the secondary battery based on the SOC estimated by using the calculated self-discharge rate and the calculated capacity deterioration rate.

2. The method according to claim 1 , wherein the correcting of the FCC comprises correcting the FCC according to the following expression:

FCC (NEW)=(calculated FCC)*(cell self-discharge rate)*(capacity deterioration by cycle rate),

where cell self-discharge rate and capacity deterioration by cycle rate represent the self-discharge rate and the capacity deterioration rate, respectively, of the second battery.

3. The method according to claim 1 , wherein the calculating of the capacity deterioration rate comprises setting each of the capacity deterioration rate values of the four state areas after one of the four state areas in which the secondary battery is present is determined when the secondary battery has been charged to 85% or higher.

4. The method according to claim 1 , wherein the FCC is updated after the secondary battery is fully charged and is reset immediately after the secondary battery is fully charged, when the secondary battery is initially used, or when the secondary battery has reached a lowest correction point.

5. A secondary battery SOC estimation system for performing the method according to claim 1 .

6. The secondary battery comprising the SOC estimation system according to claim 5 .