OCV is very important for SOC estimation. It is a function of cell SOC and temperature. There are two methods to measure the OCV, namely coulomb titration and voltage relaxation. The former discharges and charges the cell with the same low current while measuring the terminal voltage. The OCV is defined as the average value of the charge and the discharge terminal voltage. The latter determines the SOC-OCV curve by resting the battery for a suitable period after charging or discharging under specific SOC intervals.

Figure 1. OCV by voltage relaxation and terminal voltage by 1/25 C charge and discharge

Figure 1 shows the OCV measured by voltage relaxation and the battery terminal voltage by discharging and charging the battery with 1/25 C rate (OCV is got by discharge voltage relaxation. For this battery, there are little differences between OCV by discharge voltage relaxation and charge voltage relaxation.). It can be seen that discharging and charging terminal voltage is not well symmetrical about the OCV, especially in low the SOC region. Therefore, OCV measured by voltage relaxation is chosen. The measure step of the OCV is as follows. First, the cell is charged with 0.8 A (about 1/3 C) until the voltage reaches 4.12 V at the room temperature, by a constant voltage charge until the current reaches 1/30 C (SOC is regarded as 1).

After that, the thermal chamber is set to the tested temperature (0, 12.5, 25, or 45 °C), resting the cell for 2 h and measuring the OCV under SOC 1. Then, the cell is discharged with 0.8 A until the discharged ampere-hour reaches 0.2 Ah, measuring the cell OCV after it gets stable (in this paper when dV/dt is less than 1 mV/h, OCV is considered to be stable). Repeat this step to get the OCVs under other SOCs. At the last test point, the terminal voltage reaches 2.5 V and then measure the stable OCV.