1. Capacity characteristics of lithium-ion batteries
The capacity test obtains the relationship between the discharge voltage and the capacity of the lithium-ion battery at different rates. The voltage of the lithium-ion battery during the entire discharge process can be divided into 3 stages:
1) The terminal voltage of lithium-ion battery drops rapidly in the initial stage, the greater the discharge rate, the faster the voltage drops;
2) The battery voltage enters a stage of slow change. This period is called the platform area of the battery. The smaller the discharge rate, the longer the platform area lasts. The higher the platform voltage, the slower the voltage drop. In the actual use of lithium-ion batteries, it is hoped that the battery will work in the platform area as much as possible;
3) When the battery is nearly discharged, the battery load voltage begins to drop sharply until it reaches the discharge cut-off voltage.
The discharge current of a lithium-ion battery will directly affect the actual capacity of the battery. The greater the discharge current, the corresponding decrease in battery capacity, which indicates that the greater the discharge current, the shorter the time to reach the termination voltage. So when talking about battery capacity, you should specify its discharge current (discharge rate).
2. Li-ion battery open circuit voltage characteristics
The relationship between the open circuit voltage of a lithium-ion battery and the battery SOC, the OCV-SOC curve of the battery is basically the same as the battery discharge voltage curve.
In the middle range of SOC (20%<SOC<80%), the OCV of the battery changes very little, and the battery is in the plateau area; while in the two ends of the SOC range (SOC<10% and SOC>90%), the OCV changes The OCV-SOC curve of the entire lithium iron phosphate battery is flat in the middle, and the head and tail ends are steep. The open circuit voltage method uses this stable correspondence to estimate the SOC.
The OCV-SOC relationship curve of lithium-ion battery is less affected by temperature, discharge rate, and aging degree. However, there will be a certain difference between the two characteristic curves in the two states of charge and discharge.
Third, the internal resistance characteristics of lithium-ion batteries
The characteristics of the ohmic internal resistance of lithium-ion batteries during charging and discharging: In a wider SOC range, that is, within the range of SOC=[10%, 100%], the ohmic internal resistance of the battery changes very little, while at a lower In the sOC range, the ohmic internal resistance increases significantly with the decrease of SOC. This is because the battery's internal chemical substance activity decreases at the end of the battery discharge; in the entire SOC range, the charging ohmic internal resistance is generally greater than the discharging ohmic internal resistance. This is because the discharge of lithium-ion batteries is a spontaneous reaction and is relatively easy; charging is caused by an external power source, which makes it difficult for lithium ions to be inserted into the negative electrode. It should be noted that the internal resistance of the battery changes very complicatedly and is affected by factors such as temperature, depth of discharge, charge and discharge rate, and cycle times. Different battery cells of the same type are also related to the inconsistency of the battery and the different working environment.
