What do SOC and SOH mean for lithium batteries.

What do SOC and SOH mean for lithium batteries.

SOH refers to the capacity, health, and performance status of a battery. Simply put, it is the ratio of performance parameters to nominal parameters after a period of use. For newly manufactured batteries, it is 100%, and for completely scrapped batteries, it is 0%. It is the ratio of the capacity released by a battery when discharged at a certain rate from a fully charged state to the cut-off voltage to its corresponding nominal capacity, which can be simply understood as the maximum capacity of the battery. There is a certain relationship between the internal resistance of batteries and SOH. The lower the SOH, the greater the internal resistance of the lithium battery. By detecting data such as voltage, current, and temperature, the internal resistance value of the battery can be indirectly calculated, and then the SOH can be calculated based on the relationship between SOH and battery internal resistance. However, the internal resistance of the battery does not change significantly when the SOH variation range is not large, while the resistance value changes significantly when the battery ages severely. Therefore, this method has a larger measurement error when the SOH variation is small.

The influence of SOC state on lithium batteries
1. The cycle life of lithium batteries follows the expected trend of SOC variation, with a significant increase in cycle life as SOC decreases.
2. SOC is the most important parameter in BMS, involving all other operational tasks of BMS, so the accuracy and robustness of SOC are extremely important. If there is no precise SOC, BMS cannot function properly, and the battery will often be in a protected state, resulting in a shortened battery life. The higher the accuracy, the greater the range of a battery with the same capacity.
3. During the thermal runaway process of lithium batteries, a considerable amount of heat comes from the stored electrical energy. Usually, when the SOC state is high, it can also lead to an increase in the reaction activity of the positive and negative electrode materials and a decrease in thermal stability. Therefore, the SOC state of lithium batteries has a crucial impact on the safety of lithium batteries under abuse conditions.
4. Different temperatures and SOC have varying effects on the storage of lithium batteries. 55 ℃ has the greatest impact on the capacity degradation of lithium batteries, while 0% and 100% SOC are not the optimal states of charge for optimal battery performance. The optimal storage conditions for lithium iron phosphate batteries were investigated by considering various factors.
The above is the definition of SOC and SOH of lithium batteries, as well as the impact of SOC state on lithium batteries. The aging of lithium batteries is a long-term gradual process, and the health status of batteries is affected by various factors such as temperature, current rate, and cut-off voltage. The assessment of battery health status has guiding significance for the use, maintenance, and economic analysis of batteries, but there is a lack of targeted organization and summary of battery health status assessment