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Factors Affecting SOC Estimation of Lithium Power Packs
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Factors Affecting SOC Estimation of Lithium Power Packs

The real-time SOC of the lithium-ion power battery pack is a variable that cannot be directly measured, and cannot be directly measured by sensor devices. It will be affected by various factors in the external environment during operation. Including temperature, discharge current, discharge rate, internal resistance, self-discharge rate, attenuation degree and so on.

1. The degree of attenuation directly affects and determines the estimated SOC value and usable range of the lithium power pack

Among the above factors, the most influential factor is the attenuation degree of the lithium battery packs. The attenuation degree is only an external manifestation, which is actually caused by the consistency problem. The SOC value of the battery pack depends on the unit cell with the smallest capacity, that is, the most severe attenuation in the battery pack, which is similar to the shortest wooden board in the "cask principle". Even if the capacity of other batteries does not decay, the capacity of the battery exceeds the part of the decayed battery capacity. It is also unusable, which not only affects the actual battery life, but also wastes valuable capacity.


Distribution diagram of the capacity of decaying lithium power pack

2. The impact of SOC estimation on lithium power pack

It can be seen from the schematic diagram that after the attenuation of the battery pack, the 7# battery has the smallest remaining capacity in the entire battery pack due to the most serious attenuation. Therefore, the SOC value of the battery pack depends on the 7# battery. Although the degree of attenuation of other batteries is relatively Small, the remaining capacity is much higher than the 7# battery, but due to the existence of the BMS, it is necessary to protect the 7# battery with the smallest capacity from over-discharge, and the capacity of other batteries will be ignored by the BMS even if it has more capacity, and it will not play any role. .

Therefore, the maximum value of the actual discharge capacity of the lithium power pack is the capacity of the 7# battery, that is, the maximum value of the usable capacity of the battery pack is the capacity of the lowest capacity line in the figure. In the graph, there is also an average capacity line, which is between the highest and the smallest capacity, and approximates the average of the sum of all capacities. Due to the existence of the consistency problem, there is a large difference between the minimum capacity and the average capacity. The more serious the consistency problem is, the greater the difference will be, which will eventually lead to a larger difference between the available SOC and the average SOC, which will directly affect the actual battery pack. battery life.

SOC estimation needs to use parameters such as current, single cell voltage, and single cell internal resistance, and finally convert it into the actual SOC value. Under the influence of the consistency problem, regardless of the SOC estimation strategy, it is limited by the balance management function of BMS. The SOC estimate can only be infinitely close to the SOC of the smallest capacity battery in the group, which is lower than the average SOC value. That is to say, in the entire battery pack, the SOC values of one or two batteries that have decayed determine the SOC value of the entire battery pack, and are basically independent of the SOC values of other batteries. The more serious the consistency problem of the battery pack is, the lower the SOC estimate will be, and the more serious the deviation from the SOC average will be, which will not only lead to a serious decrease in the SOC utilization rate of the lithium power pack, but also seriously affect the actual battery life and utilization efficiency of the actual battery pack. .

Attenuated batteries generally have the characteristics of high internal resistance. When they are charged and discharged, the voltage fluctuation range is significantly higher than that of normal batteries. This feature will seriously affect the real-time estimation of SOC, and even make the calculated value seriously deviate from the actual value. value, which affects the actual user experience.

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