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What is capacity retention?



Recently, there have been a lot of fire accidents caused by lithium-ion batteries, and I think the interest in lithium-ion batteries is increasing. However, there are certain aspects of lithium-ion batteries that are not well known, such as their composition, various safety features, and related terminology, which I have also described.

1.What is the capacity retention rate?

When secondary batteries such as lithium-ion batteries undergo cycle tests, floating charge tests, preservation tests and other battery evaluation tests, the battery will deteriorate, and its capacity will gradually decrease from the initial capacity [mAh or Ah]. The capacity retention rate is the ratio of the capacity at a point [mAh,Ah] to the initial capacity [mAh,Ah]×100, expressed as a percentage.


2.cycle test capacity retention rate

An example of the capacity retention rate of a lithium-ion battery during cycle testing is shown below. In simple terms, the cycle test is to evaluate the deterioration of the battery during repeated charging and discharging, and one of the evaluation items of the cycle test is the capacity retention rate.

 A battery that can maintain a high capacity retention rate even after multiple cycles is a good battery. The capacity retention rate of the battery depends not only on the number of cycles, but also on the charge and discharge rate during the cycle, the ambient temperature, etc.

The reduction in the number of known cycle tests and the capacity maintenance rate can be obtained by the degradation coefficient Kc in the cycle test and the degradation coefficient Kf in the float test. Reduction of capacity maintenance rate = degradation coefficient Kc× number of root cycle tests + degradation coefficient Kf× route passage time. Here, the degradation coefficient Kc in the cycle test is strongly related to the DOD(depth of discharge) of the battery.

In other words, the greater the DOD in the cyclic test, the greater the expansion and contraction of the active material due to charge and discharge, which tends to increase the contact resistance between the active materials and between the electrode foil and the active material. In addition, it should be noted that when conducting a cycle test, if the amount of electrolyte in the battery is too small, there will be a phenomenon called drying (liquid depletion), which is not good for the battery.

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