12 State of Health (SoH) Estimation of a Battery
12.1 Definition and Indices/Parameters of SoH
To study a subject, we should first define the basic concept of the studied subject. In this section, such concepts as battery degradation, battery life, and battery SoH will be differentiated and analyzed. The question about what indicators can be used to describe the battery degradation will also be discussed in this section.
12.1.1 Relationship Between Battery Degradation and Battery Life
The battery life can be seen as being conceptually similar to a human lifetime in two ways. First, strictly speaking, the exact duration of the battery is not known until the end-of-life span. Before this, its duration can only be predicted. Second, the mentioned battery life is only suitable for the individual battery. That is to say, the cycle lives of two different batteries with the same brand and the same model are different if they are used in different ways.
A battery life covers a cycle life and a calendar life. This section first defines these two life spans and then makes an analysis on the relationship between battery degradation and these lives.
Cycle life of a battery
Generally speaking, the cycle refers to the process of charging and discharging a battery for one time. The cycle life of the battery refers to the number of cycles during the period calculated from the time when a battery is manufactured to the time when it is no longer in use. The discussion is made as follows:
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