From a vehicle perspective, the most important and relevant things for a battery system are the capacity and power performance, which needs to be accurately estimated as SOH by the BMS. Therefore, the attenuation effect of the battery is usually manifested as the change of the battery's electrical properties, especially the change of the capacity and power. In general, usable capacity and usable power decay as the battery ages.

     In energy applications such as battery electric vehicles, high-energy batteries are usually used, and the basic function of batteries is capacity storage. Therefore, the degradation of the battery can be evaluated by capacity fading. For applications such as hybrid vehicles, high-power batteries are usually used, and the basic function of the battery is to meet high-power requirements. Therefore, more attention needs to be paid to power attenuation. For PHEVs, both capacity fade and power fade should be considered. Usually, the main reasons for battery capacity fading are LAM and LLI. When the charge-discharge cut-off voltage and rate are the same, the increase of the internal resistance of the battery will also affect the battery capacity. The main reason for battery power attenuation is the increase in internal resistance.

     At present, for high-energy batteries, when the battery capacity drops to 80% of the initial capacity, the battery is considered to have reached the end of its service life because the battery cannot meet the requirements of the vehicle. For high-power batteries, the service life is usually determined by the available power reaching 50% of the initial value.

     Basically, battery life can be divided into two parts: schedule life and cycle life. The schedule life refers to the battery degradation caused by storage without cycling; while considering the battery degradation caused by charge-discharge cycles, it corresponds to the battery cycle. For actual EVs, the battery can be charged while driving or at a charging station; when parked, the battery may be suspended. Therefore, both schedule life and cycle life should be considered.

     Generally speaking, most batteries currently used in electric vehicles usually exhibit nonlinear attenuation characteristics, which can be roughly divided into three stages. In the first stage, LLI occurs due to SEI formation on the negative electrode, resulting in a rapid decrease in battery capacity during the first few cycles, especially during the first charge. The initial coulombic efficiency of the battery may be low. The initial Coulombic efficiency problem is of great value to the study of battery design and production. In the second stage, the battery performance gradually decays due to various side effects inside the battery. In the third stage, at the end of life, the capacity drops rapidly and the impedance rises rapidly. The reasons may be rapid depletion of lithium ion reserves due to lithium deposition, or loss of active material due to electrolyte loss, binder failure, or volume change. This rapid capacity drop phenomenon greatly affects the secondary use potential of the battery.

Also, sometimes the battery capacity may increase substantially. This phenomenon is often observed early, or the cycling test is interrupted, and an increase in capacity may occur after prolonged storage. The reasons for this phenomenon need to be further analyzed and discussed. One possible explanation is the passive electrode effect, arguing that a geometrically excess negative electrode may provide additional capacity (in fact, Li-ions) after storage, leading to an increase in capacity. Another possible reason has to do with charge redistribution (ie no charge or discharge force acting on them). That may be due to the improved wetting properties of the electrode electrolyte. The lithium plating/stripping process can also lead to abnormal improvements in battery performance.

    In addition to the electrical properties, the mechanical and thermal properties of the battery also change. For example, the thickness of the battery may increase due to gas generation and other reasons; during the battery decay process, the heat transfer coefficient and entropy may also change.