Many users find that during use an electric vehicle battery seems to hold less and less capacity, and the riding distance keeps getting shorter. What causes this decline in capacity? Besides factors within the battery itself, it is also related to your daily usage habits and the environment in which the battery works. Below we explain in detail the causes of capacity decline in electric vehicle batteries.

1. Sulfation

When an electric vehicle battery is over-discharged, or is not recharged promptly after discharge, lead sulfate forms recrystals. After recrystallization the lead sulfate becomes dense and coarse, and unless overcharged it is hard to reduce back completely. These incompletely reduced lead sulfate crystals lower the discharge capacity and make the battery even more prone to over-discharge, producing more dense and coarse recrystals in turn. This cycle leads to early capacity degradation of the battery.

2. Passivation

On the positive plate, passivation mainly shows up as a float-charge voltage that is too low after deep discharge, or overcharging above 120%. After many cycles the thickness of the barrier layer increases and its resistance grows, so that under constant-voltage charging the current becomes small, under constant-current charging the terminal voltage rises quickly, and a full charge cannot be reached under normal conditions. During discharge the terminal voltage drops fast, leaving the discharge capacity far below the designed capacity and causing early capacity degradation.

3. Formation of the barrier layer

When a battery works for a long time in a deep charge-discharge cycle environment, and the bond between the grid alloy and the active material is not good enough, the expansion and contraction of the active material in each cycle lets a corrosion layer form on the interface between the grid and the active material during float charging. Depending on the alloy used, this layer may be a good or a poor conductor. If it is a poor conductor, the charge-discharge performance of the battery deteriorates rapidly, causing early capacity degradation.

In summary, there are many reasons for the decline in electric vehicle battery capacity. We should take proper preventive measures so as to extend the life of the battery.