The following description assumes a sealed lead acid battery charging.

The first stage is referred to as “bulk absorption”; the charging current will be large and constant and is limited by the capacity of the charger.

When the voltage on the battery reaches its outgassing voltage the charger switches to the second stage and the voltage is held constant.

The delivered current will decline at the maintained voltage, and when the current reaches less than certain level the charger enters its third stage and the charger output will be held constant. In the third stage, the charging current is very small and at the voltage the battery can be maintained at full charge and compensate for self-discharge.