 Some chargers may keep the voltage at "Umax" for some time and allow the current to drop to 80% of the constant-current value, before proceeding to the next stage.Stage 2 is called the "Uo-phase", "constant-voltage boost stage", "absorption stage", or "topping charge".In this stage, the battery is continued being charged at a constant (over)voltage Uo, but the charge current is decreasing.