In some chemistries like Ni-Cd when using constant current method，the voltage increases up to the fully charged state point and then the voltage begins to decrease.This is due to oxygen build-up inside the battery.This decrease is significant，so the derivative of the voltage versus time can be measured to indicate overcharge.When this parameter becomes negative it shows that we have passed the fully charged state and the temperature begins to rise.After this point the charging method can be switched to trickle，or float charge，or be terminated completely.