If DC current is fed into the battery again, then the processes that were described above under 'discharging' will be reversed. The lead sulphate formed during the discharging process will turn back into lead dioxide, lead and sulphuric acid. The sulphuric acid is formed in the cavities in the plate. When the battery is drained, sulphuric acid is fed into the interior of the plate from outside. When the battery is charged, this process is reversed and the sulphuric acid moves out (diffused), and becomes available again as a supply of acid below the plates.
If DC current continues to be fed into the battery until the lead sulphate has turned back into lead dioxide, lead and sulphuric acid, this means that the battery is back to its starting point and is fully charged.
Charging a battery correctly and protecting it against over- or undercharging make a considerable difference in prolonging its service life. By using the most efficient charging methods, it is possible to achieve a long battery service life and short charging times.
In the simplest case, the charger can be switched off or switched over to conservation charging after a specific length of time. However, this simple approach to charging is now no longer state-of-the-art. As well as fully charging a battery, the charger should also evaluate the battery's charge state and detect any malfunction in it. A charger is also responsible for minimising the ageing effect.