What happens if you undercharge or overcharge a lead acid battery?

Both overcharging and undercharging are not good for the lead acid battery which we commonly use in this region at home. Overcharging with high voltage will cause current to flow into the battery even after being fully charged. This will lead to decomposition of water. High current during overcharging causes heating of the battery. This sets up a vicious cycle as a hot battery can accept a higher current. Finally it leads to thermal runaway and severe damage to the battery even within hours. Oxygen evolution occurs at the positive plates and oxygen recombination occurs at the negative plates in VRLA (Valve Regulated Lead Acid) batteries. Oxygen recombination is an exothermic reaction.

If there is no temperature compensation in the charger, when it reaches the constant voltage pattern towards the end of charging, higher current flows into the battery. Usual temperature compensation is a decrease in the constant charging voltage for every degree rise in temperature. You may note that the initial phase of charging is constant current mode in CC-CV method of charging of VRLA (SMF) batteries. The vicious cycle setup during overcharging causes thermal runaway.

Too low a charging voltage causes current flow to stop before the battery is fully charged. This allows some lead sulphate to remain on the electrode and eventually reduce the battery capacity. In my previous post on My New Sealed Lead Acid Battery, I had mentioned the maximum and minimum charging current needed. Similarly the degree of temperature compensation for charging voltage was also mentioned in the battery specification label. If sealed lead acid batteries are stored in fully discharged state for long time, sulphation can cause high internal resistance and the battery may appear to be not taking charge. This can be sorted off by charging for a longer period when the battery starts accepting the usual current. If it does not accept the regular charging current even after waiting, then it is likely that it has suffered irreversible damage due to prolonged storage at discharged state.