[DonRotolo]

To expand on the above:

A Lead-Acid battery works with a chemical reaction between (what else?) Lead and Acid. In this case, Lead Oxide (PbO2) and Sulphuric Acid (H2SO4).

There's a bit more detail than I mention here, but when the Sulphue atoms move from the acid into the lead plate, you get PbSO4 (Lead Sulphate) and the acid becomes 2H2O (Water), and a release of electrons. Those electrons are your electricity coming out of the battery. Add electrons (= charge the battery) and the reaction reverses.

The problem is that the chemical reaction is not 100% reversible, and the longer the Lead Sulphate has to form crystals (= sitting not fully charged) the more difficult it becomes to convert the Lead Sulphate back into Lead Oxide. And so, your battery loses capacity, and cannot be fully recharged any more. So a battery sitting discharged gets ruined.

The charging process also generates heat. Charge too fast and you get too much heat, which hurts the battery.