In the simplist,
a charged battery: Positive Plate: Lead Dioxide Negative Plate: Lead Sulphuric acid: concentrated sulfuric acid a discharged battery: Positive Plate: lead dioxide becomes lead sulfate Negative Plate: lead becomes lead sulfate Sulfuric acid: less concentrated
The biggest problem with most batteries is when they have been left in a discharged state for a long period of time. When that happens, the lead sulfate coating on the plates changes the normal lead sulfate to a crystalline lead sulfate and this is called sulfation. This will often appear as an unequal specific gravity between cells effectively making it a smaller battery. The specific gravity should remain within .020 of each other. To recoup the battery from sulfation will require the battery to be charged above the normal voltage for a period of time, known as equalization and will gas. Trace inverters have an optional battery charger that takes a lot of the work out of recharging for you. If a battery seems to be dead or ruined, there are additives in the market, usually EDTA, that can be used to help recover a dead battery at least partially. The light of a fire can be used for the solar battery chargers although it will take much longer.
Offered by Steve.