Being very user friendly, a lead-acid battery can be conveniently discharged and recharged quite a number of times. With each cycle, the charging process stores energy in the battery in the form of potentially reactive compounds of sulfuric acid, lead and lead oxide. The discharge process is another chemical reaction among those components that release the stored charge in electrical form. Since no chemical or physical process can ever be 100% efficient, more energy is always used to charge the battery than can be recovered from it. Thus, determining the optimum conditions for battery charging grows in importance as the cost of energy increases.
If you force a direct current into the cell in the reverse direction, you would replace energy drawn from the cell during discharge. The effect on the electrolyte and the plates during this charging process is essentially the reverse of the discharge process. Lead sulfate at the plates and the water in the electrolyte are broken down into metallic lead, lead dioxide, hydrogen and sulfate ions. This re-creation of plate materials and sulfuric acid restores the original chemical conditions including, in time, the original specific gravity.
The amount of energy it takes to re-create the original specific gravity is at least the same as the energy produced by the chemical reactions during discharge. This energy is supplied by the charger in the same form that it was removed from the battery: as volts and ampere-hours (or kilowatt-hours). Thus, if the battery produced 36 kilowatt-hours during discharge, it takes at least 36 kilowatt-hours to recharge it, plus additional kilowatt-hours to make up for losses in the energy-transfer processes.