strength or specific gravity can be measured using a hydrometer. A battery that is fully charged has a specific gravity of 1.265 (1.265 times pure water density). When the battery discharges, its specific gravity also drops until the electron flow is not enough for the required load.

But if a regular C or AA cell discharges, it is an irreversible process; the cell cannot be charged again. A deep-cycle battery bank's discharging process is reversible, allowing the electrons to be returned to the battery by reversing the chemical discharge process. A photovoltaic panel in direct sunlight can do this: the panel generates a voltage and, when the voltage at the PV panel becomes greater than the battery voltage, electrons flow into the cell plate from the panel. They combine with sulfur compounds on the plate to force these compounds back into the electrolyte and increase the sulfuric acid's specific gravity. Thus, the battery is recharged for later use. The charge and discharge concept is the same for all batteries, regardless of their specific chemistry.