What is the minimum allowable discharge voltage for maximum life of a standard 12-volt lead-acid battery?
Why A 12-volt lead-acid battery is really six cells in series, and each cell is considered fully discharged at about 1.75 volts. Six times 1.75 gives 10.5 volts, which is the accepted end-of-discharge point. Taking a cell below that drives sulfation of the plates and permanently reduces capacity, so 10.5 volts is the floor for maximum battery life.
Watch out The choice of 12 volts is tempting because that is the nameplate rating, but a healthy lead-acid battery actually rests near 12.6 to 12.7 volts and 12 volts already represents a partly discharged state, not the discharge limit. Values like 6 or 8.5 volts are deep into damaging over-discharge.
Six cells times 1.75 V per cell = 10.5 V. Stop there, or you sulfate the plates.