A lead acid battery is incredibly crude and old tech as it could get, but people still use it because it provides super cheap power storage solution, and able to withstand very harsh working environment. The disadvantage is the charge must be constantly on max level. Especially on shallow cycle lead acid battery, it cannot be fully discharged, not even once. Sulphation will occur where the sulphur in the acid will coat lead plates with a solid layer, making your battery useless. Two techniques is used to revive dead batteries, either thru pulsing resonating currents thru the plates to vibrate and remove the tough deposits, or by using a sulphur dissolving material (EDTA) to dissolve the sulphur deposit, but you had to pour in new acid solution. Either way, a dead lead acid is almost certainly doomed.