So many factors. Battery needs care to maintain its chemistry and its energy efficiency. The aging process starts at the moment the battery leaves the manufacturing factory. Self discharge ,sulphation, capacity loss, overcharging, dendrite growth, grid corrosion, excessive heat and water loss etc are some common reasons for the premature battery failure.
If the battery is not cycling for long period, self discharge takes place @ 4 % per week at 27 degree. For example, a 125 Ah battery shows self discharge @ 5 Amps current per week if it is not discharging through the Inverter. Sulphation is another important factor affecting battery efficiency. The lead sulphate accumulating on the plates prevents charge release/ acceptance cycles. Usually the lead sulphate crystals will be removed during water filling and charging. So it is better to charge the battery as soon as possible to prevent sulphation.
Another important reason is overcharging. The battery used in backup systems should have a charger cut off facility, otherwise the battery will get over charged due to float charging. When the battery is overcharged, the charge voltage increases above the Gassing voltage. This causes formation of excess hydrogen. The Gassing voltage of a 12 V lead acid battery is 14.3 at room temperature. The battery voltage crosses the gassing voltage level if one or more cells are damaged or weaker. The weaker cell will charge last since its resistance is lower than the good cells. So higher voltage appears across the good cells and may increase the voltage above the gassing voltage level.
By using the battery equalization method, it is easy to make the battery fully charged. All the cells in the battery will not be equally good due to aging. Some cells accept charge very fast and some in a slow rate. If this happens, the battery will not hold sufficient charge and will discharge very fast through the load. Equalization is done by giving slight overcharge to the battery even after the terminal voltage rises to maximum. Due to this overcharging, the weaker cells also get time to accept full charge. A fully charged 12 volt lead acid battery will show 13.8 V terminal voltage and a 12 V tubular battery will show 14.8 V in the fully charged state.