When a battery is allowed to be at a low level of charge for too long, it can become damaged.
Allowing batteries to stand for long time after a discharge without recharging will cause plate "sulfation." Sulfation is the formation of large crystals or hard lead sulfate on the battery plates. Excessive sulfation of this kind is difficult to reduce and may cause premature damage (i.e. overheating or inability of the battery to retain charge). Lead sulfate is difficult and some times impossible to reduce by conventional charging methods.
A charged battery, disconnected from a UPS or load, can maintain ideal voltage for approximately six months before recharge is necessary. A discharged battery needs to be recharged within 72 hours of the discharge to avoid permanent battery damage. Storing a UPS with the battery connected can cause permanent battery damage within a few days to a week. A UPS should be stored after complete battery recharging with the battery disconnected.
Per manufacturer's recommendations all Valve Regulated Lead Acid (VRLA) batteries should be immediately recharged following a discharge. VRLA batteries should recover fully in terms of capacity and service life if recharged within 72 hours following a discharge.
No VRLA battery should ever be allowed to drop below 1.0 volt per cell on discharge (i.e. 6 VDC for a 12 volt battery). If a customer's situation falls within the previous scenario the best course of action is to replace the affected batteries.
APC UPSs will charge the battery when the UPS is plugged in and the battery is connected to the UPS. In addition all APC UPSs will protect their batteries against over discharge when possible, but, as implied above, the storage life of the battery will be shortened relative to the amount of charge removed.