Stationary batteries are almost exclusively lead acid and some maintenance is required, one of which is equalizing charge. Applying an equalizing charge every six months or after 20 cycles brings all cells to similar levels by increasing the voltage to 2.50V/cell, or 10 percent higher than the recommended charge voltage.
An equalizing charge is nothing more than forced overcharge. It removes sulfation that may have formed during low-charge conditions. Battery manufacturers recommend first measuring sulfation. One method is to apply a saturated charge and then to compare the specific gravity readings (SG) on the individual cells of a flooded lead acid battery. Only apply equalization if the SG difference between the cells is 0.030. During equalizing charge, check the changes in the SG reading every hour and disconnect the charge when the gravity no longer rises. This is the time when no further improvement is possible, and a continued charge would cause damage. The battery must be kept cool and under close observation for unusual heat rise and excessive venting. Some venting is normal and the hydrogen emitted is highly flammable. The battery room must have good ventilation.
Equalizing VRLA and other sealed batteries involves guesswork. Good judgment plays a pivotal role when estimating the frequency and duration of the service. Some manufacturers recommend monthly equalizations for 2 to 16 hours. Most VRLAs vent at 34kPa (5psi), and repeated venting leads to the depletion of the electrolyte that can lead to a dry-out condition.
Text taken from batteryuniversity.com