The battery is fully charged when the cells are gassing freely at a low charging rate and no change in specific gravity occurs over a 3-h period. Batteries that have stood in a discharged condition for long periods of time without a recharge have become sulfated and must be recharged at a low rate to avoid overheating and excessive gassing. It may require 2 or 3 days of slow charging to bring a sulfated battery to a fully charged condition. Do not give sealed low-maintenance batteries this type of treatment. Some batteries are so badly sulfated they cannot be restored to a normal operating condition, regardless of the rate of charge or the length of time the charge is applied.
Some special procedures used to remove the hard sulfate from the plates softens the active material.
The battery then fails prematurely because of shedding of the active materials from the plates. Therefore, if a battery cannot be restored to a fully charged condition by slow charging, it should be rejected.