If the battery has a faulty cell, it’s charge voltage will not reach the charger set point to reach constant voltage mode. This results in the remaining cells being overcharged until the overrun timer terminates the charge after 18 hours. The best solution is to measure the battery voltage when its in constant power stage and confirm it is correct for the battery type. To enter constant voltage mode, switch the charger on and off to reset it and wait for the charge light to flash, then use a digital volt meter. If the battery is supporting a higher load than the rate of charging, contact the factory for modification.