Optimum battery life will be obtained if a "GREEN" hydrometer condition can be maintained, batteries should not be left in a deeply discharged state. Batteries should be recharged as soon as possible once a "BLACK" hydrometer is observed. Once the state-of-charge has reached 100% charging should only be continued for long periods at a reduced rate (on-charge voltage of 13.5/13.8 volts) to prevent long term electrolyte loss.
Voltage regulator settings should be in the order of 13.8 - 14.4 volts the higher setting preferred. On applications with repetitive deep cycling, a charging voltage of at least 14.8 volts is recommended, however the on-charge voltage should not exceed 15.8 volts.
Chargers with charge rates up to 50 amperes are generally satisfactory. If any battery becomes hot to touch, or starts to spew electrolyte, charging should be reduced or stopped to allow the battery to cool before continuing.
How to Charge a Flat Battery
A very flat or completely discharged battery may initially only accept a very low charge current (milliamps) which may not be detectable on the chargers ammeter. If the open circuit voltage is below 11 volts it may be necessary to override any reverse polarity protection on the charger.
The time required for the battery to accept measurable charge current may be as follows: