read this on optimas web site ? Pretty interesting
Safety Information:
Always wear safety glasses when working with batteries.
Always use a voltage regulated battery charger with limits set to the above ratings. Overcharging can cause the safety valves to open and battery gasses to escape, resulting in premature failure.
These gasses are flammable! You cannot replace water in sealed batteries that have been overcharged. Any battery that becomes very hot or makes a hissing sound while recharging should be disconnected immediately.
D51 & D51R - D35 - D75/25 - D34 - D34/78
These batteries are dual purpose. They are designed for engine start and cyclic applications and for use in vehicles with large accessory loads.
Recommended charging information:
Alternator:
13.65 to 15.0 volts, no amperage limit.
Battery charger:
13.8 to 15.0 volts, 10 amps maximum, 6-12 hours approximate.
Cyclic Applications:
14.7 volts, no current limit as long as battery temperature remains below 125°F (51.7°C). When current falls below 1 amp, finish with 2 amp constant current for 1 hour.
Rapid Recharge:
Maximum voltage 15.6 volts (regulated).
Maximum current: No limit as long as temperature <125°F (51.7°C).
Maximum recharge time: Charge until current drops below 1 amp.
Float charge:
13.2 to 13.8 volts, 1 amp maximum current, time indefinite (at lower voltage).
All limits must be strictly adhered to.