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Some Good to Know Battery Facts
In general, batteries will self discharge when not in use at the rate of around 4% a week. Keeping batteries charged when not in use is a good idea. Look into “trickle” chargers – small solar panels usually from 3 to 5 watts.
A battery contains water and acid. Acid is heavier than water. The longer a battery sits with no charge the more likely acid will begin to settle at the bottom of the battery. This concentration causes a build up of lead sulfate (sulfation) on the plates which reduces capacity and battery life - another reason to keep batteries charged. And another handy feature available on some charge controllers discussed elsewhere.
Discharging a battery below 80% of its rated capacity can severely damage it. At 10.5 volts a battery may be forever dead.
For those batteries requiring maintenance, it’s best to check battery water levels frequently. Hot temperatures require more frequent watering, just like your plants. Always use distilled or de-ionized water. Never use tap water. Don’t let the water level drop below the top of the plates. Add water only after it’s fully charged, not before. The water should be ½ to ¾ inch above the top of the separators. Under normal operating conditions you never need to add acid.
When it gets cold battery capacity goes down, as much a 20% when freezing (a reason why your mechanic tells you you need a new battery but the one you have will be ok until winter). When it’s warm, capacity goes up.
A pair of 6 volt golf cart batteries is no better than one 12 volt battery of the same size and quality. However, sometimes it’s difficult to find a true deep cycle 12 volt battery if that’s what you are looking for. Many people use two 6 volt “golf cart” batteries for that reason. Physical size may also make a difference. 12 volt deep cycle batteries can get sizeable. Although so can some 6 volt batteries.
Although a Deep Cycle battery can handle a 80% discharge it’s much healthier for it to go no more than 50%.