You are correct assuming the voltage of the battery and the load is the same. I would shoot for 50% capacity, depends on the battery type. You can buy more batteries and buy a cheap electrical multimeter that can read the actual draw in Amps. Usually the label on the load is the max, which many not equal the typical.
You can buy more deep cycle lead acid, sealed lead acid, AGM, or gel cells. Then you can take them to Snow Koan Solar for charging. If you have a lot of heavy batteries, it's useful to bring a little red wagon, which will also tow behind a bicycle. They can explain how to use the meter to tell when the battery is charged or discharged.
Sure there are battery stores, and solar or marine supply stores nearby you. Car batteries are not deep cycle, they are max cranking batteries. But a deep cycle marine battery can physically look the same.
Last edited by some seeing eye
on Tue Aug 21, 2012 2:26 pm, edited 2 times in total.
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