Hi Mark! I use two group 31 deep cycle batteries -- one to use, and one as a spare. I never let the charge drop below 12.1 volts, measured when the battery is resting (not in use).
One battery lasts us about three days. Then I switch to the spare.
They say 6 volt golf cart batteries are great, but then you have to use two of them at once. If one dies, that is a problem. So that is why I have one to use and one to spare.
I also have a 120 watt portable plug and play solar panel -- if there is sun, it keeps the battery full and I never have to use the spare.
