If you have access to 110 volts where you store, just use a battery maintainer, be sure you use an intelligent charger that knows to go into float mode when the battery is fully charged. If you have no power access you will have to visit it periodically during the winter and run it long enough to keep things charged up. Not a good idea to let batteries go completely dead. Another possibility, use a rechargeable power station and plug your battery charger into that, then take it home every month or so to recharge it.
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