I installed a 15 watt 12 volt solar battery charger/maintainer several weeks ago on a building with no electric service. A 9n & 8n with 6 volt batteries were hooked up in series with 25 ft of cord going to tractor #1 and 12 ft jumpers to tractor #2. Average voltage is 6.4v on #1 and 6.7v on #2. SE indiana outdoor temps and a lot of gloomy days, I consider this a success. For comparison an A C 6v battery tender has kept 2 6v batteries hooked up in parallel @ 6.5 volts each. Things you do once you retire...