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...

Glad to hear that is working for you.
I thought of doing the same thing but could not decide if I needed to disconnect the batteries from the tractor.
So I took two 12 volt rv batteries I had hooked them to a 100 watt collector.
Took a 700 watt inverter and run my 6 volt charger that I had.
It is probably not real efficient, but it works great. I also run some 110 volt led lights with the inverter. And I run a 12 volt charger/maintainer for my truck.

fun project, I might try the same to keep my battery tools charged. The lights wouldnt hurt either.

During these trying times, anything one can do to keep body & mind active is a good thing!