I use those HF solar maintainers. There not chargers as they wont charge a battery but they will maintain it. I keep them on my stuff that I keep at the farm. THe last few years with my new son I have only gotten there a few times a summer, maybe 4. And a few years only got there every 6 months or so. And in this time my like 2 year old auto battery on the farm truck (no tags just used on property) battery would be so dead it would barely crank the truck over and same with the tractor battery (which is like 7 years old and needs replacing anyway) but with those HF maintainers the batteries stayed topped up and no issue at all.