I use this Kencove charger. It'll run at least a couple of weeks on 4 D cells or much longer on a small 12v battery. The trick is to get a good ground. During hunting season, the soil here is usually very dry and I can't get a decent ground without some work. I dig 2 shallow holes a foot or more apart and pound in 2 18" rods which I connect together with wire and clamps. Then I fill the holes with water. After it soaks down a bit (possibly refilling the holes with more water), I get a pretty good ground.

KENCOVE STRIP GRAZER
[Linked Image from images.kencove.us]

“In a time of deceit telling the truth is a revolutionary act.”
― George Orwell

It's not over when you lose. It's over when you quit.