What to use to keep battery charged while sitting?

I use one of these plastic battery boxes. I have the box for two 6V batteries, but they have single and double battery boxes for about any size.

These Century Plastics HD battery boxes are plastic but extremely sturdy. My TT isn't here so I can't provide a pic, but I welded a hold down frame from angle iron, and welded the hasps for Master puck locks on each end of it. Clicky

My batteries aren't going anywhere.