[Dr_Al]

I thought about a lawn tractor battery but figured for not much more I can get a small car battery and when the main battery dies I'll just use the car battery by itself. I'm not really sure if the battery box is even needed.

I modified my old 3 wheeler to use a garden tractor battery and never bothered with putting it in a box. That was well over 10 years ago and never had a problem. So I'm thinking all it really needs is a way of protecting the battery from stuff flying off the front tire.