If you can squeeze in a small battery, look for a deep cycle " wheel chair" battery. I bought a " mighty" wheel chair vattery about 10 years ago, use it fo my pond boat " electric motor" and for my lawn sprayer and a number of other odd 12v needs where a big heavy battery is out. This thing has been awesome, can run my trolling motor for hours, my big boom sprayer for a couple hours and still have power left. About the size of a ATV battery and fully sealed. 12v at 20 ahm. Shipped to my door for 40 bucks.