I grounded mine to the tractor with a wire. I then unhooked the connector to the work light. I turn on the work light and the sprayer sprays. When not in use, I reconnect the work light. Not much need for both at the same time.
I plug mine into aux. socket (cigarette litter style plug) You can get a universal mount from just about any auto parts store. Mine works good it even has a light on the plug to show that there is power on it.
Another option is if you got welding skills is to build a battery tray somewhere on the sprayer and then just run a toggle/rocker switch up to where you can reach it.
I did this since I use a riding lawn mower that doesn't have a battery to tow my 25gal sprayer.