I have a little machine that fires tennis balls for my dog and while I can plug it into the mains electricity, it's also portable. I can insert 6x D type standard batteries which last for a half hour or so..firing every few seconds. However I want to bring it onto a tennis court to fire balls for me and what I would like to do is rig up something externally I can wire up to it where it could last for hours. I have a 12v portable power source with a cigarette lighter type output. I was hoping I could use this in some way?