I've told this story a dozen times here but....a long time ago, I had an Eagle Ultra or whatever it was. I had converted it to being portable. I was mostly running it when I was in places where I could not bring my own boat in. The security system in the house used a 12V, 7.5A battery backup plugged into a transformer that would keep it charged. I had 2 batteries and whenever I went north to those lakes, I'd simply swap out the fully charged security battery for the other. It would last me 3-4 days on the water and when it was time to go north again, I'd simply swap them out of the security system again. The batteries were always fully charged.