Thanks to Chris (DVC500) I got the fever to build a pure sine wave battery back up. I got a 300 watt pure sine wave inverter off eBay for about $80, bought a few parts and dove in. Just 5 min ago I tested it. The magic white smoke is contained! Wooohooooo! I did not make it so it automatic charges the batteries, but I'm using wheel chair gel batteries and they keep their charge for months if not used. I can charge them manually if needed and this way I'm not running a charger all the time.
PS I ended up buying a few relays. If someone needs one, PM me. They have "solder" bases. They're a pain to wire, but at least if a relay dies it's just unplug and plug in a new one.
Those gel batteries are great! I've installed some in a few motorcycles I've worked on, & have one in my RD350. You can abuse the hell out of them (not charge them ... ever!) & they still come back for more! I have one in a lawnmower that I haven't charged in 18 months .... I can go out there now, toss it on a charger & it'll be good to go for another few years!