Hi Bob. I'm a big fan of keeping it simple, so I'm scratching my head why a $20, 8lb 12v 12ah battery (which is trivial to charge) won't suffice.
Mighty Max ML12 12 12V 12AH F2 Battery for APC UPS Computer Back Up Power | eBay
I mean you want it to be reliable it seems, but getting experimental w/it appears to be counter to that goal.