For those of you who know how to mess around safely with car batteries and other high-amperage/low-voltage applications, here’s an interesting one (please note the warning below!:
APC makes an extended-run UPS called the Smart-UPS XL, which supports external battery packs. The ones I have (Smart-UPS XL 1000) are a 24V system, which means that the internal batteries and the external battery packs are all 24V. The external battery packs and replacement batteries are expensive.