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.
So when my batteries gave up the ghost, I wondered what would happen if I replaced the 20 amp-hour gel-cell batteries with 100 amp-hour RV batteries, which were cheaper in spite of having five times the capacity.