My guess is probably not.
- The APC 1400 VA UPSes that I refurbished had 24V battery pairs and not 12v batteries. However, that objection could be overcome if you use two batteries in sequence (but read on).
- The charging current of the tiny native 1400va ups batteries is probably much less than that of the car batteries. Higher current coulld damage the charging circuit of the UPS. You could make some current limiter (see Art of Electronics) to limit charging current, but it probably is not worth the time.
Two batteries for a 1400 VA ups should cost about $35, someone is selling them from a warehouse west of chicago. I bought a pallet of them at some point.
i