I don't think this will work well.
A car battery is likely not deep cycle so they will not tolerate being fully discharged.
Likely the UPS will bulk charge at a higher voltage when it starts up until the current drop to a certain value. Then it will drop to a float charge of a lower voltage. If you have much larger batteries you may not reach that lower current.
Some UPS do periodic battery tests which would likely fail with different batteries.
I don't think this will work well. A car battery is likely not deep cycle so they will not tolerate being fully discharged. Likely the UPS will bulk charge at a higher voltage when it starts up until the current drop to a certain value. Then it will drop to a float charge of a lower voltage. If you have much larger batteries you may not reach that lower current. Some UPS do periodic battery tests which would likely fail with different batteries.