replacing a transformer instead of a ups battery
In my opinion, you should keep the original structure of the UPS. Just use the car battery with new charger for it (this new charger will ensure full charging the battery in the proper time), and a relay to switch the battery from the internal one to the new one when power fails.
ON the other hand, if the car battery is not the seal one, so be careful of the acid, it may harm you, your family and stuff. Try to use the seal one either.
nguyennam