No, don't do it. I understand his thought process because they are both 12v batteries with more capacity, but car batteries are made for high burst of energy which a car engine ignition requires, whereas UPS batteries are made for slow drains. Also, these UPS are made for charging battery cells in a certain way, if you start to stack a bank of batteries of the same model in parallel hoping for more capacity, even then its a problem for the UPS's charger, they won't charge evenly and eventually becoming a problem.