Hi,
there are some differences between the startup battery(car battery) and the standby battery(UPS battery).One of the differences is the structure of the battery, the plastic of the startup battery is stronger to resist the impacts of the car, the plastic that is used on a standby battery is fire retardant.Other difference is the internal components and structure.The startup battery is design to give big amounts of current(almost short-circuit) during a short period of time(like the startup of the car), the standby battery is design to give big amounts of current during long periods of time(UPS deep discharges).Other difference, normally, is the ellectrolit of the battery, the startup battery use a liquid electrolit and it can be maintained, the standby battery normally(on small power ups´s) uses maitenance-free battery with gel ellectrolit.
The big difference that you can see on the two battery used to power a ups is the amp/hour.The battery of car are too big for the charger of the ups, and in some cases it can damage the battery charger on the ups because the charge current is not enough to charge the battery, and if it dont´t damage the ups, the battery is not properly charged, in a long term usage is going to reduce the espected life of the battery.