Use a UPS, there are some pretty cheap.
He HAS a UPS, he wants to add some more capacity to it I suspect.
yep.. do you know how to wire two aps 12v battery to one ups ?
The link that tfferreira gave is a good one, but there are MANY caveats - your UPS has to be designed to work for an extended period without getting hot !
Use a pair of NEW batteries to begin with, like I said earlier.
i even used to think of putting a car battery instead but capacity isnt the only thing to consider here... these things have some wattage ratings and you have to take that into account. VA refers to battery capacity and W refers to how much work it can handle. let's say if you have a 500W pc case and monitor connected to 600W 1200VA of PSU and busy gaming... it wont blow up immediately or will last enough untill battery gets empty. but if you replace or add more batteries for higher capacity, certainly battery will last longer but anything else inside the box wont. they are not designed to run longer than their battery and heat up blow up.i read here and there it is better to put 70-80%max load on these kind of things but better be safe and use sth rated for two times of your max need so it wont heat and blow up even if you run it longer with more batteries.they use 6-12V sealed gel batteries. you can use a car battery as well if it is 12V. red is +, black is ground.
Some UPS are picky I would go with a larger battery rather than two.
In parallel with the existing battery might work, but its never a good idea to put a new battery on an old one. Better to put two new batteries in at once.
Your tags need cleaning up. All you need to put is battery, UPS,