As others have said, it would be better to use deep-cycle batteries.
But in the inerest of recyclying ( meaning using old not buying new) You can use car/truck batteries. Even the good large Truck 12V batteries are at MAX only 54Ahr rated.
One other thing is the recharge abilities of the UPS itself. There is a maxiumu amount of recharing that it will be able to do. This I believe is at best 3x the battery Amp hours the old batteries are rated at.
Is the one you have , one that has to external port for adding more run time for normal use? If so , the best bet would be to install correctly rated ( or as close as possible) batteries on the inside, and tap the external plug. This would be the easiest way to extend your run time. Otherwise if you have the non-exteneded run type, then you could tap the cabling coming to the original batteries to extend out to a VERY carefully insulated adapter, so you can add batteries. This works as well.
Pay careful attention to the HEAT coming off the top of the UPS, that, and the regulator clicking in/out as it tries to recharge the batteries will tell you wether or not you've exceeded its recharge rating.
I have burned up one unit doing this.
The internal X-former gets REALLY hot!!! and since APC didn't think to put insolater between the metal top and LIVE volatge, there a whole lots of danger there. There's both AC 110VAC and DC on that top circuitry board, so do be very careful!!
When I open these up, I now tape a piece of thin cardboard on the metal part of the top cover just in case.
Hope this helps;
Bruce S
BTW: These are known for buring up batteries, I 'll bet your old ones are dry and possibly cracked, look on the bottoms and around the thinner ends. The charger part isn't smart enough to turn off, just keeps charging batteries. Idea is a good one, run loads through batteries while AC keeps batteries charged, just hard on batteries.
