There are several batteries available for APC UPSs that are every bit as good as the APC replacements but are far cheaper. There are a couple which are very poor as well. I would recommend researching a bit on the brand(s) you are looking at prior to purchasing. I would tell you which ones are good but I can't recall off the top of my head and I will not be back in the office where I can find out for over a week so I am no good to you here.
It sounds as though you are having some internal problems with the unit which may or may not be related to the battery. Though, if there is indeed a short I'm sure the batteries are no longer good.
How savvy are you with hardware? These units are serviceable so you can unscrew and open it to give it an inspection. Look for anything burnt or melted. If all looks good then you will want to check the resistance levels between break points for consistency. If you are not familiar with doing this type of work then I strongly suggest NOT doing it. Chances are rather good there is someone in your area that services these units whom you could take it to.
These types of issues are the best examples for promoting the idea of owning a backup unit. The costs involved with purchasing another on to have handy are far less than other things that are possible. This being said, if you can, I would order a new one and start using it. Get yours repaired if needed and get new batteries. Keep it plugged in and have it waiting should something happen.
Even better to do if you have dual PSUs on your servers. Have both UPSs live and plug one PSU into one UPS and the other PSU into the other UPS.
