I own quite a few of the APC UPS backup power products. And usually after two or three years, the UPS no longer work under load when the power goes out.

I tried buying replacements on eBay, but I wasn't happy with the quality of the third party products. They just didn't last as long. And I tired ordering replacements from APC, but I wasn't happy with the price. Seemed better to wait for a sale or deal to come around and just by a new UPS altogether.

Well not sure what changed but now you can find APC UPS replacement batteries on Buy.com for a fair price, IMHO.

Most desktop APC UPS solutions use the APC Replacement Battery Cartridge #2. Last I had checked, the price was $29.99 with free shipping. The price is only $19.99, if you qualify for $10 off for being a new Google checkout customer. You can search for other models too.

A nice bonus included in the price is that you get a box, and a prepaid return label so you can send your near-dead battery back to the recycling plant. So you can help protect the environment, while helping to protect your PC's.