I bought a used UPS that the battery is old and flakey. It will charge to 100% but won't hold a load. I even tested it on just a house lamp. The lamp stayed lit for about 15 seconds on battery power. I know I have to replace the battery. Easy enough. The question is how often do I have to deep cycle UPS batteries? The UPS was $50, a replacement battery $30. A new UPS is $120 for the same wattage (500). So I guess I saved a few bucks. Its a APC Back-UPS pro 500 with USB interface made in 1999.