Your UPS has a battery inside it. Usually it is a lead-acid type (those are the least expensive for the manufacturer), but both lithium and gel-cel batteries are sometimes used. The battery is by far the most vulnerable and failure-prone part of your UPS. If you have your UPS long enough, you will probably have battery problems. Once every six months to a year or so you should recalibrate your UPS's battery sensor, and once every several years you will have to replace the batteries.