Quote: Originally Posted by Dwarf
With a UPS (Uninterruptible Power Supply), your computer is never powered directly from the mains. Instead, the mains powers a battery charger and it is the battery which powers the computer via an inverter (which works in much the same way as inverters that you can get for your car, albeit with greater efficiency). In normal use, the inverter is powered by the charger, and the battery is constantly kept topped up at the same time. When the mains fails, the battery takes over and can power the computer for a limited period of time - typically 15-30 minutes, enough time to enable you to safely shutdown your system.
Like all rechargeable batteries, the ones inside a UPS have a finite life and, sooner or later, they will fail to maintain charge adequately. It sounds as though this is what has happened here. It is possible to replace the batteries on some, but not all, of these units. Check the accompanying manual.
This is contradictory and confusing.
Some UPS devices power their connected devices through the battery at all times. These true uninterruptible power supplies cost a lot of money, because they do not have a switchover time where the UPS changes from mains to battery and inverter power.
Cheaper UPSs - actually a "standby" power supply - like my APC 550BE switch over to a constantly topped-up battery and inverter only when mains power goes above or below certain threshold voltages. During regular use, connected devices get filtered and surge-protected power from the AC line.
The tipoff between the two types of UPS is switchover time, usually measured in milliseconds, which may or may not be mentioned in the UPS's specification sheet. During the switchover time, the connected devices rely on capacitors in power supplies to keep them running just long enough so power is never totally lost.
Source: Upgrading and Repairing PCs, 15th edition, by Scott Muller