PC Hardware in a Nutshell, 3rd Edition

BPS Types

All BPSs have three common elements: a battery, which stores electrical energy against power failures; an inverter, which converts DC voltage supplied by the battery to the AC voltage required by the load; and charging circuitry, which converts AC mains power to the DC voltage required to charge the battery. IEEE recognizes three categories of BPS, which it terms UPS:

On-line

An on-line UPS (often called a true UPS to differentiate it from an SPS) connects the load directly to the inverter, which converts DC voltage supplied by the battery to standard AC voltage. The charging circuitry charges the battery constantly while the UPS is operating, and the equipment always runs from battery power supplied by the inverter. On-line UPSs are not often used on PCs because they cost substantially more than SPSs, described later in this list. An on-line UPS has two advantages. Because the PC runs on battery power all the time, there is no switch-over time, and no switch to fail. Also, because the PC does not connect to mains power, it is effectively isolated from AC line problems. Against this, an on-line UPS has three drawbacks. Foremost is cost, which may be 50% to 100% higher than an equivalent SPS. Also, because the system runs from battery constantly, UPS batteries typically require replacement more frequently than SPS batteries, and UPS batteries are not cheap. Finally, UPS efficiencies are relatively low. An SPS runs at nearly 100% efficiency during normal operations, and ...