Dennis DeCoster, Mission Critical West, Inc., Redondo Beach, Calif.

Battery Life

Flooded-cell batteries, sealed-cell batteries and flywheels are today's practical choices for reserve power in uninterruptible power supply (UPS) systems. Flywheels are useful for certain space-critical or extreme cycling applications but provide less than one minute reserve, cost more than most batteries and are subject to bearing reliability and seismic issues due to high spin speeds.