Why battery testing needs a real load
A UPS autonomy figure assumes full load, and battery capacity falls measurably with age. The only way to obtain the real backup time is to load the UPS to rated power with a load bank and discharge the string to its end-of-discharge voltage. Estimates from the battery monitor alone are frequently far off.
Test sequence
A typical sequence: no-load start-up check; stepped loading at 25/50/75/100% with efficiency and output waveform recorded; overload capability per the specification, usually 110/125/150% for different durations; a battery discharge run after simulated mains failure; transient capture on transfers between inverter and bypass; and load-sharing accuracy on parallel systems.
Selecting the load bank
Space inside the room is limited, so rack-mounted and wall-mounted banks are the usual choice, installed near the UPS output distribution. With air-cooled units the room cooling margin has to be checked; for megawatt-class UPS systems or rooms with little spare cooling, liquid-cooled rack loads carry the heat out through the chilled-water circuit instead.