Load bank testing
A UPS load bank test proves the power train at known load steps; a battery capacity test proves how long the batteries can carry the load. They answer different questions, and a data-centre UPS needs both, each on its own schedule.
It checks the UPS power train under a known load: rectifier, inverter, static switch, contactors, controls and every connection between them, at the steps the method sets. In Eaton's method, voltage, current and regulation are watched at 50% load for one hour and at 100% load for 3 hours; its performance-test scope adds an infrared scan of the UPS and battery cabinets at each step and a power-quality meter on the output, recording voltages, currents and harmonic content. Hot spots are noted with their actual temperatures, so a connection that is starting to fail can be fixed before it does.
It shows that the batteries pick up the load cleanly and that the UPS behaves while they do. Eaton's method opens the rectifier input for a discharge of up to 4 minutes, monitors the UPS operating parameters, then restores the input and lets the batteries recharge; its performance version scans the battery cabinets with an infrared camera during the discharge. A few minutes on battery is a good test of the transfer, the DC path and the connections. It is not a runtime test.
Runtime is proven by a capacity test: a controlled discharge at a defined rate until the batteries reach their end voltage, timed and compared with the rated value. EnerSys's VRLA manual gives its own capacity test procedure and points to the IEEE procedure as the alternative, and recommends internal ohmic measurements at quarterly intervals, following IEEE-1188, to track each block between capacity tests. An ohmic trend tells you which blocks are drifting; a capacity test tells you what the string can actually deliver.
See battery replacement and testing for how the readings are taken and recorded, and the runtime calculator for a first estimate. When a string is due, compatible replacement blocks for data-centre UPS are listed in this catalogue of replacement blocks for data-centre UPS.
It runs on the bypass path while the UPS output feeds the load bank. Eaton's method moves the critical load to bypass and then to maintenance bypass before shutting the UPS modules down, and the electrician connects the load bank to the output with the UPS locked out. On some UPS the batteries are not available while the unit is in maintenance bypass, as Schneider Electric notes for the Galaxy VS, so for the length of the test the load has utility power only. That is why the window is agreed in advance and the infrastructure is checked: it must carry the critical load and the test load at the same time.
Some large UPS can load their own power train. Vertiv's safe capacity test on the Liebert Trinergy Cube and the Liebert EXL S1 circulates power from the inverter back to the rectifier through the static bypass, so the power train runs at load with no external load bank; the engineer sets the load in steps up to an overload of 120%. Vertiv points out that an external load bank creates a hazardous environment through all the cable connections it needs, which the closed-loop test avoids. Its battery discharge mode back-feeds the utility, which must be accepted first.
For maintenance visits and contracts across Canada, see UPS battery repair and replacement service.
No. Its battery discharge lasts minutes (up to 4 in Eaton's method). Runtime needs a capacity test to the battery maker's procedure.
EnerSys recommends internal ohmic readings at quarterly intervals, following IEEE-1188, with capacity tests when the trend or the maintenance programme calls for one.
Ohmic and voltage readings, yes. A capacity test discharges the string, so the load is protected another way during the test, and the plan says how.
The load steps and readings, infrared findings with temperatures, power-quality readings at the output, UPS parameters during the battery discharge, the transfers, and the findings and recommendations.
Sources, checked 2026-10-04:
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