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Battery testing, explained
There are two ways to find out whether your standby battery will actually work when the grid drops. Both have their place, and neither is optional if the load is critical.
Impedance testing: fast, non-invasive, indicative
An impedance test injects a small AC signal into each cell and measures the response. It takes minutes per string, doesn't discharge the battery, and gives you a reading that correlates with the cell's internal condition. Rising impedance over time is an early warning of a cell that is drying out or losing capacity. The catch: impedance is indicative, not definitive. It tells you a cell is deteriorating relative to itself or its neighbours. It does not tell you what capacity the string will actually deliver.
Load-bank testing: the truth
A load-bank test discharges the string at a constant current — typically the manufacturer's 8-hour or 3-hour rate — while measuring voltage and time. When the string reaches its end voltage, you know exactly how many amp-hours you got. This is the number your UPS will see in an outage. It takes hours. It stresses the whole string, the cabling, the breakers, and the disconnects — which is a feature, not a bug: if something is going to fail, you'd rather find out under a controlled test than at 03:00 in a real event.
How they fit together
The common pattern: impedance testing every 6-12 months as a routine health check, load-bank testing every 2-3 years as the definitive capacity proof — and always at commissioning acceptance. If impedance readings spike between load-bank cycles, that's your signal to advance the next full discharge test.
Who signs the report
The value of a test is the report that comes out of it — signed, dated, referenced to procedure, and defensible to an auditor. For that, you want an independent testing team, not the vendor who sold you the batteries and wants to defer the replacement conversation.