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UPS battery testing tells you which blocks are drifting and whether the string can still carry the load for the time you need; replacement swaps the string before it fails you. Here is how each test works, what it records, and when a string is due.
You know from the trend of its readings and from a capacity test, not from its age alone. C&D's VRLA manual gives the rule for internal ohmic readings (conductance, impedance or resistance) taken on every block: a change over time of less than 20% is acceptable, a change of more than 50% needs further attention, such as a load test. When readings drift across many blocks, or a capacity test falls short of the runtime you need, the string is due.
Age sets the expectation. Eaton's battery paper notes that VRLA batteries are often sold with a 10-year design life, yet in UPS use they are replaced every four to five years on average; its aging-UPS table puts the recommended VRLA replacement at 4 years and the service life at 5.
| What happens | What is recorded |
|---|---|
| Visual inspection of jars, covers and connectors | Bulging, corrosion, heat damage |
| Measure total float voltage and current | String values against the maker's figures |
| Read each block: voltage, ohmic value, terminal temperature | Per-block readings and their change since the baseline |
| Measure connector resistance | Connections above the average of similar ones |
| Measure AC ripple where possible | Ripple current or voltage |
| Capacity test, when the trend or the plan calls for one | Discharge rate, time to end voltage, capacity found |
| Replace the string, when due | New baseline readings for every block |
Heat, charging and discharges, mostly. C&D recommends a battery room at 25 °C, within plus or minus 5.5 °C, as the best combination of performance and life; Schneider Electric's manual lists elevated ambient temperature, poor utility power and frequent short discharges as causes of shorter battery life. Eaton adds the charging side: a battery held on constant float for months is fed trickle-charge energy it does not need, which slowly corrodes the positive grid and shortens its float life. More on each factor in the UPS battery life guide.
An ohmic test is a quick reading on each block that shows change over time; a capacity test discharges the string at a set rate to prove what it can deliver. EnerSys recommends ohmic readings at quarterly intervals, following IEEE-1188, and gives a capacity test procedure (or the IEEE one) for proving capacity. The two work together: the ohmic trend says where to look and when; the capacity test gives the answer that matters, runtime. A UPS load bank test is a third thing again: it proves the UPS, and its short battery discharge is not a runtime test; see UPS and battery testing.
A planned replacement keeps the load protected, follows the maker's procedure for the UPS and the battery, and ends with readings that become the new baseline. The method covers how the load is protected while the string is out, matching the new blocks to the UPS's DC voltage and runtime, torque values for the connections, and recycling of the old blocks. Replacement blocks for data-centre UPS are listed on the data-centre UPS battery page; for battery service by brand, see UPS battery repair and replacement.
VRLA batteries in UPS service are replaced every four to five years on average, by Eaton's data, even when sold with a 10-year design life. Heat, charging and frequent discharges shorten that.
Ohmic readings quarterly, as IEEE-1188 recommends; float readings monthly; cell voltages twice a year; and a capacity test when the trend or the maintenance plan calls for one.
Sometimes, on a young string with one early failure. On an older string, a new block among aged ones tends to be overcharged or undercharged, so the decision is made from the readings.
A capacity test answers that for your string. For a first estimate, use the runtime calculator, then the maker's runtime table.
Sources, checked 2026-10-04:
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