APC SURT1000XLI failed gradually over several days. It would run normally then the red FAULT LED would light up and it would switch line voltage straight through to the output. On restart it would sometimes start normally. Other times it would respond to Power On button with a single beep but no start-up; attempt to start only after the O[ff] button was pressed following Power On; or start with a different/unusual sequence of lights (which I didn’t capture). Most commonly it turned on the FAULT LED rather than BATTERY FAIL. When it finally failed terminally, start-up sequence was to boot normally (i.e showing expected pattern of lights) and coming online for a few seconds then failing with BATTERY FAIL on.
Disconnected battery and tested voltage – 52V as expected with 4x12V in series. Checked and tightened all connectors. No change. Wasn’t until I tested battery pack on load tester that it turned out its output was dropping to 0V with no load. Pinned it down to one battery that read 13.5V when isolated but simply dropped out under the slightest load. Replaced it with an old, functional battery and all working fine. Posted in case others encounter similar strange behaviour on startup that could have a simple solution.
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