How does APC UPS determine bad battery?

By robo_dev ·
just picked up two nice rack-mount APC UPS units, free.

One seems to work fine, the other one has the 'battery fail' light on red. Tried swapping battery pack to other UPS, it too lit the battery-fail light.

The battery pack tray consists of four 12V gel cell batteries, two in series, two in parallel (24V output).

The good pack measured around 23V output, but the bad pack measured around 21V.

Each of the individual batteries in the bad pack measured in the 11V range. Neither pack has been charged.

I would assume that the UPS is looking at more than just voltage?

Internal resistance of the battery?

Or does it just try to put a load on the battery, and if the load drops, it lights the red light?

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by shasca In reply to How does APC UPS determin ...

Most Smart Ups' from APC come with management software. Try installing it and it will give you more insight as to what it sees as failing. Sometimes the warning lights can be reset for false positives. It will also let you control shutdown functionality

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Charging parameters

by mjd420nova In reply to How does APC UPS determin ...

Most all errors that occur that result in the battery fail indication are from an excess of current being needed to achieve a specific increase in the battery voltage. If that current is excedded over a certain amount of time, it indicates a failed cell in the battery that is "bucking" that cell being charged. Restting the circuit after swapping the battery packs is needed to achieve a true indiction of the battery condition. If that fails to clear the fault, then the problem lies in the charging circuit itself. Look on the board for a small "pico" style fuse that may have blown due to a bad cell. These are usually soldered to the board. When replacing them, I like to mount small standoff pins soldered to the board to hold the fuses so they will be easier to replace the next time.

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