Back-UPS - no backup on power failure

[Anonymous user]

We had a power failure yesterday and my computer, which is connected to a Back-UPS Pro 420, shut down immediately. Unfortunately, I lost my work.

This is what I would expect if the battery needs to be replaced, but the "replace battery" light isn't on.

Any ideas what could cause this behavior? I would hate to buy another battery only to discover the problem was elsewhere.

[TheNotoriousKMP_apc · Accepted solution]

When was the last time the battery was replaced? The BP420's have been discontinued for quite some time, however the batteries are still readily available.

It could be that even though the UPS is passing the self-test, the battery only has just enough juice to "trick" the UPS into thinking it can sustain a load for longer than 15 seconds, when in reality, it cannot.

[Anonymous user]

Thanks for the prompt reponse. I bought the unit in Dec 2001. The OEM battery lasted until Feb 2006. That's 50 months. The current battery is only 30 months old.

I did another test, this time with the computer powered down. There's an LED on the MOBO that remains lit as long as the power supply is active. When I pulled the UPS plug, the mobo LED turned off immediately. In fact, the UPS "on battery" light hardly had time to come on before turning off, perhaps in less than 300 ms.

As a final test, I swapped batteries with another UPS. It's definitely the battery. I guess the 'aftermarket' battery wasn't as good as the OEM...?

What I don't understand is why the self-test routine doesn't flag a battery that can't even sustain an unloaded power supply for a full second. When the OEM battery wore out, the self test worked as it should. I recall testing the unit before replacing the battery back in 2006 and even after the 'replace battery' light came on, it could power my computer long enough for a normal shut-down.

I see UPS prices have dropped quite a bit since I bought that unit (I paid $190, which was the best price at the time). I guess I should just buy a new UPS rather than replace the battery so I'll have a properly functioning self-test.