BN900M didn't charge battery after power fail until I turned computer back on

[C7238]

I have a BN900M battery backup protecting my desktop computer. I think the BN900M doesn't charge the battery after a power failure unless there is a load on the BN900M.

Here are the circumstances that make think that.

Over the holiday weekend, I had a 3 hour power failure. I wasn't home. I assume my computer shut down when the BN900M battery drained. My computer is configured in the BIOS to not start back up after a power failure. So, when I got back home my computer was still off. 

I powered the computer on, but it powered off when the logon screen appeared. I do remember hearing a relay click in the BN900M when I powered on.  I powered the computer on again a couple of more times and I got further into the boot process each time.

On the third power up, the computer stayed on. But, because I heard the click, I suspected a BN900M failure of some sort. So, I opened the Powerchute app. I noticed some interesting things:

1. It said the power failure lasted 23 hours and 57 minutes. That's not true. I was 23 hours and 57 minutes since the power failure STARTED, but it only lasted about 3 hours.
2. It said the battery about 10% charged. That didn't seem right. It should have fully charged in the prior 20 or so hours.
3. Slowly over the next few hours, the BN900M reported a 100% charge.

So, does the BN900M really not start charging until there is a load? That doesn't seem like correct behavior.

[Teken]

All APC Back-UPS Series models begin charging once AC Mains is presented. The only time this normal condition may deviate is based on the following factors / conditions:

RBC: If the Replacement Battery Cartridge (RBC) is fully depleted the system will not turn back on until it’s at a defined voltage level.

If the RBC is EOL and won’t accept a charge the unit will not turn on. If the RBC is hot and this condition is detected the system will not start / charge.

Some Back-UPS Series Models regardless of battery (SOC) require the end user to press the power button to turn back on.

This is determined by the firmware present on the system and is not user adjustable or upgradeable.

Charger: In some cases the charging circuit is failing and thus isn’t recharging the RBC. This is a vicious circle as the lack of proper charging will begin to damage the RBC and reduce the cycle life  which is 260 full cycles.

The average service life of the RBC is 2-5 years based on usage, temperature, and environmental conditions. If the RBC is near the 3-5 year mark it’s time to consider a replacement.

The APC Runtime Calculator will provide you a general idea of where you are in terms operational runtime based on a defined load (Watts) and ambient temperature of 25’C when compared to a new RBC.

NOTE: Do not conduct a Lights Out / Grid Down / Battery Calibration test if the RBC is 3-5 years old. Unless you’re ready to replace the RBC now as any cartridge that is EOL / marginal will fail in short order! ☝️

Questions Ask . . . 👍

[C7238]

So, I can indeed reproduce the problem by pulling the UPS plug from the wall. This UPS clearly doesn't want to charge if there is no load on it.

That said, the UPS only gave me three minutes before it signaled the PC to hibernate and shut down. I'll replace the battery.