Pseudorandom thoughts generator.

I'm Niccolò Maggioni.
Student, geek and developer.

NUT & CyberPower UPS

For the impatients: skip to the settings.

Not long ago my APC Back-UPS 700 started beeping at me randomly for no apparent reason. I replaced its battery to no avail. When one day the power cut out and I found the UPS completely shut down instead of feeding power from its supply, I knew it was time to get a replacement.

So I went and bought a CyberPower CP900, which I read was a good alternative brand to APC. Given my previous experiences I wanted to try out something new instead of sticking to the same hardware again.

The previous UPS was connected with an RJ50-USB cable to a server running the NUT monitor, with another server polling status data from it with the respective client. Hardware failures excluded, this system was working fine: why not replicate it with the CyberPower too? It’s supported by NUT with the same usbhid-ups driver I used with the APC. Nice surprise: no more finicky RJ50-USB cables, but instead a plain USB-B connector.

The driver found the UPS straight away, I edited some configurations to reflect the new product name in the logs, and off we go. Smoke test (cutting power):

Happy for the new change, I kept working on something else. But after a few hours I noticed that the UPS section of my Grafana dashboard was showing no datapoints.

I SSH’d into the machine hosting the NUT monitor and quickly checked the status with upsc. It thought on it for a moment, and then came up with Error: Data stale.

Uhm… What?

It turns out that many CyberPower products reset the USB interface if a driver has not connected for 20s.

After some more fiddling I noticed that NUT was set to consider an UPS “stale” (e.g.: “Hey Mr. UPS, you haven’t spoke to me for a while. You know what? I’ll just silently assume that you’re dead and not tell anyone about that.”) after 15 seconds by default.

In order to get a CyberPower USB UPS to stay connected for more than a couple of minutes, you have to change the staleness settings in NUT to give them some headroom and the polling interval in the definition of the UPS itself.

NUT Settings

driver = "usbhid-ups"
port = "auto"
desc = "CyberPower CP900EPFCLCD"
pollinterval = 15

The pollinterval key may or may not be a needed change, but since in this configuration it works I see no harm in leaving it where it is.


All the other settings are either at their default values or not relevant to the disconnection problem.

It took me a while to find the right combination of settings, I hope I saved you some time & experimentation!


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