UPS switches to battery power when plugging in devices?

Over the past week or so I've had my UPS switch over to battery power briefly when plugging in new devices (possibly limited to just AC -> DC chargers, but haven't tested anything else). I had noticed it originally when plugging in a cell phone charger - even without the phone connected, just plugging the charging cable into an outlet triggered the UPS to switch.

Last night it happened again when plugging in my laptop charger: I plugged in the charger first without it being connected to my laptop, and the UPS switched back and forth for a few seconds between utility and battery before stabilizing. Once it stabilized, I plugged the charger into my laptop and everything was fine.

The UPS is a CyberPower CP1500AVR. pwrstat on Linux doesn't seem to report any outages, perhaps simply because it's too quick of an occurrence:

$ pwrstat -status
...
Test Result.................. Passed at 2012/11/09 10:23:02
Last Power Event............. Under Voltage at 2012/10/08 12:20:01 for 3 sec.

pwrstat.log showed an event last night when I plugged my laptop in, but none of the other recent events are there:

2012/09/27 05:09:33 AM Utility failed.
2012/09/27 05:09:35 AM Utility restored.
2012/10/08 12:20:01 PM Utility failed.
2012/10/08 12:20:04 PM Utility restored.
2012/11/09 01:05:11 AM Utility failed.
2012/11/09 01:05:11 AM Utility restored.

To help narrow down causes a little, I believe the first time I noticed this was during/after Hurricane Sandy. During the hurricane I had seen a spark fly off the utility pole. The UPS had briefly switched to battery power then, but nothing else seems to have resulted from the spark (others in the house didn't notice anything at the time)

Is there any other testing I can do, or is it time to call the power company?

Jack Suter

Posted 2012-11-09T15:54:05.343

Reputation: 21

If the UPS isn't damaged, then an "under voltage" means the utility power dropped below 85 volts. I wonder if something is horribly wrong with your utility power. Do you have an AC voltmeter? – David Schwartz – 2012-11-09T16:33:36.057

I don't have a voltmeter, unfortunately. The UPS reports utility voltage as being 119-120v, but that is when everything is running normally. Given that the 'event' lasts only a few seconds, would a voltmeter be feasible, or would it be better to look into a power monitor/logger? – Jack Suter – 2012-11-09T17:15:46.820

@DavidSchwartz For now I've got a makeshift power monitor running - pwrstat is running in a while loop and recording the Utility and Output voltages. I'm not sure how quickly those values update, but hopefully it'll be fast enough to see if there's a momentary sag. I haven't been able to get it to trip yet, but hopefully later tonight I'll know. – Jack Suter – 2012-11-09T17:43:41.807

Thanks for your input. The UPS is reporting a 135 watt power draw (laptop, headless server, speakers, lamp) and is rated for 900 watts, so it's a fairly light load. The self test passes without any problems, although I realize that doesn't cover all aspects of it. I should also mention that the house is 60 years old, so I'm more tempted to try to place the blame on the wiring than the UPS. The UPS was purchased in Feb 2011. – Jack Suter – 2012-11-09T17:28:52.043

A UPS conditions power that comes in, and if anything was wrong with your house wiring, the UPS should still function correctly to devices connect to it - I am 99% sure here that the UPS is malfunctioning. – William Hilsum – 2012-11-09T18:42:02.603

What I've seen with this particular UPS is that if the voltages are still within "acceptable" ranges they will pass through - I've seen it report anywhere from 115v to 122v before. If it drops below a certain point (I think I read 90 volts before, David had said 85 volts) it'll switch over to battery power. The UPS is still providing power when the new device is connected, just that it's momentarily switching to battery power before going back to utility. Now I wish I had gotten a power conditioner as well. – Jack Suter – 2012-11-09T19:07:47.160

UPS switches to battery power when plugging in devices?

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