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I have an APC UPS that sorta does the opposite thing to what it should: it can randomly die and start screaming, cutting the power supply of my PC. The PC works fine when connected to a regular socket. The issue arose after several years of use, so I thought it was time to change the battery. However, it now happens with the new one as well.

The behavior seems completely random: unrelated to any problems with the electricity or PC load. It can even happen when the PC is off, in the middle of the night. The only thing that helps to stop the alarm then is unplugging the PC from the UPS or turning off the switch on the power unit of the PC.

I figure, if it "feels" the PC connected to it even when it's off, there must be an accumulated charge in the power unit or something. Although, it may be normal. I haven't replaced the power unit of the PC since I started using the UPS.

Does anyone have any thoughts on what can be wrong? I'm inclined to blame the UPS itself. I heard it could be due to insufficient power, but I tried to put the PC under stress, it didn't trigger the problem.

Edit: I've installed apcupsd, here's the current output of apcaccess:

APC      : 001,034,0863
DATE     : 2012-07-01 16:54:03 +0400  
HOSTNAME : lev-home
VERSION  : 3.14.10 (13 September 2011) debian
CABLE    : USB Cable
UPSMODE  : Stand Alone
STARTTIME: 2012-07-01 16:54:01 +0400  
MODEL    : Back-UPS ES 700 
LINEV    : 244.0 Volts
LOADPCT  :   0.0 Percent Load Capacity
BCHARGE  : 100.0 Percent
TIMELEFT :  43.8 Minutes
MBATTCHG : 5 Percent
MINTIMEL : 3 Minutes
MAXTIME  : 0 Seconds
SENSE    : High
LOTRANS  : 196.0 Volts
HITRANS  : 256.0 Volts
ALARMDEL : No alarm
BATTV    : 13.5 Volts
LASTXFER : No transfers since turnon
TONBATT  : 0 seconds
CUMONBATT: 0 seconds
STATFLAG : 0x07000008 Status Flag
SERIALNO : 5B0807T16145  
BATTDATE : 2008-02-13
NOMINV   : 230 Volts
NOMBATTV :  12.0 Volts
END APC  : 2012-07-01 16:54:17 +0400
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I believe all APC UPS's have a data port that you can connect to a PC and query for status information. You may need to install and configure the included "PowerChute" software for Windows, or apt-get install apcaccess for Debian Linux. On my UPS it tells the reason for the last transfer and it may give you more information.

You only have a PC connected to this, no no laser printers, right? Those can spike up to 900w or more very briefly and are NOT a friend to your UPS unless your UPS is rated for it.

share|improve this answer
I have a laser printer, but I don't connect it to the power backup circuit. Anyway the problem is unrelated to any near-PC activity, including printing. I'll try to get some info out of apcaccess, thanks. – Lev Levitsky Jul 1 '12 at 11:59
I wouldn't even connect it to the same UPS unit, even the non-power backup ones (if that's what you meant) or even the same wall power outlet. – LawrenceC Jul 1 '12 at 12:00
I'll keep that in mind, but it's definitely not what happens to me. – Lev Levitsky Jul 1 '12 at 12:05
I updated the question with the output of apcaccess, can't see anything interesting in there yet. – Lev Levitsky Jul 1 '12 at 12:59

It indeed turned out to be a hardware problem, and was fixed by the APC's authorized service.

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