how do I properly measure capacity of a UPS battery?

Question

How can I properly measure the capacity of a UPS battery? Unplugging it from the wall is a bad idea since it loses the ground, so how can this be done?

Answer 1

If your UPS has a serial or usb port, there are some open source softwares that can read information from UPS.

Which UPS model do you have? Which OS are you using?

Answer 2

This info I get from a Back-UPS CS 500 usb consumer model connected to a debian linux server using apcupsd:

server:~# apcaccess 
APC      : 001,045,1130
DATE     : Fri Aug 20 14:32:06 CEST 2010
HOSTNAME : server
RELEASE  : 3.14.4
VERSION  : 3.14.4 (18 May 2008) debian
UPSNAME  : zolder
CABLE    : USB Cable
MODEL    : Back-UPS CS 500 
UPSMODE  : Stand Alone
STARTTIME: Sat Jun 05 17:47:29 CEST 2010
STATUS   : ONLINE 
LINEV    : 226.0 Volts
LOADPCT  :  33.0 Percent Load Capacity
BCHARGE  : 100.0 Percent
TIMELEFT :  21.4 Minutes
MBATTCHG : 5 Percent
MINTIMEL : 3 Minutes
MAXTIME  : 0 Seconds
OUTPUTV  : 230.0 Volts
SENSE    : Medium
DWAKE    : 000 Seconds
DSHUTD   : 000 Seconds
LOTRANS  : 180.0 Volts
HITRANS  : 266.0 Volts
RETPCT   : 000.0 Percent
ITEMP    : 29.2 C Internal
ALARMDEL : Always
BATTV    : 13.4 Volts
LINEFREQ : 49.0 Hz
LASTXFER : Low line voltage
NUMXFERS : 1
XONBATT  : Fri Aug 06 23:50:52 CEST 2010
TONBATT  : 0 seconds
CUMONBATT: 2 seconds
XOFFBATT : Fri Aug 06 23:50:54 CEST 2010
SELFTEST : NO
STATFLAG : 0x07000008 Status Flag
SERIALNO : BB0505019948

BATTDATE : 2008-08-13
NOMOUTV  : 230 Volts
NOMINV   : 230 Volts
NOMBATTV :  12.0 Volts
NOMPOWER : 300 Watts
FIRMWARE : 808.q5.I USB FW:q5
APCMODEL : Back-UPS CS 500 
END APC  : Fri Aug 20 14:32:07 CEST 2010

Answer 3

UPS batteries age over time, so their ability to sustain a load will diminish as it ages. It's for this reason that the more expensive UPS units include electronics built into the UPS itself that monitor battery condition, and then allow that to be queried by a port on the UPS itself. Older UPSes used straight up 9-pin serial connections for this, newer ones use USB or Ethernet. You will need some kind of software to query it, either one from a vendor or there are some open-source options.

Electrically, it is really tricky to judge what condition the battery is in without having sensors on the inputs/outputs of the battery itself before the DC/AC transformer. You can get a rough idea by pulling the plug from the wall, or pulling the circuit-breaker for that outlet, and seeing how long it takes to run out of juice. This is not good on your equipment since it'll crash, but at least you'll have an idea how long you can run before kicking off shut-down scripts.