Nut upsuser - Feb 2013 - APC UPS BR550GI continuously rebooting and workaround

OS: Linux BS801-TEC-02 2.6.32-279.22.1.el6.x86_64 #1 SMP Wed Feb 6 03:10:46
UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
CentOS release 6.3 (Final)

Nut version: nut-2.6.5

NUT installation method: from source tarball
  ./configure --with-user=ups --with-group=nut --with-usb --prefix=/ups
--localstatedir=/ups/var --with-pidpath=/ups/var/state/ups
--with-statepath=/ups/var/state/ups --enable-static

UPS : APC BR550GI

Repeatable: always

Motherboard's BIOS AC back function is configured to Full-on.

Problem: UPS repeatedly turns on and off.
Here's what has happen leading to that problem

1. NUT issues upsmon -c fsd after onbattwarn timer timeout.
2. services are turned off including upsd and upsmon.
3. /etc/init.d/halt issues upsdrvctl shutdown.
4. but before UPS could totally turn off, the mains power return. this is
an unexpected event.
5. after 1 min the ups will turn off its output and display LCD. cutting
power on its load
6. after 5 sec the UPS will turn on and supplies power to the load.
7. Connected PC with turns on.
8. shortly after the POST the UPS will transfer back to battery while the
system is still booting.
9. after 1 min the ups will shutoff its output and display LCD. cutting
power on its load.
10. after 5 sec the UPS will start again and supplies power to the load.
11. and repeats steps 7 onwards.

Workaround: I modified halt to start the nut driver again(upsdrvctl start)
before root partition goes read-only. Once I did that the UPS did not
transfer to battery (step 8).

Driver Debug output:  http://pastebin.com/GgLHq1AM

UPSC:
battery.charge: 100
battery.charge.low: 89
battery.charge.warning: 50
battery.date: 2001/09/25
battery.mfr.date: 2010/11/24
battery.runtime: 51900
battery.runtime.low: 120
battery.type: PbAc
battery.voltage: 14.1
battery.voltage.nominal: 12.0
device.mfr: American Power Conversion
device.model: Back-UPS RS 550GI
device.serial:
device.type: ups
driver.name: usbhid-ups
driver.parameter.pollfreq: 30
driver.parameter.pollinterval: 2
driver.parameter.port: auto
driver.version: 2.6.5
driver.version.data: APC HID 0.95
driver.version.internal: 0.37
input.sensitivity: medium
input.transfer.high: 282
input.transfer.low: 176
input.voltage: 230.0
input.voltage.nominal: 100
ups.beeper.status: enabled
ups.delay.shutdown: 20
ups.firmware: 857.L1 .I
ups.firmware.aux: L1
ups.load: 0
ups.mfr: American Power Conversion
ups.mfr.date: 2010/11/24
ups.model: Back-UPS RS 550GI
ups.productid: 0002
ups.realpower.nominal: 330
ups.serial:
ups.status: OL
ups.test.result: No test initiated
ups.timer.reboot: 0
ups.timer.shutdown: -1
ups.vendorid: 051d
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