APC backup battery voltage puzzle

I have an APC ES-500 backup battery on a newish homebuilt PC with 19"
CRT display, 4 harddrives, and AMD dual-core CPU--not to mention the
DSL modem, wireless router, and powered USB hub. As you can imagine on
power failure--which rarely happens for real--I get a minute or two of
battery life and then it's lights out.

I have a 12-volt airplane absorbed glass mat (AGM) battery and thought
this should be an easy thing to parallel-wire to the APC unit, thus
giving me much longer backup power. But the voltages don't match!
Even though the APC battery claims it is a lead-acid gell cell. Both
batteries freshly charged and off their chargers:

Gill gell-cell airplane battery: 12.48 volts
APC gell-cell battery: 13.04 volts

Measured with a digital multimeter.

This doesn't make sense. Wikipedia
(http://en.wikipedia.org/wiki/Lead-acid_battery) says "Open-circuit
(quiescent) at full charge: 12.6 - 12.8 V" so the Gill is
underperforming which is why I pulled it from the airplane. But how
can the APC show over 13 volts? Maybe it's not really lead acid?
--
The face of a child can say it all, especially the mouth part of
the face.
- Jack Handey

On Sun, 17 Jun 2007 21:54:21 -0700, Bob Fry <bobfry@mailinator.com>
wrote:

>I have an APC ES-500 backup battery on a newish homebuilt PC with 19"
>CRT display, 4 harddrives, and AMD dual-core CPU--not to mention the
>DSL modem, wireless router, and powered USB hub. As you can imagine on
>power failure--which rarely happens for real--I get a minute or two of
>battery life and then it's lights out.
>
>I have a 12-volt airplane absorbed glass mat (AGM) battery and thought
>this should be an easy thing to parallel-wire to the APC unit, thus
>giving me much longer backup power. But the voltages don't match!
>Even though the APC battery claims it is a lead-acid gell cell. Both
>batteries freshly charged and off their chargers:
>
>Gill gell-cell airplane battery: 12.48 volts
>APC gell-cell battery: 13.04 volts
>
>Measured with a digital multimeter.
>
>This doesn't make sense. Wikipedia
>(http://en.wikipedia.org/wiki/Lead-acid_battery) says "Open-circuit
>(quiescent) at full charge: 12.6 - 12.8 V" so the Gill is
>underperforming which is why I pulled it from the airplane. But how
>can the APC show over 13 volts? Maybe it's not really lead acid?

A lot depends on measurement. After the batery is charged, it must be
allowed to rest a while and have a slight load put upon it before you
can et a reliable reading. Also, temperature is important.

>>>>> "CH" == Chris Hill <hillco@earthlink.net> writes:
>> Gill gell-cell airplane battery: 12.48 volts
>> APC gell-cell battery: 13.04 volts
>>
>> Measured with a digital multimeter.

CH> A lot depends on measurement. After the batery is charged, it
CH> must be allowed to rest a while and have a slight load put
CH> upon it before you can et a reliable reading. Also,
CH> temperature is important.

Well, before I measured them I connected them in parallel and turned
everything back on...then pulled the wall plug. APC wouldn't hold
things together for 30 secs before shutting down. I think the APC was
sensing the overall voltage, seeing it low, and sent a shutdown
signal.

Has anybody had any success in wiring an extra battery in parallel to
a battery backup device?
--
I was taking my little nephew to disneyland, but i decided to
play a mean trick. i pulled up next to an old, burned down
warehouse. "Oh no," i said. "Disneyland burned down." He started
crying, and i was about to tell him it was all a joke and drive
to the real disneyland, but it was getting kinda late.
- Jack Handey

On Sun, 17 Jun 2007 21:54:21 -0700, Bob Fry <bobfry@mailinator.com>
wrote:

> This doesn't make sense. Wikipedia
> (http://en.wikipedia.org/wiki/Lead-acid_battery) says "Open-circuit
> (quiescent) at full charge: 12.6 - 12.8 V" so the Gill is
> underperforming which is why I pulled it from the airplane. But how
> can the APC show over 13 volts? Maybe it's not really lead acid?

You have to wait about 24 hours before you measure!

>>>>> "K" == Ken <_ken_3_@telia.com> writes:

K> On Sun, 17 Jun 2007 21:54:21 -0700, Bob Fry
K> <bobfry@mailinator.com> wrote:

>> This doesn't make sense. Wikipedia
>> (http://en.wikipedia.org/wiki/Lead-acid_battery) says
>> "Open-circuit (quiescent) at full charge: 12.6 - 12.8 V" so the
>> Gill is underperforming which is why I pulled it from the
>> airplane. But how can the APC show over 13 volts? Maybe it's
>> not really lead acid?

K> You have to wait about 24 hours before you measure!

OK, fine. But why would the parallel wired combo (aircraft gellcell +
APC gellcell) show a decrease in battery battery-only sustaining time,
compared to APC battery only?

Maybe the aircraft battery was well and truly bad...?

--
When Armageddon comes, it would be good to be an Olympic athlete,
because running real fast and jumping over stuff could come in
handy.
- Jack Handey