[CTSARA] AGM Battery Questions

[Jon Perelstein]

I would like to get a better idea of how long I can use an AGM battery
while powering a laptop computer.  The battery is a 79Ah AGM battery that I
bought from West Mountain Radio a couple of years ago (before West Mountain
was sold).  When I look at the various charts for AGM batteries on-line,
they seem to be contradictory.

And no, I don't have the user instructions that came with the battery.  I
know I filed them away very carefully (and not in the circular file
either), but I can't find them in an of the places I can think of looking
for them.

Conditions - the laptop is a Dell Inspiron running Win 7 with a Celeron
processor.  I had the AGM battery connected to a Targus inverter to
generate AC, and was using that to power a standard Dell laptop charger to
run the computer.  The computer's battery was fully charged.  I had WiFi
turned off, the screen at a medium brightness, but did not allow the
machine to go into sleep mode.  For applications, I was updating and then
saving an Excel spreadsheet for about 5 minutes every 15 minutes, and
immediately after was sending the spreadsheet using Winlink for about 3
minutes.

Using a Watts Up, I found that the computer was averaging around 1.7 amps
draw - giving a power utilization of about  22 watts, which is a bit high
for what Dell suggests I should have, but not surprising given the overhead
of the inverter.  When I was saving the spreadsheet or when Winlink was in
connection, the draw was about 2.2 amps.  When I wasn't doing anything, the
draw was around 1.4 amps.

The battery had been fully charged prior to the start of operations.  After
about 1.5 hours of use, battery voltage was down to about 12.5 volts.  By
about 2 hours into operation, voltage was down to about 12.4 volts.

The various tables I see on-line suggest that I shouldn't allow the battery
to go below about 50% load, but then different sources claim anywhere from
12.0 volts to 11.0 volts for that magical 50% mark.  To make matters worse,
the various tables on-line for time was current suggest that the battery
might be good for 6-8 hours at that current draw, or might be good for
16-18 hours at that current draw, or somewhere in between, depending on
which source I used.

I would appreciate any insights.

73s
Jon, WB2RYV

P.S.  For extra credit, I would appreciate pointers to any good articles on
how to power a laptop from a 12v battery.