[CTSARA] Update on AGM Battery Questions

[Jon Perelstein]

I would like to thank the people who responded to my post about AGM
batteries ("AGM Battery Questions").

There was a lot of agreement that I should get a direct DC-to-DC converter
and not go through a combination of an inverter and computer AC-to-DC
charger.  I've ordered a DC-to-DC converter from Powerstream  (
http://www.powerstream.com/ADC-p006.htm).

Joel Hallas recommended that I look into a battery boost regulator, which
keeps the voltage constant as the battery's voltage output drops (up to a
certain point, of course).  A typical such regulator is shown at
http://www.mfjenterprises.com/Product.php?productid=MFJ-4416B (note that
Joel did not recommend or not recommend this unit, I'm just giving an
example).  Joel also pointed out that the effectiveness of a boost
regulator depends on the voltage/current needed by the computer.  If the
computer can operate down to 11 volts (for example), then a boost regulator
isn't going to buy much.  I noted that the computer's battery is rated as
11.1 volts.

Charles pointed out that any rule of thumb based on capacity is based on
Amp Hours (Ah) and not voltage - and that correctly measuring the Ah
requires some specific test equipment.  Be that as it may, most people
thought that a specific voltage could be picked for a specific battery
based on the manufacturer's data sheet for the battery - it wouldn't be
perfect but if used conservatively would help protect the battery.

A few others reminded me that the capacity of a battery changes over time
and that it's always good to test with a battery analyzer every so often.

Overall, those people who expressed an opinion felt that I should be able
to get 6-8 hrs of operation with the 79Ah battery (without a boost
regulator) given the current draw.

I have an extended life laptop battery on that computer (6.6 Ah/87 wH) that
gives me 3-4 hours in the field under similar usage patterns.  The battery
I have is
http://www.amazon.com/Extended-Replacement-Laptop-Battery-Inspiron/dp/B005U2VAW0/ref=sr_1_1?ie=UTF8&qid=1402409995&sr=8-1&keywords=inspiron+battery+1525,
but there are many other similar batteries on the web.

Two of those laptop batteries would give me 6-8 hours of operation, weigh
about a gazillion pounds less than the 79Ah battery, cost at least one arm
(if not a leg) less than one 79Ah AGM battery, and wouldn't require a
DC-to-DC converter.  That would imply that I'm better off just buying a
couple of those batteries to power my laptop than dragging a 79Ah AGM
battery into the field with me.  Now, however, I'm curious as to why a
couple of 6.6Ah laptop batteries would be so much more useful for the
laptop.

73s
Jon, WB2RYV