Date: Tue, 25 Nov 2003 02:04:46 -0500
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: An overview of 14 electric math?
In-Reply-To: <32E76014-1EA5-11D8-856F-000393DB2980@knology.net>
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At 12:39 PM 11/24/2003, Felder wrote:
>Anyone want to offer an easy-to-follow, practical way to to figure how
>much battery it will take to run lights, fans, etc. in the real 12v
>world?
>What would be helpful is a way to determine the amperage of various
>appliances
Measure it with an ammeter with engine not running (most devices will draw
more current as voltage goes up, but some things with motors running loaded
and certain devices with smart power supplies could draw more as voltage
drops).
>and then figure what the rule-of-thumb efficiencies would be
First approximation is just add the current drains together to get an
hourly amp-hour consumption. Decide how many amp-hours of battery capacity
you can/want to use, divide by hourly rate to get number of hours. Assume
that as you draw current faster and faster it will draw down the battery
more than you'd expect just by adding the drains. Assume that discharging
a deep-cycle battery to 10% will reduce its life by at 4/5 compared to
never going below 50%. Discharging a starting battery to 10% will ruin it
in a few cycles unless it's a special like the Optima (red-top?) which is
rated for 50 deep discharge cycles over its lifetime. Assume that you
won't be able to economically charge above about 85-90%, and that charging
using only the built-in automotive system will likely not get that high and
will permanently reduce the capacity of the battery by reason of it never
being fully charged.
To quantify that excess drain resulting from high discharge current
somewhat, raise the *total* drain in amps to the power 1.4 and use that
value to compute the amp-hours. With luck the actual results will be
better than that.
To get closer than that (the exponent depends on the construction of the
battery and may range from about 1.05 to 2...big difference in
answers. That exponenent is the value "n" in Peukert's equation, and may
be available from the battery manufacturer for a given battery model),
consult with mfr and/or quantify it by measurements, or adjust it over time
to match the results you get. Useful description here
http://www.amplepower.com/pwrnews/beer/ -- much more info on that site
about battery management, charge, discharge, capacity etc. and by searching
"peukert equation" on the web. Batteries and battery maintenance are a
huge subject, can spend your life learning about it. And a good hunk of
change as well, depending on what you want to do. If you use batteries to
supply significant amounts of power you will certainly spend money -- on
some combination of fuel, replacement batteries, monitoring gear and either
automatic or manual charge regulators to replace the one built in to the
alternator (and maybe extra cooling for the alternator, or an alternator
rated to run full-out for long periods at high temps. The more power you
use and the faster you use it, the more important it will be to make
choices that are expensive up front -- at the extreme, you're looking at
gel or AGM deep-cycle batteries of four times your daily consumption or
three times your maximum current drain; smart voltage regulator, hot-rated
alternator, smart amp-hour monitor, smart AC charger -- *each* of those
choices could be some hundreds of dollars. No problem at all spending a
thousand up front. At the other extreme you run bone stock and spend lots
of time charging at low efficiency and replace your starting battery a lot.
good luck and enjoy, it's pretty interesting subject,
david
>to then determine how many hours you could run various combinations of
>things
--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"
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