Date: Mon, 5 Apr 1999 19:37:42 -0400
Reply-To: David Beierl <synergx@IBM.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <synergx@IBM.NET>
Subject: Re: 2nd battery...an alternative??
In-Reply-To: <00c901be7fb6$f0fc6200$281ba2d1@newsys98>
Content-Type: text/plain; charset="us-ascii"
At 15:52 4/5/99 -0700, George Chapman wrote:
>Volks:
>
>recharged by the alternator). Both batteries can be drawn
>down relatively frequently, but the auto battery prefers to
>be immediately recharged.
I'm afraid there's a slight misconception here. A starting battery is
*not* designed to be discharged fully; it expects to run fully charged
pretty much all the time. I would not be at all surprised if an average
starting battery were ruined by twenty or fewer full discharge cycles.
What the starting battery *is* designed for is to produce enormous current
for a very short period. Starting, in fact. To achieve this it gives up
vibration resistance and deep-discharge capability, and it is lighter and
smaller and costs less than a deep-cycle battery for a given
current-delivery ability. But this makes perfect sense for a starting
battery -- supposing a vanagon draws 250 amps for 15 seconds while
starting, that is only *one* amp-hour draw. A battery rated at 600 CCA is
only promising to deliver *five* amp-hours total -- granted it's in the cold.
>
>Another thought is that deep-cycle batteries do have, in
>general, a limited number of "cycles" they will provide.
>The "deeper" a deep-cycle battery is discharged (below 50%
>discharge, in general) in each cycle, the shorter the
>overall battery life.
Yes. This is true of all lead-acid batteries. A battery that lasts 200
cycles if drawn down to fifteen per cent charge, may well last 1000 cycles
if only drawn down to fifty per cent. Also, as a practical matter it is
extremely difficult to charge a battery above 85%, so if you go by the
conservative advice you will only get to use 35% of the battery's capacity.
On our boat we discharge well below 50%, accepting that we will get fewer
discharge cycles in return for getting more amp-hours from a given weight
of battery. We're trading dollars for weight/space.
>
>In determining power usage of equipment (load in amps) be
>certain that all power loads have been converted to the same
>units. (A laptop computer rated: 1amp@ 110V would be drawing
>approximately 10x's that from 12V battery) Power (watts) =
>applied voltage x circuit current in amperes (amps).
In addition to this, you have to consider the rate at which power is being
drawn. Battery capacities are specified at the so-called 20-hour rate,
i.e. a load current which will discharge the battery in 20 hours. But the
faster the discharge, the less total power is available. At a one-hour
rate the total power available might be as little as one-tenth of that
available at a 20-hour rate -- although some additional power would be
available after a rest period, for a lower discharge rate.
Incidentally, in the marine world there are a number of systems available
which 1) control charging voltage much more intelligently than the standard
automotive regulator, which is built with the assumption that it will be
supplying a fully-charged battery most of the time, and 2) directly measure
power in and out of a battery as amp-hours or as a percentage of full
charge, etc. These systems aren't cheap (several hundreds of dollars) but
they can be very useful for people who want to get maximum charge for
minimum engine time, and who want to manage charge/discharge cycles to
maximise battery life. West Marine http://www.westmarine.com has a
selection, as well as gelled-electrolyte deep-cycle batteries (for
something under $300 each as I recall). Gelled-electrolyte batteries can
accept a very rapid charge, but they are quickly ruined by charging
voltages above 13.98 volts, so they should usually be bought in conjunction
with a management system that understands them.
david
David Beierl - dbeierl@ibm.net
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