Date:         Fri, 13 Aug 2010 16:19:23 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: 1 Solar Panel: Alternate Charging Between 2 Aux. Batteries?
Comments: To: neil n <musomuso@GMAIL.COM>
In-Reply-To:  <AANLkTimkZ6JLGJA0PGa4=ErGPjMXJbp5JTv9FbVdFQP+@mail.gmail.c om>
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At 02:10 AM 8/13/2010 Friday, neil n wrote:
>Curious if one could save some money by purchasing 1 quality higher
>output solar panel that could charge each battery by x amps/day as
>opposed to 2 panels charging these two batteries (in parallel) while
>appliance is connected.

There's a tradeoff -- increased panel efficiency vs decreased usable
capacity in the battery because of load current being a higher
fraction of nominal capacity.

The empirical equation describing this was developed by a man named
Peukert, and it looks like this:

t (hours of use) = H (C/IH)^k  where:

C is the amp-hour rating of the battery
H is the discharge time in hours that the rating is quoted for
(typically 20 hours)
I is the instantaneous discharge amperage
k is a constant that for lead-acid batteries generally is between 1.1 and 1.3.

For example:
Your fridge draws 7 amps.  Your battery is rated 85 amp-hours at 20-hour rate.

t = 20 x (85/(7x20))^1.1 = 11.55 hours, or effectively 81 amp-hours
capacity best case.
t = 20 x (85/(7x20))^1.3 = 10.45 hours, or effectively 73 amp-hours
capacity best case.


For an appliance drawing two amps the difference would be much less;
but for something drawing say 25 amps:
t = 20 x (85/(25x20))^k = 71 amp-hours best case and 50 amp-hours worst case.

For two batteries in parallel at the higher draw:
t = 20 x (170/(25x20))^k, or 76 amp-hours *per battery* best case and
61.5 amp-hours per battery worst case.

So the larger your load currents, the more disadvantage there is in
drawing from only one battery at a time.  This is the factor you have
to balance against the per cent greater efficiency of the fancier panel.

Yours,
David