Date: Fri, 13 Aug 2010 15:01:34 -0700
Reply-To: "Mike \"Rocket J Squirrel\"" <camping.elliott@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: "Mike \"Rocket J Squirrel\"" <camping.elliott@GMAIL.COM>
Subject: Re: 1 Solar Panel: Alternate Charging Between 2 Aux. Batteries?
In-Reply-To: <4c65aa49.9269e50a.51ea.ffff9335@mx.google.com>
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Most interesting. A couple years ago Mark Drillock was planning on
filling up the spaces on the underside of one of his vans with a passel
of small lead acid batteries instead of the bricks of marihuana he
usually smuggles across the border. For a fellow with fairly high
appliance current draw a bunch of little batteries might be quite an
improvement over one great big ol' battery. Not factoring the cost of
all them little batteries and the cost of wiring, etc., natch.
--
Rocky J Squirrel
84 Westfalia: Mellow Yellow ("The Electrical Banana")
74 Westrailia: (Ladybug Trailer company, San Juan Capistrano, Calif.)
Bend, OR
KG6RCR
David Beierl wrote:
> 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
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