Date:         Sun, 17 Jun 2007 23:56:33 -0700
Reply-To:     Michael Elliott <camping.elliott@GMAIL.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Michael Elliott <camping.elliott@GMAIL.COM>
Subject:      Re: Solar panels for your Vanagon.
Comments: To: roadguy@roadhaus.com
In-Reply-To:  <009001c7b156$4a25e610$febad34b@t41>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Hi Larry,

For those calculations I used 12.6 volts only as a way to derive a rough
measure of ampere-hours from the panel wattage x hours.

For my application, I picked a "float" voltage of 14.2 volts for the
unsealed lead-acid battery and programmed the controller with this
value. In the morning the controller dumps as much current into the
battery as it can derive from the panels, about 6 amps. The current
stays pretty high, typically around 4A, all morning long until the
voltage gets close to 14.2 volts, then it tapers as the voltage
approaches this float voltage. Once the voltage has reached the
setpoint, the controller adjusts the current as needed to hold it there.
Usually by noon only about 0.8A is required to stay at this float
voltage. This is my definition of a fully-charged battery: one needing
less than 1A to stay at 14.2 volts.

The battery monitor is a coulomb-counter -- it measures amps in and out
of the battery and estimates its charge condition from that. In the
morning before the sun is on the panels it usually shows the battery at
about 82% full. The voltage is around 12.4 volts, I think, unloaded.

Coulomb counters can't accurately model what goes on in a battery, so
this model is programmable to reset itself to indicate 100% charged when
voltage, current, and time conditions have been met. I "told" it that
when the voltage is greater than 14.1 volts and the charge current is
less than 1 amp, and these conditions persist for more than 5 minutes,
the battery is to be considered fully charged. This monitor gets things
pretty close. As charge flows into the battery, the monitor shows the
charge condition getting closer and closer to 100%, and by the time the
volt/current/time conditions I have specified are met, it generally
thinks I am in the red about 1.2Ah before it re-sets itself to 100% (full).

Once the battery is charged and the controller is holding it at 14.2
volts with that 0.8A trickle, the refrigerator cycles on and off, a
laptop might be connected, but the controller keeps the voltage and
battery charge current stable. It also has a thermocouple connected to
the battery to compensate for temperature. I haven't tried it, but I bet
that during sunlight hours I could disconnect the battery altogether and
run everything off the panels if the controller doesn't go wacky w/o a
big lead-acid battery hung on it.

Your assumptions are good ones, and for many cases conservative
assumptions are desirable. But let's face it: if we were trying to
design for worst-case scenarios, we probably wouldn't use 20 year-old
vehicles. For life-and-death survival my solar power system isn't designed.

--
Mike "Rocket J Squirrel" Elliott
71 Type 2: the Wonderbus
84 Westfalia: Mellow Yellow ("The Electrical Banana")
74 Utility Trailer. Ladybug Trailer, Inc., San Juan Capistrano
KG6RCR



Larry Chase typed:
> Mike,
>
> For the most part I agree with your calculations, they only thing that
> puzzles me is how your batteries can be fully charged by noon.
>
> Please define what you mean by fully charged.
>
> What is the battery voltage?
>
> In my system, the charge controller (led lights) will show a fully charged
> state in about the same time frame you reference. But in reality the
> batteries are only
> At a partial charge of around 75% or around 12.3 volts.
>
> As you suggested ... my recommendations are based on fairly conservative
> assumptions that would cover lots of solar conditions and locations.
>
> Larry
>
> - - -
>
>
> Date:    Sun, 17 Jun 2007 18:47:45 -0700
> From:    Michael Elliott <camping.elliott@GMAIL.COM>
> Subject: Re: Solar panels for your Vanagon..
>
> Using these conservative assumptions, let's analyze my system.
>
> Assume 6 hours of sunlight / day, with panels derated by 15%, as suggested.
> My panels are 2x45W, = 90W, derated by 15% = 77W * 6 hours = 450W / day of
> power production. Assuming a 12.6 voltage system, call that 35Ah of
> available power / day.
>
> Our Norcold draws 2.5 amps when running, so if it ran for 24 hours a day, it
> would use 2.5A x 24 = 60Ah in a day. But it doesn't run full-time, it cycles
> on every ten minutes and on hot days it might run five minutes per 10 minute
> cycle, and on cold nights maybe a minute per cycle. So that's about 1.25 Ah
> during the day, and a lot lower at night. So for a 24-hour period, call it
> 20Ah for the reefer.
>
> We use a laptop about 2 hours a day. Laptops generally pull 3 amps. Say we
> run the laptop for two hours -- that's 6Ah.
>
> Our little Philips DVD player draws about 2.5 amps, and a film is usually
> about two hours, so that's another 5Ah.
>
> For lighting we have some 8-watt 12-volt dc fluorescent fixtures, and a
> couple of 12-volt halogen "reading lights." Each of the various lights draw
> an amp or so when running, but we don't do a lot of that - one light at a
> time, usually, and no lighting if we're watching a DVD. For general lighting
> we use Dietz oil lamps. Electrically, we probably use less than 1Ah for
> lighting per night.
>
> All told, we use around 35Ah per 24-hour period. Add 25% as suggested, that
> comes to 44Ah.
>
> Based on these numbers, we're clearly using 9Ah more energy per day than I'm
> generating from my solar rig. But in real life, our battery is fully charged
> by noon every day and until we lose sun in the evening, we have a power
> surplus.
>
> Now if my life depended on a solar power system, I would use very
> conservative assumptions, at least as pessimistic as the ones that Larry has
> proposed. But we are only camping, and in good weather, too. If things get
> sucky, we'll move on.
>
> --
> Mike "Rocket J Squirrel" Elliott
>
>
>