Date:         Tue, 27 Mar 2007 07:24:35 -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: Trickle charging battery with 1.8W solar panel = failure
In-Reply-To:  <Pine.LNX.4.64.0703270034420.20135@birdbird.example.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

I was going to post some ideas about Martin's issue but I'm glad I held
off -- Matt's response is much more in-depth and useful than anything I
would have posted. However, Matt continues to promulgate a tired and
pernicious stereotype when he recommends fusing the panel's wires to
protect them from melting due to shorts if they are gnawed on by "... a
hungry squirrel." C'mon guys -- a few wires get chewed on by some inbred
hick distant cousins and suddenly all the squirrels in the world are
ravenous insulation munchers?

--
Mike "Rocket J Squirrel" Elliott


Matt Roberds typed:
> On Mon, 26 Mar 2007, Martin Jagersand wrote:
>> Trying to find a reason I measured the output of the charger at two
>> times: 1pm under full Alberta sun: 40mA,  at 4pm 10mA.  That makes
>> only 480mW and 120mW. Far from the advertized 1.8W.
>
> The solar panel probably really does make 1.8 W, if you are lucky enough
> to be using it on 21 June at the equator.  In Alberta in March, getting
> only 0.48 W doesn't seem out of line.  Also, you were probably getting
> a little more than 0.48 W, because if the panel was putting out exactly
> 12.0 V, it would never charge the battery - the voltage from any charger
> has to be a little bit more than the battery voltage or it will never
> work.  If you were getting 13.0 V at 40 mA, that'd be 0.52 W.
>
> Also, the manufacturer may be cheating a little - the raw panel itself
> may indeed put out 1.8 W in full sun, but because it's used as a battery
> charger, there is almost certainly a diode in series with the panel
> to prevent the battery from discharging through the panel at night.
> This diode will drop something like 0.5 V to 1.0 V - if the raw panel
> puts out 0.138 A @ 13 V (1.8 W), the diode will use 0.07 to 0.14 W of
> the available power.  It doesn't sound like much, but you don't have
> much to start with.
>
> There are different ideas on what a good long term key-off load is for
> a car battery but I seem to recall something like 20 mA being discussed.
> If your key-off load is that high, your solar cell must _average_ at
> least that much output to keep the battery charged.  This means that
> the actual peak output at your location probably needs to be something
> like 3 or 4 times that, because of cloudy days, night, etc.
>
> You could measure the key-off drain on your battery.  Make sure
> everything is shut off, then disconnect the negative cable at the
> battery.  Set your meter to measure current, then put the red lead on
> the free end of the negative cable and the black lead on the negative
> battery terminal.  If you get something much over 20 or 30 mA, you may
> want to investigate your electrical system.
>
> If you find a device that draws a lot of current with the key off but
> otherwise appears to be working properly, you might be able to reduce
> the load on the battery by pulling the fuse for that device when you
> park the van for a long time.  You might tape the removed fuse to the
> steering wheel or shifter so you remember to reinstall it when you
> come back.  This way, any charger you use doesn't have to power that
> load as well as trying to charge the battery.  Another alternative
> is to disconnect the battery from the van completely and connect the
> solar panel right to the battery terminals.  If you do this, include
> a small fuse (5 A or so) in the positive solar panel wire right at
> the battery terminal - this will help stop the solar panel wires
> from melting from the battery current if they get shorted for some
> reason, like a hungry squirrel.
>
> If I had to store a car outdoors in the winter for a long period of
> time, the first thing I'd try with the battery is to take it out of
> the car and store it in a heated location indoors.  If there was power
> available, the next thing I'd do is hook up something like a Deltran
> "Battery Tender" or Schumacher "Battery Companion" or similar.  These
> are "smart" chargers which will charge the battery and then keep it on
> a small enough charge to make up for any discharge but not overheat
> the battery and cook off the electrolyte.  Even if one of these chargers
> was drawing 2 W average from the AC line, running it for six months
> straight would use about 9 kWh, or around US$0.60 to US$1.80 at current
> rates.
>
> Most so-called "trickle chargers" are not smart enough to reduce the
> charging current to a low level and will happily cook your battery if
> you leave them on long enough.  They are good for slowly charging a
> battery overnight or for short-term maintenance, but not for long
> term.  If a charger like this was the only thing available, I'd see
> if I could come up with an electromechanical lamp timer to run the
> charger for something like an hour a day.  This would give the charger
> a chance to correct any self-discharge of the battery, but not let
> it stay on long enough to start cooking off the electrolyte.
>
> If there was no power available, but there was sunlight, I might try
> to use a solar cell.  In support of solar energy, both the US and
> Canadian governments have done a lot of research into how much sunlight
> ("insolation") you actually get at various places throughout the year.
> Using that information, and data sheets for the solar cells, you could
> pick out a size of solar cell that would definitely keep the battery
> topped up.
>
> If there is no place to keep the battery inside, but there is power
> available at the car (like maybe at a camp site), I'd probably still
> disconnect the battery cables, and then hook up an AC-powered charger.
> Having a camper makes this easier, but if you do it on a non-camper,
> do something so that you don't forget you have a charger plugged in!
> Disconnecting the battery is one clue - when you put the key in,
> nothing will happen.  A simple thing is to loop the extension cord
> for the charger through the driver's door handle, or around the
> driver's outside mirror, or similar.  If you don't do this, you will
> eventually drive off with the charger still connected, and end up
> buying a new charger, a new extension cord, or maybe a new outlet.
>
> Matt Roberds
>