Date: Tue, 27 Mar 2007 01:14:34 -0500
Reply-To: Matt Roberds <mattroberds@COX.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Matt Roberds <mattroberds@COX.NET>
Subject: Re: Trickle charging battery with 1.8W solar panel = failure
In-Reply-To: <e3e1d7f0703261716w1415297pc594356380ceef28@mail.gmail.com>
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
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
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