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Coleman® 55-W Solar Back-up Kit With 2-W Battery Maintainer

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Premium Subscriber
Oct 2, 2010
New Glasgow, NS

I'm looking at something to allow me not to have to run my generator at the trailer so much. Anyone out there have any knowledge of these units. Also can you connect multiple batteries to the set up and what about adding the generator into the mix to make sure the batteries are always topped up?



Completely Banned for the Greater Good
Mar 14, 2012
Carter County KY
A long time ago, a guy, I think his name was George Ohm or something, came out with a simple formula of Volts, Watts and Amps.

Along about that time another guy - named Fourier applied Georg's theory's and came up with simple solutions to complex problems.

Ohm's Law Calculator

Voltage (E) = Current (I)
* Resistance (R)
Power (watts) = Voltage
(E) * Current (I)

When you apply 55 watts to a battery, you do not store 55 watts.
First off you have to use some type of protection diode - which forces the power to only travel in one direction, and the diode has a forward bias drop of around .7 of one volt.

Next is the problem - a solar panel is only about 30% efficient.
The sun does not shine every day.
The sun does not shine at night.
The panel only produces about 80% of its rating when in direct sunlight.

Which means, as the sun traverses across the sky, the panel is only fully illuminated about 3 to 4 hours out of the whole day.

There has been people who have devised a system which follows the sun and aims the panel directly at the sun, but that system also uses power.

Batteries by nature only holds about a 80% surface charge.
Once the battery's are charged, they hold no more.
The battery tender circuit - which performs the function of monitoring the batteries and making sure that they are not over charged / nor under charged - also uses power.

Batteries cost money.
The more batteries you use - the more money you spend.
The more money you spend, the less efficient the system is.

In general - the cost to produce your own power, over a 10 year life span of the batteries is almost 400% more.

Wet cell vs dry cell
Wet cell batteries - either needs some type of maintenance - check the water level from time to time and monitor the water level... Or they are a sealed type and sometimes the battery uses a type of system - where it recycles the acid vented when you draw a whole lot of power. Or it vents the gas and eventually the plates goes dry and the battery fails.

Or you can use some type of Gel Cel - usually around 40% more in cost, with about the same life span.

Most solar panels - of any size, tends to be of the 24 volt variety.
It is cheaper to have two solar panels and aim one east and the other west then to try to use just one panel and have to turn it all the time.


May 28, 2009
These are also seen as Sunforce and Harbor Freight solar kits. They will do an ok job of charging a small lead-acid to run those radios in your sig - maybe led lighting etc. About 25ah min (flooded) 12ah min (sealed AGM) battery to a max of 60ah capacity reasonably. If your load can deal with that size battery, then you are good to go. (Check your kit / battery specs - these are generic values from the top of my head)

The panels themselves are thin-film amorphic, which means that they are about 1/3 larger than mono/polycrystalline for the same output power. However, with the thin-film, if any part of it is covered by a leaf, shadow, etc, just that part of the panel doesn't produce output. (Ie, cover half the panel with the shadow, and it will put out only half it's rated power). With a mono/polycrystalline panel, if a leaf falls onto it, or shadow from a big twig crosses it, the entire panel goes nearly dead. Thus, I like thin-film for camping, unless you want to babysit a monocrystalline panel all day. :)

You can put together your own kit of better parts pretty easily, but I started out with these, and keep them for camping use with my 55ah AGM deep-cycle battery. They are easy targets to criticize, but if you don't intend to make them heirlooms, permanently mount them to your RV, or try to make them your sole off-grid solution, they do fine for camping and running stuff from a small(ish) lead-acid battery.

Here are two good forums that might help you get started:

Solar Forum - Discussion Board on Solar Panels and Solar Power Products
Solar Electric Power Discussion Forum by Northern Arizona Wind & Sun

Have fun!
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May 28, 2009
With a mono/polycrystalline panel, if a leaf falls onto it, or shadow from a big twig crosses it, the entire panel goes nearly dead. Thus, I like thin-film for camping, unless you want to babysit a monocrystalline panel all day. :)
This is also a good way to test the modules in a folding panel made of mono/polycrystalline. If you measure the current going into your battery, and temporarily shade only one of the modules, you should see a very large drop in current. Shade all the other modules one by one. If you don't see a massive drop in current, that module/wiring could be bad.

Cover part of an amorphous thin-film with your hand, and you'll see only a slight drop of current. I guess the moral is that if you can put up with a slighly larger panel, the thin-films are good for the typical ground-mount when camping which can put up with small shading better than the physically smaller monocrystalline types.
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