darryl ramm’s blog

In late 2005 I installed a large (64W) solar panel on the Cobra trailer for my DG-303 glider and it has worked great. I get asked about the solar panels, so I thought I’d save some information here about it, largely stuff that has been posted in the past to private newsgroups. I’m going to be installing a similar set up on my ASH-26E trailer, but I may play with different charge controllers (more on that in future).

My DG-303 has dual 12 amp hour batteries. I have two sets of two batteries, so I can always have a set charging while the others are in use. I wanted to also be able to leave all four batteries in the trailer hooked up to the solar panel when the glider is not in use. With my glider the batteries need to be removed from the glider for disassembly so it made sense to just make a mount for four batteries in the trailer rather than charge some in the fuselage of the glider parked in the trailer.

I wanted a significant amount of solar power available; something like enough to fully charge a fairly well used pair of batteries on a good sunny day. This meant a large panel with charge controllers, a place to mount four batteries and one or more solar charge controllers depending on the charging scheme. All up costs for this project was around $600 - it could be done for less without some of the over engineering in my set up.

SOLAR PANELS

Some people mount crystalline silicon solar panels onto a trailer roof, these need to be either fairly small or mounted on a large rigid frame. I definitely did not want an ugly frame holding a panel on the roof and I wanted a large panel with lots of power. Mounting a large panel on a trailer meant looking at amorphous, instead of crystalline, silicon panels for their ruggedness. The down side is amorphous panels are not as efficient as crystalline solar panels - often by a factor of a few times.

Strobl Solar do make crystalline solar panels for trailers, and the Strobl Solar panels for installation on glider are fantastic. But I think the UniSolar are much better for my application, even if they may not be as efficient as crystalline silicon panels. I went with a Uni-Solar US64 64 watt panel. The panel is large, at 54 by 29 inches.

The US-64 is still available but smaller models in the US series are not. A possible replacement for the smaller panels could be be the UniSolar ECO-Series, and while I suspect they can I have not confirmed that those panels can be removed from their frames and used the same way as the US series described below.

A back of the envelope calculation for available charging power is -

Average charge current per day = (Peak power current) * (hours of effective daylight per day)

Peak power current for the US-64 is about 4 amps. The hours of effective daylight per day is used in typical solar power planning and is calculated for different locations and seasons. I used 4 hours as a pretty conservative number. So -

Average charge current per day = 4 amps * 4 hours = 16 amp hours per day.

Given that at any time I’m trying to charge two 12 amp hour batteries this number is in the right ballpark for being able to charge a half used set of batteries on a good day. I know it’s not this simple, but it is in the right ballpark.

I purchased the solar panel and charge controllers from Colorado Solar and found these guys great to work with, they may no longer be carrying UniSolar panels. An alternative source of the UniSolar US-64 panels may be Affordable Solar.

CHARGE CONTROLLERS

I decided to use SunGuard charge controllers from Morningstar. These are simple charge controllers, no facility for managing load running off the battery, no metering or LED lights etc. However they do have good charge regulation, including temperature compensation and are designed for sealed/AGM batteries. And they are relatively cheap, around $30 each.

I wanted each battery to have its own charge controller so they would charge independently and efficiently. MorningStar charge controllers can all be hooked up in parallel across the solar panel but you need to run a blocking diode in front of each charge controller. MorningStar has a circuit diagram (PDF) of this. Schottky power diodes should be used as they has lower forward voltage drop than conventional diodes and therefore are more efficient. There are lots of diodes to choose from, an example being something like this one from Digi-Key.

I could also have connected several batteries across one charge controller as MorningStar show here (PDF). Again the blocking diodes are important as they stop one battery charging the other if they are at different states of charge, and again these should be Schottky diodes. However I don’t like this scheme as it does not let the charge controllers most intelligently/efficiently charge the batteries if they are at significantly different charge states. Also I’d like to even avoid the small (typically ~0.3 volt) drop from he Schottky blocking diodes and let the charge controller control the voltage more accuratly.

One thing to note is the MorningStar SunGuard has a 6 to 7 volt minimum operating voltage, if the battery is below this voltage it will not power the SunGuard enough to charge the battery. This is not uncommon for PWM (Pulse Width Modulation) style charge controllers like the SunGuard. Other charge controllers including the MorningStar SunKeeper have no minimum operating voltage. The SunKeeper was not available when I did the original install, I would probably use the SunKeeper for this and other reasons like it’s status LED.

INSTALLATION

The UniSolar US panels have the amorphous solar cells sandwiched between a layer of clear plastic (probably ETFP aka Tefzel) on the outside and a thin sheet of galvalume steel on the back. The edges have the clear plastic folded over and sealed and appear very weather tight. This whole assembly comes mounted in a frame made of “L” shape aluminium with a plastic junction box bonded onto the back of the solar panel with RTV silicone.

My Cobra trailer has an aluminum top but the same technique could also apply to fiberglass top trailers as long as they don’t have a compound curvature to their top.

I decided to bond the solar panel to the aluminum trailer top using 3M VHB (Very High Bond) double sided adhesive tape. This tape is made for the automotive industry and is used to do things like bond aluminum body panels to trucks. It is also what Strobl Solar use to bond solar panels to glider fueslages. This is a distant relative of the 3M double sided outdoor tape you buy at hardware stores. VHB is strong stuff and expensive. I don’t fancy my chance of every getting this panel off! The specific variant of VHB tape I used is type 4936 in 20 thousandth inch thick foam by 1″ wide. This type is intended for using on painted aluminum. I obtained this from Uline. I went a little overboard with the amount of tape I used, putting strips about evey three inches down the lenght of the panel, and across both ends. Which about half a roll, so the tape to adhere the panel cost me about $60.

I dismantled the US-64 solar panel from it’s aluminum frame, and discarded the frame and removed the junction box behind the solar panel. After careful measuring and double checking I drilled a 2 ½ inch hole the roof of the trailer with a hole drill. Be careful not to drill through cross members in the trailer roof.

Using lacquer thinner (which if you leave it on too long will take off the white paint on a Cobra trailer) I cleaned all the wax and other stuff off the trailer roof where the panel was going to be adhered. I taped the adhesive tape to the back of the solar panel, left it’s protective backing paper on the exposed side, laid the solar panel on the trailer roof and duct taped one long edge so the duct tape would act like a hinge. I lifted up the solar panel like a book cover, removed the protective backing from the VHB tape and then laid back over the solar panel. For woodworkers this is just the same way you lay up wood veneer with a masking tape hinge. I then ran my hands over the panel, pushing down to make sure the tape was adhered and then ran a bead of white silicone sealant around the edge of the panel to keep water out and make it look nice. I then glued the the junction box back behind the panel inside the trailer with silicone sealant, and ran 12 gauge wire (cut from an old extension cord) to the charge controller panel.

The MorningStar charge controllers, diodes etc. mount in a polycarbonate box I made that bolts to the inside of the trailer in the tool storage area. See the photo above. Each charge controller sits above the battery it is charging, you want them in the same area since the temperature sensor used for charge compensation is mounted on the charge controller (more expensive controllers have a remote thermistor or similar sensor). I also made up a polycarbonate tray to hold the batteries, an aluminum angle attached to two short bungee cords hold the batteries down in this tray so they don’t bounce out.

IT JUST WORKS!

This system really worked great. I just don’t ever have flat batteries, I never plug my batteries into an AC charger, it just all works.

I was initially worried about this large black patch on top of the trailer and whether this would increase the temperature inside the trailer much, I had plans to investigate this but in practice it felt that even on hot high desert days the temperature inside did not feel like a problem. If it ever becomes a problem I assume a thin sheet of rigid polyurethane foam insulation under the panel area would help cure any problem.

After I had this basic four battery set-up working for a while I also added another charge controller and a 33 Ah AGM battery permanently mounted in the trailer. The fixed battery battery powers RV style ThinLite 190 Series interior fluorescent lights at the inside back and front of the trailer and a Hella Marine deck floodlight mounted high up on the vertical fin part outside back of the trailer. These lights have also worked out well. The rear floodlight is geat for helping derig a glider after dark. I’ll post more information on the light setup in another post.

The 33 Ah battery could also used to charge a glider battery through a DC to DC charger such as a Great Planes Triton or Graupner Ultramat (made for charging radio control electric model batteries from a 12 volt battery or car electrical system) or run a 12 volt air compressor.