Don Williams KD6UVT WBCCI #14207, SNU (#157)
note: Written in 2007 - This is a work in progress. I am working on more detailed version of the installation process
My wife and I enjoy boondocking and camping in our Airstream at local, state and national parks that don’t typically have hookups. We purchased a Honda EU3000is generator for those times when we might want to run the air conditioner or the microwave but otherwise prefer to operate on battery power. The Group 27 battery in our old motor home would last 4 full days with intermittent use of the furnace in the evenings, however, we found out what most of you know. The modern Airstreams have a lot of parasitic electrical demand for our appliances. We can squeak out 3 days with the two Group 24 batteries and careful use of power. We decided to install a solar charging system so that we could extend our stays somewhat indefinitely.
I looked at several sites on the Web that offered
systems/ kits for RVs and chose the 260
watt DC3 RV solar power kit from Solartron Technologies in
Victorville, CA. Our previous Airstream, a 22’ CCD, had a 50 watt
panel on it and I wanted more charging capacity, especially during
low light conditions. Solartron had solar kits up to 520 watts of
charge capacity and a 2500 Watt inverter to power heavy 120 volt
loads for those that can’t leave anything at home.
The DC3 kit included:
Two Kyocera130 watt photovoltaic (PV) solar panels rated 7.39 amps at 17.6 volts with a 25 year warranty;
two US made flat solar module mounting kits with stainless steel mounting hardware;
a US made BZ Products MPPT250 250 Watt high efficiency Multi Power Point Tracking charge controller with a 5 year warranty;
25 ft of sunlight resistant wiring;
25 amp inline photovoltaic fuse and instructions;
(optional) double breaker box with 15 amp and 30 amp breakers.
The first thing I did was read the directions and check the parts list to see everything was included. The next thing was to carefully plan the layout for all components and confirm the locations and dimensions where everything will be installed:
locate the solar pre-wire installed by Airstream, yellow (positive) and green (negative), the photovoltaic end is terminated in the lower part of the refrigerator space accessed from the outside hatch on the side of the trailer, the battery end is a coil of wire near the 12 volt power bus under and behind the sofa against the front wall of the trailer;
locate where the charge controller will be installed, in our case on the street side of the shelf between the sofa and the front of the trailer;
locate where the photovoltaic panels will be installed on the roof, between the air conditioner and the rear roof vent.
I spent a lot of time measuring and laying out where the panels would be located. I loose fitted one panel on the roof in both planned locations to make sure they would fit. I found some issues that needed careful attention. While the panels fit, the curved roof of the Airstream had to be considered in the installation. The electrical junction box on the back of the panel would almost touch the roof. The second item was the bottom of the “Z” shaped mounting brackets did not sit flat on the roof. Lastly the mounting holes in the panel frame didn’t line up with the spacing of the ribs or spars in the Airstream roof.
I found one more issue to address. I believe Airstream pre-wire the trailers for about 100 watts of solar power. I was installing two 130 watt panels. The installation manual recommended that 6 gauge be used for wire distances of 11 to 20 ft. The 10 gauge wire would create some voltage loss at higher current levels and reduce the efficiency of my system.
The BZ Products Charge Controller incorporates maximum power point tracking technology that can be used to minimize the voltage losses in the 10 gauge wire. There are two design features in MPPT that were important to me. The LC130 PV panels produce the most current, 7.39 amps, at 17.6 volts. If the panel were connected directly to the batteries being charged it would operate at the battery voltage, typically 12 to 14 volts depending upon its state of charge. Panel output at this voltage would be would be higher, 7.8 amps, however, total power, volts times amps, would be lower; 14*7.8=109.2 watts; 17.6*7.39=130 watts. The MPPT charge controller masks the load voltage connected to the panel and causes it to operate at a voltage that maximize total power output. The controller also includes a high speed switching network and inductor to transform the 17.6 volts to the load, in this case the battery at 12 to 14 volts. In this example charge current would be 9.2 amps at 14 volts and 10.8 amps at 12 volts, both higher than the 7.8 amps that would have delivered if the battery was connected directly to the panel. The most recent MPPT designs can transform even higher mismatches in voltage with little loss of efficiency, 24, 50 and even 100 volts to 12! This feature is very useful in large fixed high power solar installations such as a business or home.
I decided to wire my panels in series as a nominal 24 volt system to take advantage of the MPPT features; current flow or amps at 24 volts would be reduced 50 compared to a 12 volt system, resulting in less voltage drop through the 10 gauge wiring. The higher voltage of the panels in series would also help in low light conditions where output of a parallel system might be too low to charge the batteries.
After all the hard work the satisfaction is in how well it works. We and our 5-year-old grandson endured four cold nights in the Mojave Desert Preserve this past Thanksgiving. Low temperatures were 31 to 36 degrees so the furnace ran a lot to keep us warm through the night.
I really want to emphasize the need for accurate measurements of the roof before you order your kit, to the nearest 1/8 of an inch. I measured the available space between the air conditioner shroud and the rear ceiling vent and the distance between the rivet lines fot the internal ribs from the ground without going up on the roof. I was concerned with the length of the space and didn’t pay much attention to the width, in my case the space between the refrigerator vent on one side and the drip rail above the awning on the other. While it turned out that I had sufficient space it was an extremely tight fit. I ended up with 3” between the two panels and 2” clearance from the panels to the vent shroud and to the drip rail on each side respectively. You can order PV panels in different widths and lengths to accommodate your space but it is much easier if you don’t have to take or ship them back. I was Lucky!
parasitic electrical demand – this is the electricity needed by the propane and smoke detectors, refrigerator controller board, and similar things
BZ Products MPPT250 250 Watt high efficiency Multi Power Point Tracking charge controller
Anderson Power connectors
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