As was pointed out in some of our other RV Solar Education pages, panels laid flat on the roof of an RV operate at a much higher temperature than panels in lab conditions at STC and NOCT. There will be a voltage reduction because of the resulting high cell temperatures. There will also be small voltage losses in the wire harness and the charge controller system. This means your panels need to be at a substantially higher voltage than your battery bank in order to overcome this drop and still be able to store power. Standard 36 cell solar panels intended for nominal 12 volt battery charging typically operate between 16.5 volts and 18.5 volts (according to their STC ratings). When you factor in the voltage drop from the heating of the cells and the other known system-wide voltage drops, some of these solar panels may not be able to fully recharge batteries in hot conditions.
A couple methods exist for overcoming the voltage drop described above. One method is to start with solar panels that have operating voltages that are above 17.5 volts and use a heavier gauge wire for the system wire harness. The panels we have built for us operate at 17.7 to 18.5 volts which will provide the necessary potential to overcome the other system voltage drops and still have enough voltage to finish charging your batteries even in the hottest conditions.
The other method for overcoming voltage drop is to place two or more standard 36 cell solar panels in series with each other. This increases the operating voltage of the solar array while keeping the amperage low. This higher voltage is then fed to an MPPT type charge controller which steps down the voltage and boosts the amperage. This is a viable way to overcome the voltage drop, but it comes with some drawbacks. One is, that when you series connect two or more solar panels, you run the risk of dramatically reducing the energy production if any part of those panels gets shaded. Another drawback is, that the higher the voltage entering the controller, the less efficient the conversion process is to lower the voltage coming out of the controller to level safe for feeding onto a 12V battery bank.
For example: We have two customers with the same solar panels and charge controllers. One day they ended up parking next to each other in the desert. One customer had four 100 watt solar panels, installed by AM Solar, connected in parallel. The other customer bought the same equipment from us, but had their panels installed by someone else in series. The customers decided to see how the two systems compared since they were both being exposed to nearly identical conditions. They looked at the charging amperage when both systems were in full sunlight, with no shade on the panels, and found that the panels connected in parallel produced a slightly higher charging amperage. Then they monitored the charging amperage while standing on the RVs’ roofs and casting similar shadows on a single panel in their arrays. The series connected system had a very dramatic drop in charge current while the parallel connected system only dropped by about 25%. The three unshaded panels in the parallel connected system kept on producing power, while the shaded panel in the series connected system disrupted the entire array. The moral of the story is that in RV applications (where partial shade may be very common), it is best to have your solar panels wired in parallel.
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