My CMS Sharing our experience about RVing

March 24, 2008

Solar Charging Thoughts

Filed under: Uncategorized — donwood49 @ 5:07 pm

Our electrical system is described in another article. Here are a couple of additional thoughts.

If you are going to add panels to your system they should be compatible with your existing panels. The wattage of the new panels doesn’t matter. All that does is change the amount of charge current provided. The voltage rating of the new panels is critical. This usually means you shouldn’t mix different manufacturers panels in the same circuit. The charge controller can’t individually optimize for different rated (voltage) panels and the lower voltage panels would not provide as effective an output as the same wattage panel of a matching voltage.

Having panels that can provide more current than the rated output of the charge controller isn’t a problem since the charge controller can limit the maximum output. It isn’t necessarily a good idea though since you may waste some of your expensive solar panel output current. In our case, we do have more panel (400w) than is needed for our charge controller rated output of 25 amps. It does allow us not to have to raise the panels to still get some reasonable charge to the batteries. When our travel style changes to longer stays, a bigger or additional charge controller will probably be beneficial.

If you want to mix panels with different voltage ratings (different manufacturers) you should feed the panels to two different charge controllers either to the same or different battery banks and you’ll get more charge for your panel investment. Assuming two 120 watt panel cost $1600 and you get 75% efficiency due to a voltage difference you would realize maybe 38 amps of charge per day.If you invested another $200 for another charge controller¬† you would get 100% output or maybe 50 amps of charge per day (more if your charge controller uses MPPT). Pardon the short cuts on math and the wild guesses on outputs. I’ll try to add actual figures when I get a chance but the estimates are close to our experience per my memory.

What this all really relates to is the same thinking as is used to get more output using Maximum Power Point Tracking (MPPT) charge controllers. If you match the controller to the panel you can get the maximum output. The method of matching is using the voltage.
You can choose panels with different wattage ratings from the same manufacturer to allow the panels to fit more easily on your RV’s roof in order to avoid roof top obstacles. Our Kyocera panels are all the same width but the length changes with the wattage rating.

To get the most from your panels and generator use it is best to estimate the potential charge available in the morning (is it cloudy, how much charge have you been getting recently, etc) and run you generator in the morning to bring the batteries up to the point that the solar will top off the batteries. Our freedom inverter/charger charges at 75 amps when I run the generator on discharged batteries. The higher the charge on the batteries, the less output the charger will provide to the batteries. The solar charge controller does the same, as the batteries are closer to fully charged, the charge current is reduced. But, running the generator in the evening for 1.5 hours to top off the last 30 amps at 20 amps doesn’t seem as effective as charging at 75 amps in the morning for a total run time of 30 minutes. The solar charge current is closer to what the batteries want to receive in the afternoon.

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