PV wiring when in shade


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cak

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Joined
Mar 14, 2021
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133
So much of this depends on cost benefit analysis which if you have the time and interest in doing detailed per panel full day and year solar exposure analysis you could figure out the most optimum setup. Or you could be lazy like me and just go for good enough and then install couple extra panels to make up the difference :) My current setup I have about twice the production capability with my panels as my charge controller which means the MPPT controller is pegged to it's max a lot more of the day and manages to meet my needs for now. It sounds like you know your stuff and are on the right track and have gotten some good ideas from previous comments, multiple MPPTs or bypass Diodes.

For me understanding how and why the system works a certain way helps me to figure out solutions. Solar panels are made up of lots of solar cells in parallel and series and each solar cell functions as a diode which produces electricity when hit by light but also produces light when a reverse current is applied. Solar cells and LED diodes are actually structurally the same just put in reverse and optimized for producing or consuming light. As long as your panel voltage is higher than your battery at full charge plus a little the MPPT will do it's job. The reason to put even more panels in series is actually more about cost savings than efficiency because if you series to a higher voltage you can get the same amount of power to the MPPT using smaller wires. The tradeoff with more panels in series is more dangerous voltage levels and if any one panel(or part of panel) in series is shaded or otherwise compromised then it will dramatically decrease the whole series voltage and thereby power generation. There is also potential for problems when connecting multiple series in parallel because they will try to equalize voltage to all be the same so if one of the parallel strings is more shaded or worse angle then that string will be lower voltage and drag the others down. There are panels sold that are designed to be shade tolerant which is usually done by adding diodes within the panels. I have seen this most with portable and flexible panels. The diode causes any suboptimal(damaged, shaded, etc) solar cells(or strings of cells) to not drag down the other cells because even though they are lower voltage than the neighbors the diode blocks power from entering that cell although they still won't contribute to the whole system. Many modern panels do not have diodes because they reduce the overall efficiency due to the forward voltage drop of the diodes. The MPPT also has a diode(or a fancy mosfet circuit to achieve the same result with less voltage drop). The MPPT "diode" stops the batteries from discharging into the panels while they are shaded or at night. So the MPPT functions as a fancy and expensive diode keeping a lower producing panel or string from dragging down a different one.

So all of that is to say that is all comes down to which tradoff makes more sense in your situation as far as adding diodes to specific panels that you know will cause problems and accept that it will slightly reduce the overall efficiency of those panels or putting the problematic panels in their own MPPT. Most often a major factor becomes how many wires you want to run from your roof and how much will that wire cost relative to other solutions.
 
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