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Old 09-02-2018, 07:22 AM   #18
heiser252
Member
 
Brand: DRV
Model: 40KSSB4
State: California
Posts: 46
THOR #5723
Clev asked me his original question from this topic (with a little more detail included) in a pm on another forum. I haven’t been monitoring this forum as closely recently, but with the pm, I came here and decided to add some additional information. I also wanted to post my response to Clev’s pm here so that others might benefit from the information. (Clev, hopefully I am not overstepping my boundaries here.)

Basically, Clev has an existing system with four 150 watt panels installed for a total of 600 watts. His existing MPPT charge controller is limited to a maximum wattage input of 800 watts and he wants to add two additional 150 watt panels to his system. This would give him a total of 900 watts. He wants to know if there is any way to wire the six total panels to keep them within the specifications of his existing controller. Here is my basic response to that general question:

“Sorry, I haven’t been keeping up with the Thor forum that much lately. To start with, here is a basic equation that should help:

Watts = Volts x Amps

For example, my panels are Canadian Solar 235 watt panels. I have six panels for a total of 1410 watts (235 x 6 = 1410). The Vmp rating of my panels is 29.8 and the Imp rating of my panels is 7.9 (29.8 x 7.9 = 235.42). This is why my panels are rated at 235 watts.

I run my panels in three parallel strings of two panels in series (six total panels). When you run in parallel, the voltage stays the same and the amperage is cumulative. When you run in series, the voltage is cumulative and the amperage stays the same. So in my case, I have two panels in series. This gives me 29.8 + 29.8 which is 59.6 volts at the same 7.9 amps. I then have three of these series strings. This gives me 7.9 + 7.9 + 7.9 which is a total of 23.7 amps at the same 59.6 volts. Again, Amps x Volts = Watts, so 23.7 x 59.6 = 1412.52 total watts, or basically the same 1410 watts (when you account for rounding). In other words, series or parallel makes no difference in my total watts. Hopefully this example helps you with your calculations.

If you go to six of your 150 watt panels, no matter how you wire them you still have a potential of 900 watts. In other words, you can overpower your existing charge controller. You can re-wire your system (with six total panels) into any combination of series/parallel and it will always come out to 900 watts, which is more than your particular controller is rated for.

MPPT controllers in general can take high voltage input and convert it to lower voltage at higher amperage output. Your controller could take the higher voltages of various series/parallel combinations of your six total panels (assuming they were within the maximum wattage rating of your controller) and still put out the appropriate voltage to your 12 volt battery system. This is what MPPT controllers are designed to do. They take the additional voltage input from high voltage panels and convert that extra voltage to additional amperage going into your batteries at the proper voltage.

You also do not want to mix one series string into your existing parallel set of panels. You will be mixing different voltages, which will pull down the voltage to the lower of the two. When you mix panels of different voltage (your two additional panels in series essentially become one panel with a higher voltage), the maximum output will be the lower of the two voltages. This is based on electrical theory that is too difficult to explain here, but it is what happens. This, in essence means you would not be getting any benefit from this new string any more than just adding one single additional panel (of the same ratings) to your existing array.

Hopefully this helps. Let me know if you have any more questions. I am happy to help.

Chad”

The Renogy tech that Clev mentioned in his original post did not give Clev completely correct information. He was correct in that running panels in series can be more efficient. This is because you can use smaller wire and components related to the wiring than the same wattage array run in parallel. Higher voltage can use smaller wire than lower voltage (this can be seen by plugging numbers into any wire size calculator). However, when the tech said running in series would lower the wattage of the array, he was incorrect (as I explained above).

Hopefully this information is useful. Again, Clev, I apologize if I overstepped by mentioning our PM conversation.
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