Mixing Panels (open circuit voltage)

[Croft]

The panel install is going great. Yesterday I mounted the panel on the roof and ran the wires down the fridge vent. One thing I did discover when I checked the wiring to the panel was that after the voltage drop over 15 - 20 feet of #10 the voltage of the new panel had dropped to +/- 19.5V, much closer to that of the old panel. There is less of a mismatch than we feared. Today I will finish the wiring.

[Croft]

Everything connected. After voltage drop the two panels were putting out 19.7 and 19.8 at the controller respectively. Thanks for all the help everyone. Converting to MPPT is still under consideration. I know I should but do I really want to spend $126. But then again it is only a tank and a half of gas for the motorhome.

[mapmaker]

One thing I did discover when I checked the wiring to the panel was that after the voltage drop over 15 - 20 feet of #10 the voltage of the new panel had dropped to +/- 19.5V, much closer to that of the old panel. There is less of a mismatch than we feared.

After voltage drop the two panels were putting out 19.7 and 19.8 at the controller respectively.

It has nothing to do with voltage drop across the cable. When you put two panels (or anything else) in parallel, their voltages are the same. --mapmaker

[Croft]

That is why I measured them before I tied them together. My original measurement was right off the new panel while the old panel was measured at the wires I took off the CC, 15 - 20 feet away. The initial difference disappeared after I added the 15 - 20 feet of #10 to the new panel and introduced this voltage drop to the equation. Of course when I physically tied them together in parallel they became exactly the same (because they were tied together).