Do solar panels produce their voltage even when the sun is low?

[awheeler]

I'm trying to decide between a parallel or series-parallel setup for my camper. This is for a 24v system with an MPPT charge controller and the 175 watt panels product 35 volts. I will be using 6 panels.

In series/parallel I can have 70 volts going to charge controller or a straight parallel with 35 volts going to charge controller. Would I be better off going with the 70volt option since I am charging a 24v battery? In the evenings when the sun is low would the 35 volts potentially drop to below my 24v charging level?

Thanks

 

[Rando]

This is one of the common misconceptions with solar panels - they are current sources, not voltage sources. The panels will produce very close to their rated voltage even at very low illumination, but the current will decrease dramatically (see the figure below). For your situation there will be little if any difference in late day power production between series and parallel - once the output voltage is too close to the battery voltage for the MPPT to continue to function, there is almost no power left to harvest as the available current will be close to zero.

 

[Alloy]

You'll loose more (if the wiring is sized properly) from series connect panels that are affected by shade.

 

[awheeler]

Great thank you! That settles it, parallel I go.

 

[broncobowsher]

I do series. Reason being is the current is cut in half. I have less loss for a given size of wire. Get more power out of a smaller gauge wire. Looking at my charge controller I will see 1A input current but 3.5A of output current. The wire run for the cells is 12 gauge. Even at 200W of power I am only pulling 5-6 A through the cell wiring. That is almost on voltage drop. And if I loose a volt, only getting 69 instead of 70V, I am ahead. At 5A, a 1V drop is a loss of 5W.

If running parallel I would double the amperage, and double the voltage drop. Now 10A of current and that would cause a 2V drop. That is a loss of 20W. You quadruple your losses with a lower voltage system.

(Above numbers are rounded and cleaned up to make easier mental math. Actual numbers will be slightly different. Parallel wiring losses will be less due to the added mass of additional parallel wiring. Effectively up-sizing the wire size to the charge controller. But a higher voltage system is still more efficient at transfering power with less loss.

 

[roving1]

I have two 80 watt panels. I did not do super exhaustive studies or anything but in series mine will start putting half an amp into the batteries before the sun even technically rises and comes over the horizon. I also found in all but the crappiest darkest overcast the series panels will get above the cutoff for charging to kick in on my Victron 75/15. Wasn't as much the case in parallel. The biggest thing is with the panels pushing 40 volts into the MPPT it doesn't work very hard. Pushing 12 amps into the batteries for hours doesn't even make my unit warm to the touch. It feels ambient temp on an 80 degree day. I was also able to make the panels portable with longer wires to be able to angle them or run them into the sun if the vehicle is parked in the shade with no real down side.

 

[Alloy]

Series / Parallel depends on location.

These are our conditions 80% of the time. Each panel may see an hour of sun. For these condition I wanted the panels connected in parallel so I looked for panels with a high VOC.

This morning at 8:30AM the panels are producing 0.2% of the rated output with the voltage (parallel) at 52-58v.
The max VOC is 70V.