Combining strings where one is shaded

I have 8 panels @ 40V and max 10A and during the day 4 of the panels are likely to become shaded but the other 4 should get full sunlight all day.

Obviously if I parallel all 8 panels into a single string then the amps will drop for all panels when the shade hits. However if I split into 2 strings of 4 (both in parallel) and run them into a combiner box, will the fully sunlit array continue to produce 10A and be combined with the shaded array at maybe 2A? When the shading happens, will the combiner box allow me to have a system producing 320V @ 12A as opposed to a single string solution which would be 320V @ 2A?

Thanks!

Y2JB said:

I have 8 panels @ 40V and max 10A and during the day 4 of the panels are likely to become shaded but the other 4 should get full sunlight all day.

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Gotcha

Y2JB said:

Obviously if I parallel all 8 panels into a single string

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A string is typically in series. Parallel is parallel. It is unclear if you mean 8P or 8S.

Y2JB said:

then the amps will drop for all panels when the shade hits. However if I split into 2 strings of 4 (both in parallel) and run them into a combiner box,

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This sounds like 4S2P.

Y2JB said:

will the fully sunlit array continue to produce 10A and be combined with the shaded array at maybe 2A?

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In almost all cases, yes.

Y2JB said:

When the shading happens, will the combiner box allow me to have a system producing 320V @ 12A

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No.

4S2P is 160V/20A max (12 A in your scenario)
8S is 320V/10A max

8 panels in series, if four are shaded, voltage should drop in half but current remains the same.
That is OK if the sun is off-angle. If near full sun so near full current, bypass diodes may fail, and may damage the panels.

I have long strings, and they do get some shading but not during middle of day.

4s2p could be better.

In either case, consider voltage range over temperature, also current, vs. SCC specs.

sunshine_eggo said:

4S2P is 160V/20A max (12 A in your scenario)
8S is 320V/10A max

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Assuming both 4s are same orientation. If two different orientations, lower peak current.
Can you use a different orientation that gives more hours of good sun?

By the way, a 4S2P array is fine, even if one 4s string is shaded or oriented differently. With just ambient light, Voc is about as high as Vmp of the panels in full sun, so no current is backfed into the shaded string.

Many apologies, I meant all the panels in series not in parallel.

So my question is: if I have 2 strings of 4 panels in series and run them into a combiner box, 4S2P, if one string becomes shaded (goes to around 2a) and the other gets full sunlight (10a), will this configuration allow the fully lit string to continue to provide full output without being dragged down (too much) by the shaded string?

Y2JB said:

So my question is: if I have 2 strings of 4 panels in series and run them into a combiner box, 4S2P, if one string becomes shaded (goes to around 2a) and the other gets full sunlight (10a), will this configuration allow the fully lit string to continue to provide full output without being dragged down (too much) by the shaded string?

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Hedges said:

By the way, a 4S2P array is fine, even if one 4s string is shaded or oriented differently. With just ambient light, Voc is about as high as Vmp of the panels in full sun, so no current is backfed into the shaded string.

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Yes, I've measured panels open-circuit in the shaded and compared to panels operating MPPT in the sun.
The good string won't be dragged down at all.
If other string is partially shaded, e.g. 1 or 2 panels shaded while other panels still have sun, the remaining panels in partially shaded string will contribute near nothing.

If you had two separate MPPT, then the partially shaded string would still contribute what its unshaded panels (or unshaded diode-bypassed portion of a panel) produced, but only if voltage sufficient.

A single 8s string would contribute 100% of any unshaded panels (or unshaded diode-bypassed portions of panels) because current simply pushes past the bypass diodes. i.e. no benefit going from 8s to 4s2p. However, they does depend on MPPT algorithm doing the right thing; half-cut panels in particular could trick less sophisticated MPPT so they select a local maxima instead of the highest peak.

Everything within SCC's voltage and current limits, of course. And temperature changes Voc & Vmp of panels.

Interesting. So you’re saying I may be better going with 8S rather than 4S2P? My panels are half cut and they are next to each other so I can easily go with either config.

I just assumed that a single shaded panel would drag all 8 down if in 8S. Why is that not the case?

I prefer 8s. I have 8s "36V" panels, 12s "24V", 24s "12V". That for inverters with 600V max input spec. My strings are 480Voc, 380Vmp under nominal conditions.

Bypass diodes.


This is not good if some panels have direct sun (max current) while others are shaded. Some brands have undersize diodes, considering they don't get proper heatsinking. At full current those fail, overheat, melt or crack the panel. They're OK with sun at an oblique angle when there is shade.

If a regular (not half-cut) panel gets shade blocking one cell, current drops so low that MPPT algorithm reduces voltage until diode bypasses that section.

Half-cut panels are wired internally as 2p3s (not 3s2p). Each diode-bypassed section has two strings of cells in parallel.

If a half-cut panel gets shade blocking one cell, current drops from 2x current of a cell to 1x, because the parallel connected cell is still working.
Array now delivers half the current, and somewhat higher voltage than before. A dumb MPPT algorithm happily operates at that point on the I/V curve. A smarter MPPT algorithm makes periodic sweeps to lower voltage, finding an operating point where full current is available and voltage is only a bit lower. The power/voltage curve has two humps, and only some MPPT look for the higher hump.