Mottled shade on solar panels not good?

[howardwheeler]

I may have misunderstood, but I read something while on a trip (I can’t remember where I read it) that left me with the impression that it was damaging to the diodes on my panels to leave them operating if stored under mottled shade. I don’t remember exactly why, but reasons were given that involved how the bypass diodes worked. Does anyone know if I should switch off my panels while it is stored? When we aren’t out somewhere it rests under a tree that even after it loses it leaves ensures erratic shading all day long on my panels. Thanks.

[rjsurfer]

When in the storage facility I cover my camper with a fairly heavy duty cover and if I had to quantify the loss of voltage I'm getting from the panels I would place it at only 50% maybe even less.

Ron W.

[howardwheeler]

That is the issue I had seen something about. My panels are Canadian Solar that I believe have three bypass diodes in each panel. So I am going to assume I’m okay.

[pianotuna]

bpounds,

Nice link. Thanks!

[bpounds]

This explains the theory of a hot PV cell. In practice, it isn't really a concern. But maybe this is what you've seen referenced.

Shade on solar cell

[howardwheeler]

So diodes are not damaged by repeated partial shade. Great. I have a Morningstar mppt controller but I had to go parallel on my three 315 watt panels because of the voltage cap if in series being exceeded when the panels would first get light. By the way, I had my first opportunity this last week to truly drycamp depending on the solar array to keep my residential fridge running. Despite continued cloudy skies, it worked like a champ.

[BFL13]

With no shading, I had my three 100w panels in parallel with MPPT and also tried with them in series with the MPPT. Made absolutely no difference to amps to the battery in full sun.

With shade coming in where the shadow from the trailer (ground mounted solar) started across the array from one end late in the day, I started to lose amps to the battery.

I chose to be in parallel for that, since I kept more amps coming in from the remaining part of the array, whereas being in series meant losing more sooner. The shadow was at an angle somewhat moving across, so it might have been a race between what diodes could do for you and what keeping unaffected panels going does for you. You still have diodes in an affected panel even when in parallel of course.

Perhaps it is another of those things that "depends".

[pianotuna]

mike-s my controller can not deal with more than 50 volts input. It is a Blue Sky 3024di which was, at the time, top of the line. When panels are new their output may be a bit higher--I would not trust it to have 2 24 volt panels in series.

[mike-s]

pianotuna wrote:
Unless the panels are 24 volts or higher--then MPPT is a good path to follow.

So, why would you put multiple 24 V panels in parallel rather than series through an MPPT controller (other than a crappy one which couldn't handle 48V)?

[BFL13]

mike-s wrote:

BFL13 wrote:
I see what you mean, but if your example started with the two 12v panels in parallel with an MPPT controller, then the type of controller would be the same in each case.

Not much point paying for MPPT then using parallel panels, though. I was talking about common, real world systems.

Yes, but the wording seemed to suggest the cause was the controller type and I just wanted to make that go away and leave the reasons as you meant, which I hope is now all clear.

[pianotuna]

mike-s,

Unless the panels are 24 volts or higher--then MPPT is a good path to follow.

[mike-s]

BFL13 wrote:
I see what you mean, but if your example started with the two 12v panels in parallel with an MPPT controller, then the type of controller would be the same in each case.

Not much point paying for MPPT then using parallel panels, though. I was talking about common, real world systems.

[BFL13]

I see what you mean, but if your example started with the two 12v panels in parallel with an MPPT controller, then the type of controller would be the same in each case.

[mike-s]

BFL13 wrote:

mike-s wrote:
It won't damage the diodes. They're there to bypass shaded strings of cells. But, especially if you use a PWM controller, shade on even a single cell can stop all power output from that panel for as long as it's shaded. But nothing gets damaged.

What does PWM vs MPPT have to do with it?

Simple, with PWM shading on a single cell will take out that whole column, the voltage for that panel will drop below what's needed to charge the battery, so you effectively lose the power from the whole panel.

With MPPT (assuming you have 2 or more panels in series, or a higher voltage panel), losing a string just results in a partial loss from the panel, the remaining columns continue to contribute.

e.g. 2x 100W "12V" panels wired in parallel to a PWM controller. Shade one cell, you lose the output from the entire panel, and instead of 200W you now have 100.

The same 2x 100W "12V" panels wired in series to a MPPT controller. Shade one cell, you lose 1 column (commonly 3 per panel). The diode bypasses that column, but the other 2 continue to contribute because the voltage is still above the minimum needed by MPPT. You only lose 1/6 of the potential power, and instead of 200W you get 167.

(I probably should have said "column" in my first response, "string" more often refers to series panels)

[BFL13]

mike-s wrote:
It won't damage the diodes. They're there to bypass shaded strings of cells. But, especially if you use a PWM controller, shade on even a single cell can stop all power output from that panel for as long as it's shaded. But nothing gets damaged.

What does PWM vs MPPT have to do with it?

There was something about cells in panels being damaged by some light effect, can't remember what that was (not the water one either), but it wasn't mottled shading or the diodes. Where a cell got too hot ISTR.