Solar PV Array Switching

[Warpspeed]

Try fitting diodes where you join panels. See if it makes any difference ?
What I use are the thirty five amp metal diode bridges.
The metal case is insulated from the diodes, and you just use two of the four internal diodes.
Each panel positive connects to one of the ac terminals, and the + output of the bridge is the output.
The -ve terminal is not connected.
It will probably run quite hot, but you can bolt the metal case to one of the panel mounting rails as a heat sink.

 

[gotbeans]

looks like a full bridge rectifier, bridge is short for it? thought it was for ac to dc ?
for this setup I got those diodes inside the cable plug things for panels

hopefully my drawing is obvious cuz I am bad at explaining it (pretend there's a black wire with all the red wires..)

I have some panel issues it's supposed to be 10 on each thing but gotta return 3... so didn't hook up the 1 extra cuz it doesn't work like that

 

[Warpspeed]

Yes its a bridge rectifier, but the diodes can be used for other things beside rectification.
They are good because its mechanically strong, weatherproof, and cheap.

 

[gotbeans]

I'll keep it in mind if my things die. I got a few replacements cuz the 30 amp diodes are like $1-2 each
my drawing is actually wrong, I got diodes where the fuses are. So I have 4 fuse 4 diode total

 

[ricardocello]

Try fitting diodes where you join panels. See if it makes any difference ?
What I use are the thirty five amp metal diode bridges.
The metal case is insulated from the diodes, and you just use two of the four internal diodes.
Each panel positive connects to one of the ac terminals, and the + output of the bridge is the output.
The -ve terminal is not connected.
It will probably run quite hot, but you can bolt the metal case to one of the panel mounting rails as a heat sink.

I’ve done exactly this for my 4s3p setup.
Big metal bridge rectifiers with two diodes in parallel, with significant heat sinking, 1000V reverse rating.

These are Blocking Diodes. Most solar advice out there today says you don’t need them.
I just sleep better knowing that no power will ever backflow to other strings during shading.

In my case, the bridge rectifiers can put out 12W heat max when full current.
Meaning I’m losing 36W of power in full sunlight due to diode voltage drop on three strings.
I’m fine with that, but i suspect others would not.

 

[gotbeans]

Why do you both have the diodes in series, for even less leakage?

 

[Warpspeed]

Not less leakage, but NO leakage.
And as Ricardo above^^ says, it also solves the problem of shaded panels pulling down good panels.
It solves quite a few unsuspected problems, even though the diodes do introduce some extra slight losses, it appears that is outweighed by the advantages.
Its definitely well worth a try.
If you gain nothing, you can easily remove them, but I doubt you will do that.

 

[gotbeans]

Not less leakage, but NO leakage.
And as Ricardo above^^ says, it also solves the problem of shaded panels pulling down good panels.
It solves quite a few unsuspected problems, even though the diodes do introduce some extra slight losses, it appears that is outweighed by the advantages.
Its definitely well worth a try.
If you gain nothing, you can easily remove them, but I doubt if you will do that.

well "no leakage" is less than some leakage ?
I didn't know single diode would have even noticeable leakage.. maybe I have to get 2 now

I'd have to do this type of test by putting blankets over one array and checking the power draw to get absolute worst case scenario? Or what would you guys do. I assume the diode is a lot hotter doing that.
I have a lot of 30 amp and 50 amp diodes, the 50 amp will get less hot obviously, is there a con to using one so large? (my arrays in parallel is 16 or something close amp)

 

[Warpspeed]

If you are only putting 10 to 15 amps through a thirty amp rated diode it is still going to get hot.
A fifty amp rated diode dissipates the same power and might get just as hot.
If you bolt either to a heat sink, both will then have a way to get rid of that heat and operate at a much lower temperature.
The thirty amp diode will work fine. Only reason to use something larger is if you can get them free !!

 

[gotbeans]

If you are only putting 10 to 15 amps through a thirty amp rated diode it is still going to get hot.
A fifty amp rated diode dissipates the same power and might get just as hot.
If you bolt either to a heat sink, both will then have a way to get rid of that heat and operate at a much lower temperature.
The thirty amp diode will work fine. Only reason to use something larger is if you can get them free !!

Yea I got a lot of them. I have drawers full of stuff from working, I figured the larger one more surface area so can handle the heat
I was also just looking at things like this which are similar to yours but bigger

https://www.aliexpress.us/item/2251832726856632.html

with this big fat guy he should stay pretty cool, more heat is more resistance so maybe I'll save 1 watt per year

I'll have to look at it in summer, right now the heat of the diode is pretty non existent.
You think they'd melt my solar wire connector things? they're plastic

some things like these sites, I guess they're called MC4 connectors.. My other array pre-dates when these were on panels I guess, seems they're on everything now lmao

MC4 In-Line Diode for Reverse Current Protection

In-Line reverse current blocking diode with solar MC4 connectors for positive (+) solar input. Can be used to prevent batteries from back feeding to panels at night, and to optimize systems with multiple solar panels wired in parallel. Rated for 20A max.

powerwerx.com

https://www.amazon.com/gp/product/B0B4DSHB3J

 

[Warpspeed]

Use whatever you already have, but bigger will not make any difference electrically.
Those big 100 amp units are only rated at 100 amps when bolted to an appropriate heat sink.
Completely naked, the biggies get just as hot as something smaller.

 

[gotbeans]

It is a big heatsink, the other smaller ones have no heatsink at all, but yea if I even went above 50amp I'd have an active fan on it etc
with the inverters now though with such high voltage I will never see such an amperage

 

[ricardocello]

It is a big heatsink, the other smaller ones have no heatsink at all, but yea if I even went above 50amp I'd have an active fan on it etc
with the inverters now though with such high voltage I will never see such an amperage

i used the 25A variant of these: Vishay Bridge Rectifiers
and bolted them to some weatherproof solid metal Hammond boxes used as heatsinks, with some heatsink compound.
My arrays are ground mount so I have plenty of room.

 

[Hedges]

With 2 strings in parallel, neither diodes nor fuses are needed, even if one string is oriented different so shaded.

It is said to fuse all strings if more than 2 in parallel. If you have 3 in parallel and one shorts, the other 2 dump their current into it. But the recommended fuse side (e.g. 15A for panels with 7.5A or so Isc) probably won't blow.

If one string is shaded, ambient light still causes it to have Voc about the same as Vmp of other string in full sun (I've measured it), so no current flows and diode doesn't make a difference. If it gets no light at all, it still doesn't carry much current when driven to Vmp by other string. I tried 2 panels in series in the sun for 2x voltage, connected to 1 panel in a dark room, and current was about 40% of Isc.

Manufacturers have always said all panels have to be same orientation, whether in series or parallel. But examining the IV curve, we can see that Vmp is only slightly reduced. SMA ran a field test to determine the difference, and published a white paper saying that two strings of different orientation in parallel delivered about 2% less power than with separate MPPT.

Given the low cost of PV panels today, compare to cost of SCC or inverter, getting up to 50% more kWh through the SCC at the cost of 2% reduced efficiency makes sense.

In the event of a shorted string, the diode would keep it from dragging down the good string.

 

[Hedges]

yea but from what I've seen the panels draw power and radiate infrared light if they have no light coming in.. so they'll take from the other array
(and thus the arguments I see all over begins)

"Photoluminesece", but I wasn't able to see anything except IR from heating. And in a pitch black room, hardly any current drawn until I drove voltage above 2x Vmp toward 2x Voc, even then about 40% of Isc.

Any ambient light on one string and it's Voc was about Vmp when in full sun.

Measuring PV panel performance

I suppose I could buy a solar panel tester like Will reviewed, but that doesn't feel DIY enough to me. Besides, I'm a cheapskate, and I try to do things in the most difficult (and expensive if the equipment wasn't on hand) way. I'm presently working on an underperforming string of Sharp 165W...

diysolarforum.com

If a string 10s is in the sun, parallel with 10s in the shade, approximately zero current drawn while operating on an MPPT. The IV curve seen would be affected when controller does sweeps at higher or lower voltage, but peak voltage and power about the same.

When both strings have sun but at different angles, they will be pulled slightly off Vmp, produce slightly less power. But probably more from the array than if all current pushed though blocking diodes, causing power loss. Using an "ideal diode" (FET with drive circuit) might save drop through diode, but won't make up for the slightly shifted Vmp. Only separate MPPT would, but only about 2% extra power available, not worth the cost.

 

[gotbeans]

I do sometimes see almost double the voltage on one of them than the other. The first one in the 2nd array gets shade from a fence & building which is why I separated them into parallel squares instead of long rectangle horizontally
The ambient light isn't giving it max voltage, but you say it's supposed to be sitting at absolute max? I don't have this, a cloudy day has more voltage than the afternoon for this section.
Yea I wish I could have MPPT per panel ? one day they'll be $10

 

[AntronX]

The ambient light isn't giving it max voltage, but you say it's supposed to be sitting at absolute max?

They should be near Voc even in the shade. What panels do you have?

 

[gotbeans]

They should be near Voc even in the shade. What panels do you have?

Blemished Canadian 250W Solar Panel | SanTan Solar

These Canadian 250W Solar Panels have water intrusion, which has caused discoloration on the vinyl and busbars. Please see pictures for more details.

www.santansolar.com

I think these ones, they were $45 when I got them

 

[AntronX]

santansolar.com/product/blemished-canadian-250w-solar-panel
I think these ones, they were $45 when I got them

OK, that explains it. Worn out blemished panels can develop cell to frame leakage due to moisture ingress and i suspect old cells can wear out and develop internal leakage current which would pull your low light open circuit voltage down. You probably still have very insignificant amount of leakage current noticeable only in low light and in open circuit measured with your voltmeter. Chances are your diodes produce more loss at full light vs. just letting shaded panels "steal" couple of milliamps from working array.

 

[400bird]

They should be near Voc even in the shade.

Just to add some detail here, they should be near VOC when unloaded. A PWM or other load can pull them down significantly lower. A bad MPPT algorithm can also pull down the voltage, a good MPPT should hold them near VMP under most light conditions (if there is enough light to make power)