Differing voltages of used panels

[MountainmanBill]

I bought 9 used Jinko solar panels today. Here are the specs:

295-watts
VMP 36.8 V
IMP 8.08 A
VOC 45.2

The voltages of the panels are varying anywhere from 28-41 volts. The amps are roughly the same at 6-amps. It was hard to get accurate readings with varying cloud cover, but I took the reading with roughly the same amount of irradiance. The nine panels will be connected in 3S3P. I understand that the best I can hope for is the lowest voltage and lowest amps in each string. If the lowest voltage is indeed 28 volts per string, then the total voltage for the array will be (3 X 28-volts) 84 volts. I have two charge controllers connected to the same battery bank. The other array has around 100 volts going into its charge controller. This additional array will have 84 volts going into its charge controller. That's a 16-volt difference between the two arrays feeding their individual charge controllers, which feed the same battery bank. Is that 16-volt difference acceptable or not?

 

[sunshine_eggo]

With a spec of 45.2Voc, I would consider anything below 40 a bad/problematic panel.

I would repeat the test by laying all the panels nearly flat on the ground (prop up one end with a brick or something - flat panels can kill grass due to the heat) in the sun for 30 minutes. Then take measurements of all 9. That way you know they are all good and warm, and you can compare apples to apples.

A list of all 9 voltages vs. a range is more useful.

Diode failures in the junction box can cause low voltage readings:

Checking a PV panel for open bypass diodes

Panels subjected to regular shading conditions are prone to have blown out open bypass diodes. Bypass diode can fail open or short but most often they get overheated and fail open. A shorted bypass diode is easy to detect as Voc of panel will be low due to the shorted bypass diode shorting...

diysolarforum.com

 

[MountainmanBill]

What is this?

 

[SamDeleted]

What is this?

View attachment 143534

Looks bad to me! I'd be getting back onto the seller asking for a refund, that is a problem

 

[MountainmanBill]

I brought one of the low voltage panels indoors, removed the junction box cover, then removed all the caulking type material sealing its innards. I see no diodes to replace.

What I did find, whether or not it means anything, are four tabs that were bent over one another. The two center tabs appear to be soldered together and do not separate. The top and bottom tabs, marked by my fingers, opened quite easily. I thought maybe the diodes were hiding inside the tabs. No such luck. I don't believe these panels have diodes. But back to the tabs.

Indoors, with very little light, the panel has a voltage of .61, with the upper and lower tabs separated. When I touch the top tabs together the voltage goes up to 2.5. When I touch the bottom tabs together the voltage reads 1.8 volts. When I touch both top and bottom tabs together the voltage is 3.01 volts.

My guess is that all four tabs should be soldered together but for some reason the top and bottom tabs separated, separating some of the cells from the circuit perhaps? I'm thinking of soldering the tabs back together to see what happens. Thoughts? Anyone ever deal with this type of situation before or know what's going on exactly?

 

[MountainmanBill]

So here are my findings...

In full 11:00 a.m. sun the panel had a voltage of 13.9.
Connected the top two tabs together and got 28.0 volts.
Connected the bottom two tabs together and got 28.1 volts.
Connected them both together and got 42.3 volts.
So I re-soldered the top and bottom tabs together. The panel has a voltage of 42.7 volts and 7.7 amps. That's better than the 28 volts the panel had when I got it. So for whatever reason the tabs somehow got separated and lowered the voltage and amps of the panel. It probably had a poor solder joint from that factory and all the jostling around over the years the solder joints failed.

Now, should I refill the junction box compartment back up, and with what, or should I just leave well enough alone? The compartment's cover doesn't have a rubber seal.

 

[sunshine_eggo]

Those tabs are where the diodes would go.

Have you opened up the junction box of any other panels? I would do this before wrapping up this panel. A panel without bypass diodes will completely cripple a string if there is any shading anywhere on the panel.

 

[MountainmanBill]

I spent the day in the warm sunshine (boy it's nice to see the sun again!) repairing the rest of the panels. I decided on running a bead of caulk around the cover perimeter to seal the junction box back up. Here are the numbers-

Panel #1. 42.8 volts, 8.80 amps
Panel #2. 42.2 volts, 8.62 amps
Panel #3. 42.5 volts, 7.95 amps
Panel #4. 44.1 volts, 8.86 amps
Panel #5. 44.3 volts, 9.04 amps
Panel #6. 44.3 volts, 8.32 amps
Panel #7. 44.7 volts, 8.70 amps
Panel #8. 43.0 volts, 8.03 amps
Panel #9. 43.3 volts, 7.43 amps.

So back to my original question about feeding the same battery bank with two different voltages from two different arrays. What is the tolerance there?
If I go off the lowest voltage in the strings, that being 42.2 volts (panel #2) X 3 strings, that will give me 126.6 volts going into the charge controller, if I understand this correctly. My other array puts out right around 100 volts, give or take, and into the second charge controller. That's a difference of 26 volts. If that's too big of a difference, is there anything I can do to get the voltages closer together?

I have some 100 watt panels if it matters any.
Vmp 17.8
Imp 5.62
Voc 21.6

 

[sunshine_eggo]

The range of your 9 panels is NOT a concern. New panels typically have some kind of tolerance on their specifications - ±3% or something like that.

Vmp of panels/strings in parallel on the same MPPT need to be within about 5-10% of each other. The 126V difference would either cripple the 126V array or completely negate the 100V array.

EDIT: Added NOT

 

[MountainmanBill]

Then how about this. I understand that I can mix and match solar panels in any way I choose. The drawback is that each string will use the lowest voltage and amperages of each string involved. So in essence I would be losing potential energy that could otherwise be gained by using like panels. The trouble is, I don't have like panels. I have two different makes and models with differing voltages, watts, and amps.

If I put together strings with the same make and model and volts and amps, I get an unbalanced voltage going into each charge controller. Apparently not a good idea.

Going strictly off the Vmp of each brand I have 36.8 volts (brand #1), and 30.3 volts (brand #2). If I used two brand #1's and one brand #2 for each string, three times to make up the array, put the three strings together in the combiner box, I'd have the lower voltage of brand #2 of (3X30.3) 90.6 volts to send to charge controller #1. For the other array I'd do the same thing but use two #2's and one #1 for each string for that array. I'd still have the lower voltage of brand #2 of 30.0 volts and 90.6 volts for the other charge controller.

This is ok to do right? Without any issues created by such a configuration? It's either this or I just spent a butt-load of money on panels I can't use in my system and the better part of a day repairing them.

 

[sunshine_eggo]

Then how about this. I understand that I can mix and match solar panels in any way I choose.

For panels on the same controller:

1. Series Imp must be nearly identical. ALL panels in a series strings will run at the lowest rated current.
2. Parallel Vmp must be within about 5-10% of each other. At this level, you can be confident you will get reasonable function from both strings, and your statement below will be mostly true:

Sorry, I didn't pick up on the 2nd controller in your post.

Looping back to your original post:

The other array has around 100 volts going into its charge controller. This additional array will have 84 volts going into its charge controller. That's a 16-volt difference between the two arrays feeding their individual charge controllers, which feed the same battery bank. Is that 16-volt difference acceptable or not?

Acceptable.

The individual MPPT have no idea what the other is doing. As long as each array is acceptable to the MPPT it is connected to and the voltage is high enough to charge the battery, then any difference in the two arrays doesn't matter.

 

[MountainmanBill]

So you're saying that I can connect my three strings of three panels each of the same brand into one array, and the three strings of three panels each of the other brand for the second array, feed them into their own separate SCC, then into the same battery bank, even though they have differing voltages? That is good news indeed!

Now for the right hand not knowing what the left hand is doing (happy Easter), according to the manual of my Morningstar 45-amp 150-volt charge controllers, it says something about them talking to one another, so I think they do know what the other one is doing. But maybe they don't care. I have noticed that when the batteries are absorbing that one controller holds back some amps from the smaller array while the bigger panels feed the battery the bulk of what the arrays are producing according to the battery bank's needs. Maybe the same will be true with this set up?

 

[sunshine_eggo]

So you're saying that I can connect my three strings of three panels each of the same brand into one array, and the three strings of three panels each of the other brand for the second array, feed them into their own separate SCC, then into the same battery bank, even though they have differing voltages? That is good news indeed!

Now for the right hand not knowing what the left hand is doing (happy Easter), according to the manual of my Morningstar 45-amp 150-volt charge controllers, it says something about them talking to one another, so I think they do know what the other one is doing. But maybe they don't care. I have noticed that when the batteries are absorbing that one controller holds back some amps from the smaller array while the bigger panels feed the battery the bulk of what the arrays are producing according to the battery bank's needs. Maybe the same will be true with this set up?

Any connected charger "communicates" in response to the battery voltage, which can be influenced by other chargers, so they can "see" the effects the others have on the battery voltage, but they have no idea where it's coming from, and it doesn't propagate to the array.

In some cases, some MPPT can actually communicate. Victron smartsolars can be in a bluetooth VE.Smart network where a master controller regulates the charging phase and current of its peers; however, even in that case, you could have completely different arrays on each controller, and this "master controller" regulation ONLY matters in absorption and float. In bulk phase, it's all-balls-out for all chargers.

In absorption, it's voltage limited, so only so much current can be drawn from the chargers.

Might be worth a quick read:

Charging from multiple sources

One can have as many independent charging sources running concurrently as they like provided they are programmed properly for the voltage of one's battery bank. Examples (some or all at once, even multiples of a single type, different brands, etc.): Solar PV via MPPT Solar PV via PWM AC-DC...

diysolarforum.com

 

[MountainmanBill]

Thank you kindly sir! I'm going to sleep really, really well tonight.

 

[sunshine_eggo]

Sorry for the confusion.