Mixing Solar Panels, check my work

[ADDvanced]

So I have a bus with 2 100w Renogy panels on the roof. It's not cutting it anymore. I have an extra 230w santan panel laying around doing nothing... so mocked this up, does this look correct to everyone? Also yes F Renogy, but this system already exists, so I'm using it.

 

[sunshine_eggo]

Vmp must be very close. your mixed array will underperform significantly, and there's a good chance your 2X renogy panels will simply discharge into your Santan panel giving you nothing.

I missed some detail on the parallel and series.

Parallel would provide about 360W, but there's a risk that the SanTan would simply discharge into the renogy, and you get nothing.
Series would provide about 345W.
Mix would provide about 360W, but there's a risk that the renogy would simply discharge into the santan, and you get nothing.

I would go series.

 

[ADDvanced]

I get that it technically 'underperforms', but all 3 of those mixed diagrams above are at least 145 more watts of solar than I have now. I've been reading a lot about mixing panels, but nowhere did I read about smaller panels discharging into the larger one, have a link?

 

[Mattb4]

After a quick think (and being it is afternoon my thinking is not all that good) I would go for the parallel setup. The thing is that your MPPT SCC might produce better than a simple drop to the same PV voltage might suggest. It could establish a higher Vmp point for the 3 panels at the same Imp.

 

[740GLE]

Can you have one array one direction? And another the other?

 

[sunshine_eggo]

I get that it technically 'underperforms', but all 3 of those mixed diagrams above are at least 145 more watts of solar than I have now. I've been reading a lot about mixing panels, but nowhere did I read about smaller panels discharging into the larger one, have a link?

I revised:

Parallel would provide about 360W, but there's a risk that the SanTan would simply discharge into the renogy, and you get nothing.
Mix would provide about 360W, but there's a risk that the renogy would simply discharge into the santan, and you get nothing.

As you can see, it's not about smaller. It's about voltage.

Current flows from high voltage to low voltage. The higher voltage string of a parallel can see the lower voltage string as a short circuit.

If you're close where The Voc of the lower string is still higher than the Vmp of the higher string, less chance. You are proposing strings that are 33-50% different where ~10% is typically the max.

 

[ADDvanced]

I revised:



As you can see, it's not about smaller. It's about voltage.

Current flows from high voltage to low voltage. The higher voltage string of a parallel can see the lower voltage string as a short circuit.

If you're close where The Voc of the lower string is still higher than the Vmp of the higher string, less chance. You are proposing strings that are 33-50% different where ~10% is typically the max.

I understand what you're saying, I guess I just don't understand why this is. I've been googling and reading mainly posts on this forum about parallel strings of different voltages, but nobody ever really says why.

For almost every other solar theory/basic thing I've had to learn, there are a lot of articles that go into detail on the subject, but for this one, it seems like it's mostly discussions in forums.

If anybody has any links, I'm willing to read them.

 

[sunshine_eggo]

I understand what you're saying, I guess I just don't understand why this is. I've been googling and reading mainly posts on this forum about parallel strings of different voltages, but nobody ever really says why.

For almost every other solar theory/basic thing I've had to learn, there are a lot of articles that go into detail on the subject, but for this one, it seems like it's mostly discussions in forums.

If anybody has any links, I'm willing to read them.

You accept that a charger works by providing a voltage slightly higher than battery voltage so current flows, yes?

i.e., battery at 12.0V, charger at 12.05V, current will flow into the battery. You have two sources, yet one "discharges" into the other.

Why do you not accept a slightly higher voltage source not flowing current through a slightly lower one?

 

[yodamota]

What about another charge controller for the santan panel?

 

[sunshine_eggo]

$67

 

[740GLE]

Get like a dozen of those and you can power the world!

 

[ADDvanced]

You accept that a charger works by providing a voltage slightly higher than battery voltage so current flows, yes?

i.e., battery at 12.0V, charger at 12.05V, current will flow into the battery. You have two sources, yet one "discharges" into the other.

Why do you not accept a slightly higher voltage source not flowing current through a slightly lower one?

No, it makes sense. I just want to read more about it, the 5-10% voltage thing is mentioned a lot on forums, but I'm not finding any nice articles that go into more detail. Like, a lot of the articles about mixed panels don't even mention this, which I find surprising.

 

[sunshine_eggo]

No, it makes sense. I just want to read more about it, the 5-10% voltage thing is mentioned a lot on forums, but I'm not finding any nice articles that go into more detail. Like, a lot of the articles about mixed panels don't even mention this, which I find surprising.

The 5-10% is because if your lower voltage string Voc is above the Vmp of the higher voltage string, so there's a good chance the higher voltage string won't discharge through the lower voltage string. Voc vs Vmp difference tends to be about 17%. 5-10% is just a rule of thumb to keep you under that limit.

 

[Mattb4]

If I had some panels of different voltages to play with it would be a interesting experiment. I expect you do not see many articles about such hookups because most folks simply add another SCC rather than chance damaging their panels.

 

[Hedges]

If you have access to enough panels (or similar panels), what about:

3s Renogy 100W || 2s Trina 230W

That would be 60V, 12.5A, 760W

Best of the two strings have quite different orientation, say 90 degree apart. Peak wattage would then be a good fit for 520W max of SCC.


I am considering a system with 12s || 11s, due to quantities I've got. That's < 10%, and I'll put the longer string in location more likely to get one shaded.