Mix and match Solar Sells

HI Guys,

Im looking for advice.
I need to add more panels to my setup. I currently have 4 JA390w Solar pannels.
I got the 390w, because it “should” be available for some time to come. Got them last year September.
It suited my needs perfect, in summer… Winter, not so great. (you learn as you go on!!)

BUT, i need more panels, , and trying to get those panels is like searching for chicken teeth.

SO, can i add other panels to it? This is what currently have:
JAM72S09 PR 375-395 Global_EN_20190515A (what i have).pdf (397.8 KB)

I can get these units, (405W) this is the specs:
JAM72S10-MR-390-410-MC4-EN.pdf (407.4 KB)

Will they work with each other?

Or If someone have 4 of my panels… I take them!!

Having the new panels in a separate string is not an issue. It is only if you want to add the panels in the same string that you need to be careful to match them.

Any panel will work, but it is like mixing new batteries with old ones.

The issue is with the Voltage that the panels work at. The power (Watts) does not really matter but the voltage is critical.
You are looking at panels that work at the same voltage that your current panels work at. Small differences is not an issue, but big voltage differences will be working against each other.

thank you @Louisvdw
How many volt is small differences?
On the Victron Excell:
390W: @min temp: 215.2v @max temp 142.9v
405W: @min temp: 213.4v @man temp 150.0v
Each will be on there own string.
Charger is Victron 250/70.

If the new panels will be on their own string, then it does not matter at all. See the post about east/west facing panels for more info and a doc with calculation if you are interested.
Solar panel direction East/West vs North

thanks @Louisvdw .

I have to substantially disagree with Louisvdw on this one. (Unless I misunderstood something).

Voltage on panel strings does not matter. The voltage is cumulative if you add the panels to the same string, the problem here is that the panels themselves are “current sources” so if you add the panels in series the voltages will pick up (30V + 50V = 80V) but the currents will be limited. So your new 405W panel would act as if it is a 390W panel.

You can at any time put new panels with old panels, the new will just follow the MPPT and act as the old. So you will lose out in the end.

(Note my comment comes from having practical experience on Utility Scale sites).

The best and ultimate is to replace entire strings or to create a second string as was advised.
What I do find interesting here is that you note that the Charger is a Victron 250/70, so effectively the 2 parallel strings will be on 1 MPPT?

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My understanding is that adding panels together in series adds up the voltage but limits the current to the current of the weakest panel.

When adding panels together in parallel, you will add up the current but get the lowest voltage of a single panel.

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Yes, I can see that.

No, I don’t see this, not without such a voltage severe mismatch that allows current to circulate between panels.
So, if the current can’t circulate the lower voltage panel cannot drop the voltage of the higher one. But the higher voltage panel can dictate the voltage of the lower voltage panel without current circulation.
So, I say it is the higher voltage panel that sets the no-load voltage.
Once an MPPT and load are applied, I suppose the voltage will settle somewhere that is between the ideal for both panels. Supposing the voltage difference is close that shouldn’t be a whole hill of beans.

This is indeed the case. We have on our plants 240Wx24modules parallel with 280Wx24modules. Once the MPPT kicks in the voltages are exactly the same.

What @Rautenk said, it makes no difference. I told Nelius so but asked him to confirm here.

Mix a 100w and a 400w panel, now THAT is an issue.

Just to be pedantic, but for the sake of people that are learning about electricity, (not yourself @Rautenk ) the two panels (strings whatever) will always be at the same voltage by virtue of being connected in parallel. No load, full load, MPPT it doesn’t matter.
It’s just that your answer implies they could be different.
They can’t otherwise, Mr Kirchoff would be upset.
I was only correcting @jykenmynie that it would be the higher voltage for no-load conditions that would reign, not the lower voltage.
@Rautenk I know that you know this, but there are people that read this forum that might take it wrong, and every little bit of knowledge helps.

Correct, under no-load conditions, the voltage would be the open-circuit voltage of the higher voltage string which shouldn’t matter because you get no power at that point. What will happen though during operation is that the power vs voltage curve for the array may look a bit odd, and if the MPP voltages of the 2 strings are very different you could even have 2 peaks. The MPPT will find the point where it gets the most power from the combination of the 2 strings, which will most likely not be exactly the MPP for either string but rather somewhere in-between. If the string voltages are very similar as these seem to be, I don’t think it would be a big problem.

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I tried to get a bit more details on this online, to better understand, but found this:

*Edit: Deleted that link

Now I know better than to trust random blogs and possibly take it out of context, but here they state that a parallel connection will operate at the lower voltage?

Yeah, a lot of BS.
I didn’t make past how a 375W in series with a 180W panel, gives you 609W.
I am not even going to read further.
Life is too short.

Here seems to be a better source (got the link from 4x4 community) what at least got their basic maths correct:

Also saw something similar being said on the other forum when I googled.

Better, but still wrong.

So it seems these guys talk about join solar panels up without an MPPT of any kind? When you add an MPPT I assume it gets a little bit more complicated.

If I correctly understood a post from @plonkster on the other forum, “If you put them in parallel and their Vmp is close, then there should be no derating. If Vmp differs between the strings, then the mppt will pick the voltage that makes the best power, which will likely be the lower of the two Vmp values, so one of the strings will suffer a little, but overall you should still see a net increase in power.”
it does seem to indicate the same thing?

Sorry for continuing this, but I would really like to understand it better myself.

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A PV module is a constant current source (proportional to the level of light of course). If you put them in parallel, the currents will add up.

But power is the product of current AND voltage. So the voltage at which you make them work is what decides the power.

If you run without an MPPT (or a cheaper PWM controller), they panels all work at the battery voltage. Since this is generally 20% or more below the Vmp of the panel, it doesn’t matter that the panels are dissimilar (just make sure they have the same number of cells, don’t parallel a 36-cell with a 60-cell or a 72-cell!). The currents add up. It works as you would expect.

But when you add an MPPT into the mix… then it searches for the optimum voltage to work at. This will most likely latch onto the lower of the two Vmp ratings (because moving above that voltage will cause a rapid loss in power from half your panels). Because the other half of the panels now run at a sub-optimal lower voltage, they will make less power. But they will still make the same current.


So I understand then, putting them straight on your batteries, in a parallel connection, all the panels will operate at the lowest voltage in the DC system (being that of the batteries, most likely) and move current into the batteries at a rate that is proportional to the sunlight on each of the panels (their amps will add up, but the watts moving into the batteries will be [battery voltage]x[panels’ amps])?

Then adding an MPPT allows the panels in parallel to operate at the voltage of the lowest panel and not of the battery, increasing overall power generated?

So, stupid question, but then at night time, the voltage of the panels will be zero, would you need to disconnect your batteries from them else current will flow out of the batteries to the panels?


Usually people put a diode inline to prevent this happening. Or that is what we did back in the 80s/90s before MPPTs existed and when PWM was still expensive :slight_smile:

I have no idea what happens exactly at night. The PV cells themselves are diodes, and they are reverse-biased at night. But I believe they do have a bit of reverse leakage. There’s a whole science to that, not all diodes are completely “electron-tight” in reverse. In fact, Schottkys are known to have leakage… and so do PV cells as far as I know.

Edit: Just wanted to add… obviously at the very least put in a fuse if you’re going “direct to battery”.