Mixing and matching panels (with Solis AC Inverter)

After a lot of thinking, and asking a reputable Victron supplier too, I’m not going to bet the replacement of a 250/100 on 5S just to add 3 x 350w panels (1050w -20% = 840w) to the existing array.

As it is really primarily to heat geysers, removing 1-2kw off the existing system, to be able to recharge a 560ah bank, that this is a much better more efficient option: Solis and Victron on same system

I have 4S on my 250/100 and so far this is the max volts I got so not near the 200V I expected which means I probably could have gone 5S but also didn’t want to risk it.

You guys are making me think that I should do a wiring change. Max I have seen is 146v on my 150/45 MPPT. 3S Longi 455w panels. (49.5 VOC STC).

Above 145V it stops making power completely, so definitely a good idea, even if you don’t care about blowing up the MPPT.

What’s the temperature coefficient on those panels? It would be interesting to back-track from 49.5V/25°C and see at what point you might get damage.

And on the 250’s, 245v?

Not sure… check the documentation. Probably something like that.

LONGi_LR4-72HPH_425-455w_-DS2.pdf (2.1 MB)


I didn’t know about the 145v cut off. I can’t say I’ve often noticed it going over 145v often in an open circuit state consistently. Normally sits on 140v but the peaks hit around 145v-146v. This was Sunday:


I’d say you are on borrowed time.

It is rare to have peak sunlight without getting at least some heat into the panels already, that is to say often the panels are warmer than ambient. That gives you a bit of leeway, but I have also been at a place in Southern Namibia (called Koës), and even in Windhoek you get days like that where you have a sharp cold wind which will cool panels down significantly, even in the full sun.

If I take my own setup, the datasheet says my old Canadian Solar modules have a Coefficient of 0.31%/°C, and a Voc of 45.5V at 25°C. So I have 4.5V or 10% room at the top, and 10 divided by 0.31 gives me 32 below 25°C, or -7°C.

Now the lowest temperature recorded in my area in the last two decades is 0°C.

From another angle: The highest panel voltage (3-series) recorded is 140V, or 46.67V per module, about 2.5% over, so between 8°C and 9°C below 25. So the coldest my North-facing panels have been by the time they got enough sun (in my area), was around 16°C.

So doing the same math. You have 0.5V room at the top, or about 1%, which translates to around 4°C room, or 21°C. If I ran those panels on my roof… using my earlier numbers, I’d have blown an MPPT already. Even though my lowest calculated panel temperature is a good 15°C above the lowest recorded ambient, I’d have been screwed.

I concur with Phil. Borrowed time.

OK, will have them changed to 2S 3P.
Thanks for the info.

When making this wiring change:
Disconnect the PV from the MPPT at night, or you may force the condition you are trying to avoid by disconnecting the battery from the MPPT during the day.


Or at the very least, drop the PV before disconnecting the battery, not the other way round.

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This whole argument confuses me, I have put the following question on the Victron Community, as data from many many installations tells me that in our Sunny South Africa the alignment of all conditions needed to blow your MPPT might just never ever happen…

Maybe I stand to be corrected, but in 30 years in the electrical field I have never seen the specks of electronic components within an Device been advertised as the operating specs of the device. I can not believe that Victron will do this… There is always some kind of safety factor in the design… I will definitely loose my respect for Victron If their products blow up if you exceed the rating By a mere one or 2 volts… I would expect 10 to 15volt safety band above advertised specks…

Lets see what their answer is…

Answered @JacoDeJongh and I think its probably the right answer… :slight_smile:
Warranty is their issue and I guess we all have to make this “call” at some stage or another.

I have done this on close to 200 installations, I have monitored them extensively for at least 12 months after installation, I do random monitoring every winter and I can tell you, to exceed that Voltage is not easy…

On one specific installation my guys wired 4s 72 cell panels unto a 150 Volt MPPT, VOC by time of connection was sitting at 179Volt DC, My guys resetted the MPPT at least 5 times before informing me its giving an alarm… I went out to site, fixed the issue and the same MPPT is still running two years and 2 winters later… That is why I say, These just dont blow up for nothing… Man really??

I accept Guys answer, and I can imagine why they want to give this answer… but dont (fully) agree. Most of you know, I have given up on trying to blow Blue equipment for one reason, I just couldn’t…

And that’s probably why we like the BLUE kit - don’t we… The fact that they can be pushed hard speaks volumes for the quality of the components…

Again, I feel like I need to add a little bit of perspective here.

Voc voltages for most 72-cell panels used to be around 45V for a LONG LONG time. That gave you a 10% margin at the top, and in South Africa, that is ample (we don’t get snow and all that). It is the latest and greatest high-power PV modules that’s spoiling the broth (so to speak), because they push 49.5V. That’s a 1% margin.

Any installation done with the <400Wp modules is going to be just fine.

The MOSFETs are most likely 200V rated devices. In the topology used you need to be able to handle the PV voltage+ripple. So as long as the MPPT is disabled, which it should be when it exceeds 145V according to @plonkster , it should be fine up to 200V in theory. (unless they are 180V devices, but they certainly are not 150V devices).

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Ill send you the figures from 540’s, east array in Bloemfontein… the max voltage ever reported to VRm is 137.81 Volt. One would expect that East facing on a cold morning would create higher voltages, but early morning the batteries needs charging so going over voltage will be very very difficult…

That is exactly my point…