Mixing and matching panels (with Solis AC Inverter)

I KNOW this has been discussed over the years, but maybe this can be a clean clear thread.

The existing panels 4S/3P:
12 x Canadian Kumax CS3U 350P
Pmax: 350W
VMPP: 39.2V
IMPP: 8.94A
Voc: 46.6V
Isc: 9.51A

New additions, as there are NO 350w with the same specs as above.
1 x Canadian Kumax CS3U 355P
Pmax: 355W
VMPP: 39.4V
IMPP: 9.02A
Voc: 46.8V
Isc: 9.59A

2 x Canadian CS3W-420P
Pmax: 420w
VMPP: 39.5 V
IMPP: 10.64 A
Voc: 48 V
Isc: 11.26 A

Due to space constraints, I can only fir 15 panels.

Can I connect the 1 x 355w and the 2 x 420w panels 5S/3P with the 12 x 350w?

Alternatively, I just say phukkit, get a Solis 700w grid-tied inverter, and connect the 1 x 350w and 1 x 420w in series onto it, East facing.

Yes (as long as your MPPT can handle the voltages). Off course those panels will act mostly like the 350Ps that you already have ITO power output.

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This brings ANOTHER “thing” to the forefront.

To date, I see the max volts the MPPT has ever seen on 4S, was 180v on the data I have.

I always use Zero for safety, like so, and this is way to close to comfort IF one wants to play it save all the time.

BUT, CoCT has never seen Zero degrees on a cold sunny day, so this is more than fine.

Thing is, the 4.2kw array has had spikes of like 5750 watts. I did not catch the volts on those days.

I think I’ve seen a -7°C in weather data, from several decades ago. It is exceedingly rare, but it does happen. I have experienced a 0°C in Somerset West in the last decade. Once.

Same here, once in S/West, a solidly frozen windscreen and boat cover, early one morning, hence me always going for Zero on the calcs.

This means, that if I follow my own “rule”, I should not do that … added to that, the weather is changing around us.

Most of the time when you see a -7 degree like that it would be at night and when the sun starts shining it warms the place up again during the day and by the time you get good angles on your panels it should be a bit warmer.

So I tend to look at the avg min, more that the extreme min if it is one of those once in a 50 year occurance.

Disclaimer: I am not an authorised financial service provider of solar service provider and I don’t have any registration numbers. There is no reason you should listen to my advice.

Historically, I can get -6C, which pushes me just over the threshold, pity.

@TheTerribleTriplet, as I have mentioned many times I am a fan of paralleling E and W arrays.
Now, I use the same panels in both arrays. But it is similar, to the question you are asking because those arrays have different maximum power points at different parts of the day.

This is also taken care of automatically by the MPPT which selects an optimum throughout the day and weights the optimum towards the most productive array (at that time of day).

So I don’t believe paralleling these (close-ish) panels will be that far from optimum, the MPPT will settle on the optimum maximum power point. And that compromise maximum point will be very close to the per string maximum power point.
I think to minimise the effect even further, I wouldn’t have an oddball panel on every string, but have 10 identical panels on 2 strings and all the oddballs on a third string.

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I agree with Phil.
Those specs are close to the original so that they will not be a big impact. You will get a bit less from the larger panels as the current will not be as high.
I will also put the odds all in one string and the match in other strings if you can. Then you know the matched will be working the best they can and the odds will try to get there as well.

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Excellent idea ito the strings. Thanks, @Phil.g00 and @Louisvdw

Would anyone go for the 5S ito the volts, betting on it never exceeding 249v?

I do that often…

I even have a site with 6s 330 monos on a 250 mppt, site running for almost 3 years now.

If you read my disclaimer above, I would yes.

If you look at the PV voltage and current graph in VRM you will see that it takes a bit of time to get to the working voltage for the panels. So even though dawn was 06:23 for me this my panels only got the their working voltage about an hour later which should be warmer than any negative temp by then.
Also by then your SmartSolar will start using the panels alrady, which means you will use the Vmp and not the Voc voltage of the spec.

The panels used in the picture below has a Vov of 49.5V and a Vmp of 42.0V. If you ever open the breaker during those very cold mornings the open voltage Voc will reach your MPPT until it will start pulling the current that will lower to the working voltage. That will not be a good idea.

You’re on your own there. I don’t push it, but I see -6C every 5 years.
Panels get up to Voc with very little light, so I don’t even see the merit of doing it on a West string. ( Where theoretically the panels will be warmer by the time they kick in).
On the other hand, a Victron MPPT should kick in at 5V+ of the battery voltage, (and therefore >Voc never happens), but these days with BMSs disconnecting batteries can we actually rely on a battery being available?

Even if a BMS disconnect the normal draw of a home is in most cases a few 100w that should keep the panels from reaching Voc. But that is a good point to remember.

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Titbit to share:
The new model BMS, during top balancing, has disconnected a time or two, Victron never registered that like the model before, that infamous DC Ripple I become so familiar with.

I hear the relay open, charging is disconnected, yet the system happily continues working powering the house/loads.

Think it has to do with these little wires:

I have seen a clip where a Victron MPPT will turn on with just the PV voltage, but many MPPTs need the battery voltage first. There has been some discussion about the BMS being locked out from restarting because of this issue. I also think the Deye/Sunsynk inverters with built-in MPPTs have had this issue after switching off on low battery voltage, maybe I’m wrong though.

Yes, I believe those wires sense that there is a DC voltage on the bus. This in turn releases the BMS to turn on. If this happened at night say, a Victron MPPT would wake up (and provide a voltage to those wires) when there was sunshine, but a lot of other MPPT brands don’t.
Therefore there is no voltage on those wires, therefore the contactor never closes, and subsequently, those MPPTs never turn on. The system is effectively locked out.

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Ill make a little video this weekend… Wil explain why I am Comfortable with 5s on 250 and 3s on 150mppt’s

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What I saw this morning:

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