# MPPT Sizing

Will a Victron SmartSolar MPPT 150/100 be sufficient for 3S5P 445W Canadian Solar panels or should I get a 250/100? (Paired with MultiPlus II 48/5000 and 10kW BSLBatt Powerwall)

I donâ€™t think 3S would work well. Would get quite close to 150V? Rather go 250/100. What is the Voc on those panels?

Not sure if my math/understanding is correct

Voc/Isc
3x48.9V = 146.7V
5x11.54A = 57.7A
146.7V x 57.5A = 8,435.25W

Vmp / Imp
3x40.9V = 122.7V
5x10.89A = 54.45A
122.7V x 54.45A = 6,681.015W

The MPPT 150/100 says the following

> Nominal PV power, 48 V 1a,b) 5800 W
> Max. PV short circuit current 2) 70 A (max 30 A per MC4 conn.)
> Maximum PV open circuit voltage 150 V absolute maximum coldest conditions 145 V start-up and operating maximum

> 1a) If more PV power is connected, the controller will limit input power.
> 1b) The PV voltage must exceed Vbat + 5 V for the controller to start. Thereafter the minimum PV voltage is Vbat + 1 V.
> 2) A PV array with a higher short circuit current may damage the controller.

Not sure what to make of all of this

For further insight, see: New member from Durban - #36 by Phil.g00

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Thatâ€™s at 25Â°C. If you take into account the temperature coefficient, you may well find that it exceeds 50V at a temperature thatâ€™s entirely within the realm of possibility. Philâ€™s other linked thread has a good example of accounting for this.

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Thanks for everyoneâ€™s input. It has turned into a very interesting conversation.

Staying withing all recommended parameters, which of these 12 panel configurations would be the most beneficial? Victron Calc

• 2S6P - 150/85
• 3S4P - 250/85
• 4S3P - 250/85

Is there a benefit of using a /100 over a /85 in these configurations? i.e. Will the battery be charged faster or will it be wasted money?

Additional context, I have a flat IBR roof, so can mount panels with an optimal tilt.

Thatâ€™s the setup I have: SmartSolar 250/100 with Canadian Solar CS3W panels in a 4S4P configuration.

Hereâ€™s the yield curve for yesterday:

The jagged curve after 1pm is because the batteries were fully charged and it only had to carry the loads. I have Pylontech batteries with a charge voltage of 52V, so it clips at 5200W in summer around noon. My panels are 400W, so I expect youâ€™ll experience more clipping on fair weather days.

Be aware that if you run a SmartSolar at a constant 100A that it gets very hot, 80C+, which scared me a bit. I mounted mine on a large aluminium plate which brought temperatures down to about 60C.

The advantage of the 3S5P configuration is that one shaded panel will only drag down two other panels with it, whereas with the 4S4P configuration a shaded panel will drag down 3 othersâ€¦ but you do have an extra panel to make up for it. Either way, if you mount them all facing north at a 30 degree tilt youâ€™re going to experience significant clipping over the peak hours of the day. If your roof allows it I would recommend you point a couple of sets NE and a couple of sets NW. Thatâ€™ll reduce the clipping losses, and give you better yields in the early morning and late afternoon. (Just make sure that all panels in a series set point in the same direction.)

I donâ€™t see the benefit of the MC4 version. The battery cables are screwed down regardless, so itâ€™s not like the MC4 plugs are going to allow you to plug and unplug the SmartSolar quickly. Perhaps someone else can come up with a scenario where the MC4 version saves time and/or money, but Iâ€™ve always gone for the Tr.

That is the safest option (if you truly believe you can blow your Mppt with 3s). I have done over 100 installations with 3s on 150/? And never lost one. If you are looking at a 100 amp outputâ€¦ I would rather go for the 250 and oversize up to 30%

I am running 150s with 3s 47.9 voc in Bloemfontein for over 3 years now with no issue. People tend to forget, the moment the Mppt starts working it pulls the voltage way down. Reaching 150voc even below zero under load would be the exception, not the rule.

In SA we use our batteries, so the Mppt starts working the moment the sun allows it toâ€¦ Againâ€¦ Reaching 150voc in Bloemfontein is almost impossible, Pretoria even more so.

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Tr

Yes, but it also rescans every half hour and when it does, the voltage temporarily shoots up to Voc.

Mainly, I think the reason people get away with 3s on the 150V controllers is

1. those do allow a bit more room at the top, to mention just one component, 160V is a standard capacitor size and is what is used in those chargers, while on the 250V chargers, it has 250V caps. Way less margin.
2. Typically in South Africa, the panels have time to heat up before full sun heats them, so even if ambient is at a low-ish temperature, the panel typically is not.
3. If youâ€™re installing smaller panels (in the 350W to 450W range) on the smaller MPPTs (which is probably what youâ€™d do), then you also find that typically those panels have a Voc thatâ€™s a bit lower than the larger panels (which are quite close to 50V already). The 47V Voc panels you mentionâ€¦ those are not the ones that worry me. It is the 49V ones!

With all that said, I have 3S arrangements on my own roof, on a 150V charger. But I live in a coastal area where Iâ€™ve never seen lower than 0Â°C, and the coldest in the last 50 years was -7Â°C. So Iâ€™m happy with my calculated risk.

Jaco, you are correct. The MPPT will start to pull down the voltage as soon as it makes the first watt.
This meant it really only became an issue if, on that cold morning, the user disconnected the battery, and that power had nowhere to go, exposing the MPPT to the total Voc.
As long as the user left things alone, he wasnâ€™t at significant risk on a cold morning.

But things have changed quite a lot over the last 5-6 years, Lead-Acids are out, and Lithiums are in.
And with Lithium batteries came the BMS, which has half a dozen reasons to switch the battery supply to the MPPT off.
The more recent, sophisticated BMSs actually have low-temperature protection that prevents charging around 0 deg C. This form of protection all but guarantees the MPPT will be hit by the full Voc, and nothing will be available to pull down the PV voltage.
So it is quite explainable why the legacy successes may not be the same future success stories.

Mine are 49.9V (JASolar 525â€™s) - to close for me and I went to 250/xx MPPTâ€™s.

Tr for the current rating limit of an MC4 connector (30A I think?)

@Phil.g00 and @plonkster all im am trying to say, although the possibility is there on paper and in calculations, to reproduce that perfect moment and for all the contributing factors to line up for the MPPT to pop, is almost impossible in SA, especially in Pretoria.

Theory and what happens in practice are two different things. I have so many installations all over SA from way up North, through the Freestate and Karoo right down to Capetown, and non of their data makes me worry for one second.

Even though BMSâ€™s can disconnect the batteries from the system, if the design was done correctly, this will not happen.

Not to labour a point, but you are saying that it is OK to not follow design criteria correctly in one area if design criteria are followed in another. Which is an inconsistency.

What I will say is this: Weâ€™ve seen the voltages of panels steadily rising over the last decade. There are panels out there with a Voc of 49.9V at 25Â°C. Youâ€™re an idiot if you put that into a 3s arrangement

Not only because of the risk of blowing something, but also because above 145VDC, the MPPTs wonâ€™t even attempt to start up.

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To me (Raka) 146.7 is < 150, the changes of that ever exceeding 150 is extremely slim.

When during the day do will this happen? And then, where in SA will you experience sub zero temps at that time of day?

I would like to see real world data showing 3s on panels smaller than 500Watt exceeding 150V. If you have some, can you please share it?

Phil, I hear what you say, but I base my designs on Calculations AND real life experience. I am pretty sure that with almost 500 installations I should have a pretty good idea of how the installations perform in practice. In Europe I will never ever attempt 3s, but in Sunny SA, its a total different ball game.

I rather leave you to your own opinion, no amount of data I can supply will convince you that the risk as you perceive it, is much much smaller than you think.

Lets stick to the safe design parameters, 2s on a 150, 4s on a 250 and 8s on a 450.

Not to labour the pointâ€™s above and below.

1. Mine are to close for comfort. 49.9v
2. Warranty issues can come into play.
3. The price difference is significant but I think the room at he top end is worth it. ie Not running the MPPT at full throttle all the time is a good thing.

BUT - if an installer specs and implements I would expect they take up the slack IF there is an issue. So no problem with a 3P on an 150/xx as long as installer doesnâ€™t disappear.

Again, I am not arguing that there isnâ€™t many many factors in SA that means we (almost) never see those high voltage, and I have zero beef with older installs (one of them being mine). The reality is that even that 145V-150V do-nothing region is very rarely observed.

My point is simply that the margins have been shrinking, and I think the time for 3s on a 150V controller is coming to an end quickly as the panels get bigger.

In fact, even the 250V controllers are getting a bit small these days. Which is why we have the 450V ones nowâ€¦

I cannot in good consciousness advise someone to do this (here, today, with current 650W panels) without a massive massive disclaimer.