mlink avatar image
mlink asked

Exceeding capacity of MPPT 150/100?

I’m building an RV system using (6) 250w panels with the following specs:

Max Power: 226.92w

Vmpp: 21.052v

Imp: 11.01a

VOC: 24.34v

Isc: 11.75a

While researching for the appropriate Victron controller, I attempted to use the MPPT Calculator (Excel) to size the charge controller, but my panels don’t seem similar enough in specs to the Victron panels for inputs.

I plan to use MPPT 150/100 with the above panels (which I already have) in parallel and 12v. But now I’m confused as to whether the panels exceed the controller’s capability, and/or whether I should wire it differently in some combination of serial/parallel.

Can anyone verify this unit will work or recommend a better course of action or a different hardware configuration. Any help greatly appreciated.

BTW: I currently have a 12v battery bank using (4) Trojan T-145 6-Volt batteries. I realize this is small for the above, but plan to replace this after I install the above.

charge controller compatibility
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boekel avatar image boekel ♦ commented ·

this will work fine, I'd put at least 2 panels in series (so 3 strings of 2 panels) you won't miss much power with a 100A MPPT on 12V with 6x 225Wp panels, especially if they're mounted flat.

edit: didn't see that WKirby already replied...

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lnanne avatar image lnanne boekel ♦ commented ·

Boekel, so many people keep suggesting to run panels in series or series/parallel rather than all parallel on a 12v system. We don't see the advantage (if you're good to install the heavier wiring). With parallel, we get max amps in a shorter period of time and shade is less critical. The shorter time is really important when the sun might be blocked by clouds. What an I missing? And sorry to hijack this thread. Let me know if not appropriate!

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Mark avatar image Mark ♦♦ lnanne commented ·

For the panels in question the short circuit current (Isc) is 11.75A. So IF all PV is wired in parallel, then the total Isc = 70.5A. However, the max Isc allowed for a 150v/100A MPPT is 70A. So although just over it would not be recommended to proceed with this arrangement.

The only advantage of wiring all panels in parallel would be during times of PARTIAL shading.

Neglecting losses - there should be no difference between the instant MPPT power or current OUTPUT of a system wired in parallel or series.

The significant advantage of a series configuration is much higher voltage & accordingly lower current INTO the MPPT.

As a result the PV cable can be much thinner/cheaper & with lower power % losses.

But more importantly, the charge time will be greater since MPPT 'wake up' voltage (Vbatt +5v) will occur earlier & it will shut down later.

'How much' better depends on the ratio of V open circuit provided to the MPPT vs batt voltage +5v.

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Mark avatar image Mark ♦♦ Mark ♦♦ commented ·

Just to clarify my comment about 'partial' shading (in case it is misunderstood) I was referring to different levels of shading between panels that are in the same series string.

This would not occur with unobstructed panels during times of cloud cover or low light.

If this is an unavoidable common occurrence then it IS desirable to limit the qty of panels in series or avoid it all together (providing there is still a good margin between Voc & batt system voltage).

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mlink avatar image mlink commented ·

I'm fortunate because the solar install is in conjunction with a roof project on a 5th Wheel, so I'm planning to run 2 gauge cable the 10' from roof combiner that will be directly above the charge controller location.

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6 Answers
wkirby avatar image
wkirby answered ·

It is possible to input the parameters for your own panels into that calculator.

However, you have maximum 1500W of PV power. That controller can make 1200W@12V and when your battery is in the bulk stage, it will be able to make 1440W, so that ammount of PV is very good. It will be rare to see the full 100% from any PV array anyway unless the sun is strong and totally perpendicular to the array.
Even if your array does have the perfect environmental conditions to make 1500W you controller will still be fine because it will limit to 100A battery current anyway.

Until you upgrade your battery bank, you can limit the maximum charge current to a lower level for your small bank.

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Mark avatar image Mark ♦♦ commented ·

You mean 1440w once in absorption phase? :-)


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kevgermany avatar image kevgermany ♦♦ Mark ♦♦ commented ·
Once it reaches absorption current will drop. 1440W is a theoretical max at the point when the charger sees 14.4V at the battery and switches to absorption/fixed voltage. But by this stage the battery won't take the full current unless there's a large bank or it's lithium.
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JohnC avatar image
JohnC answered ·

Hi Mlink. Let me throw you another possibility - 24V.

Your batts will appreciate being in a single string, rather than paralleled, and a /60 mppt would be plenty. 12V loads via a Victron Orion dc>dc converter. Maybe you have loads that can work with 24V too?

I lived with a 12V system of similar size as yours for years. Too much grief with batts, and I went all the way to 48V. Most of my loads are ac, so when my 12V inverter croaked (not a Victron) I jumped at the chance to get away from 12V.

Look to the future with what batts you may end up with. Multiple strung daily cyclers can be a nightmare, And I'm never going back there.. :)

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Mark avatar image Mark ♦♦ commented ·

I also strongly agree with this recommendation, providing it's not too late... My system is 24v (@ 560Ah = 13.4kWh total) & in hindside I wish I went 48v instead. It would have cost me far less in cable & equipment + the related efficiency advantages.

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mlink avatar image mlink commented ·

Yes, since this system will be mobile and flat, so unavoidable partial shading on occasion. Most RV loads (lights, water pump, hydraulics) are 12V. But the heavier loads (fidge, home theater, satellite) are 110.

From above, I see the benefits of series install for higher voltage. All new everything - so cable size not a consideration.

Can anyone suggest an equipment list at 48v for these panels (above) and, which Victron model 48/3000/120v inverter or inverter/charger would make the best system. I assume that this is a six battery install...

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Mark avatar image Mark ♦♦ mlink commented ·

To make up a 48v system the most common battery arrangements are 4x12v, 8×6v or 24x2v.

In terms of battery sizing, you really need to work out your daily loads, the max depth of discharge you want to achieve (cost vs batt life trade off) & how many days of autonomy you want.

Keep in mind that IF you go 48v with those particular panels then you MUST wire in strings of 3 in series - otherwise it will never 'wake up' the MPPT & work at all. Accordingly if you went 24v then the minimum would still be 2 panels in series.

About the equipment, just size the inverter/charger based on the max power you want to draw from it + a buffer. With the MPPT this is directly linked to the batt system voltage & the total PV power. With 24v you only need 1/2 the current output of a 12v system to deliver the same charge power. Likewise, with a 48v system you can use a MPPT with 1/4 of the current output.

This can save you some good money & better than wiring batteries in parallel.

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Mark avatar image Mark ♦♦ mlink commented ·

That said, if your not totally confident yourself, it's probably best to get some advice about the system design & products from your Victron dealer.

If you have a suitable design for your needs & the most appropriate bits from the start then you will be much happier with the performance & spend far less in the long run...

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mvader (Victron Energy) avatar image
mvader (Victron Energy) answered ·

Hi, the authorative document for sizing is this one:

I see though that it says nothing about series vs parallel. We’ll update it.

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Gary Pacey avatar image
Gary Pacey answered ·

Although a 24 or 48-volt system has some benefit, doing so on a 12-volt chassis adds complication with respect to cross-charging from the engine alternator. You would then require a DC-DC charger which will have complications of its own.

The decision on battery capacity and arrangement is best made after looking at your system loads and inverter charger decision. Say your inverter charger was 3000 VA or under, then 12-volts for the battery makes sense. On the other hand, a Multiplus 5000 would require a 24-volt battery system.

For a 12-volt system with your panels, I would recommend using two 100/50 controllers, each handling a string of 3 panels.

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bossrox avatar image
bossrox answered ·

I think I can help you on this. The Victrons wattage input max in the the low volt range like 24v is much less than if you were to string 3 or 4 panals in series. It handles way more wattage at 100v so that's your best solution. It doesn't matter how high your input is as long as it's under it's max of 150v. The charger will only put out the voltage to charge your batteries at what you set it for. Hope that helps you out.

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lightful-mind-vet avatar image
lightful-mind-vet answered ·

So, I have a similar situation. I have four 340 watt (Hyundai HiS-S340Ti) panels, and 1120 ah 12 volt DIY battery (LiFePO4) on & in my travel trailer. My inverter/charger is the Victron MultiPlus II 120x2 12 volt, the charge controller is a Victron MPPT 150/100, and pretty much everything (except for the DIY batteries/BMS) is being managed by the Victron Cerbo GX. With the four panels in full/direct sunlight in Arizona, I'm making 7.5 kwh, and consuming around 7 kwh per day - yeah, don't judge me :). I have the panels in a 2s/2p configuration. I want to add two additional panels, since it was my original plan and the cables are already up there - two of the six were destroyed in shipping. But, I'm concerned about the max wattage limitation of the MPPT (1450 for a 12 v system). I realize I'll only be clipping/exceeding capacity at the peak of the day when the sun is high in the sky provided I'm in a very clear and sunny environment - and that it a real potential from time to time. Most of the time, it will stay under that - especially on cloudy days, and in places where I have some blockage. Currently, at the peak of the day in AZ, I'm generating just over 1000 watts - Vmax around 85 volts. I just want to be able to keep the system breaking even, or providing a net gain, when I'm in the forest and/or experiencing a lot of clouds. At present, like I mentioned, I'm making only slightly more (about .5) kwh more than I'm consuming in an almost ideal solar generation environment. That said, how big of a risk of destroying something would I be taking by adding the two additional panels in a 2s/3p array?

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