Help selecting a MPPT Controller

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Christopher A asked Dec 14, '18

Help selecting a MPPT Controller

Hi, I am designing a solar system for a RV. My intention is to have Two parallel strings of 600 watts each (2X300 Watts panel in series) for a total of 1200 watts in a 12V LIFEPO battery system which I know will require two parallel charge controllers.
Right now, I want to start with one string of 600; 2X300 watts in series to a Victron MPPT controller.
The second stage of the project will be an identical second string to a second controller making it a total of 1200 watts in two parallel strings. (two identical 600 watt strings to two charge controllers).

I have a couple questions:
1. Which controller would be practical for this application?
2. I am also considering adding a third panel to each parallel string, in the future, bringing the total of each string to 900 watts. Is there a charge controller that will handle these parameters of expanding the 600 Watt string to 900?
3. ALSO considering making this a 24V system. Is it possible to select a charge controller that will be flexible to handle any of these parameters? IE, 600-900 watts, 12 or 24V?

4. Is what I'm intending possible?

300-->300-->(+300 future)--->MPPT1------12 or 24V combined\
300-->300-->(+300 future)--->MPPT2------12 or 24V combined /

Thank you,

Chris A

MPPT Controllers

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Answer 1 · 2018-12-14T04:07:45Z

JohnC answered · Dec 14, 2018 at 04:07 AM

Hi Chris. A single 150/70 will handle 900W into 12V.

It will also handle 1800W into 24V. Yes just one, you don't need two.

And really, 1800W into 12V is beyond practical usefulness for 12V, so yes 24V is where I'd head when you add that second string of pv.

Again, just one mppt, but your panels should be all the same..

Edit/: Sorry I didn't answer your question directly, I just assumed one battery system, and that may not be so. If you really must have 2x mppt's, then a pair of 150/45's (Smartsolar assumed) would be 'adequate' for 600W into 12V, and just a little oversized for 900W into 24V.

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Christopher A commented · Dec 14, 2018 at 04:46 PM

I'm still deciding on the final design of this system voltage. The coach is a 12V house system for many items, like stock fridge, ignitors, lights, water pump, ect and there would need to be a converter to make a 24V bank work with the 12V systems already in place. I also have to figure out the alternator that charges both a 12V house and 12V chassis battery.

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Christopher A commented · Dec 14, 2018 at 04:50 PM

I have not decided on the final system voltage. I understand that 12V is limiting, although the systems in the RV are 12V and would require a converter to output those voltages. That's easy. My biggest question is the Shore power conversions (probably a Victron Inverter/charger-Hybred) that will take care of that switching and how to tie it into the alternator, which I cannot change from 12V. Not sure this makes all sense, but I'm still putting this together and calculating the most optimum configuration.

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Christopher A commented · Dec 16, 2018 at 04:14 AM

Will a single 150/70 handle 1200W into 12V and/or 24V?

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JohnC ♦ Christopher A commented · Dec 16, 2018 at 05:06 AM

Almost. You can work it out yourself - 70A x 14.4V (typical Pb absorb level) = 1008W. Sure it falls short in theory, but it may never reach that anyway in practice. This is hard to explain, but your pv will display a 'bell curve' behavior peaking at 'midday'. If your system is well balanced it may/should reach Absorb a couple of hours earlier, and will be clipping by midday.

70A is to me a pretty good practical match for 1200W of pv (@12V). Just look upon that extra 200W as insurance for cloudy days, and to widen the bottom of the bellcurve.

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JohnC ♦ Christopher A commented · Dec 16, 2018 at 05:14 AM

Character limit beat me.

Please take care paralleling Pb batts if you really want 12V. It's not recommended, and balancing them can be a serious issue.

Li's with an onboard BMS may/may not suffer this. But be aware..

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Answer 2 · 2018-12-14T15:31:53Z

John Rushworth answered · Dec 14, 2018 at 03:31 PM

Don’t forget the new webpage MPPT sizing calculator here:

https://www.victronenergy.com/solar-charge-controllers

If you’re not using Victron panels then use the spreadsheet in that page, as it allows you to edit and add panels of your choice.

Also there’s a Whitepaper in that page or if you prefer similar info in a blog format, see here:

https://www.victronenergy.com/blog/2014/03/28/matching-victron-energy-solar-modules-to-the-new-mppt-charge-regulators/

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Answer 3 · 2018-12-14T16:33:49Z

Pat Davitt answered · Dec 14, 2018 at 04:33 PM

I agree with JohnC about battery voltage. You don't state the size of the battery bank you are planning, but with 1200W of solar I assume it would be 600-800AH @ 12V. You should seriously consider a 24V battery system. The component (wire, breakers, fuses, etc.) size for that powerful of a system is reaching the limits of practicability. In my case, with 1400W of solar I went to a 48V battery system.

I did a series of articles for Truck Camper Adventure magazine about my system. The URLs are below if you want the details.

Pat

http://www.truckcamperadventure.com/2018/07/a-mad-scientists-take-on-the-ultimate-electrical-system-part-1/

http://www.truckcamperadventure.com/2018/08/a-mad-scientists-take-on-the-ultimate-electrical-system-part-2/

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Christopher A commented · Dec 14, 2018 at 04:41 PM

That's interesting information. So, with 24V system, it's more practical to run and step down to the 12V House voltage?. I just have to figure out the alternator charging to a 24V battery bank. It's a US made stock new RV, So I have a clean slate to work with.

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Pat Davitt Christopher A commented · Dec 14, 2018 at 04:55 PM

With that much solar on the roof, I would not worry about alternator charging. Remember the solar works when you are rolling down the road.

Pat

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JohnC ♦ Christopher A commented · Dec 15, 2018 at 04:27 AM

Selecting 12V vs 24V is also dependent on what batts you want to use. Like you don't want a lot of parallel strings of Pb's, maybe ideally one or perhaps two. Doing more than that will likely break your heart..

I too went all the way to 48V, mostly for the battery thing.

Your alternator could be stepped up to 24V (even 48V) with a Victron Orion DC/DC box adjusted for (say) the float V of your batts.

And stepped down to 12V with another one for your house loads

Note too that @ 48V, the mppt can be smaller again.

Blue boxes on the wall make for a pretty sight.. :)

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Christopher A JohnC ♦ commented · Dec 15, 2018 at 05:47 AM

After more thought I'm leaning towards a 24V Lifepo4 Battery system. roughly 800 amps but some calcs still need to be done to balance cost and complexity.

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Answer 4 · 2018-12-16T09:06:34Z

Michael Connolly answered · Dec 16, 2018 at 09:06 AM

I went s different way to domebof the answers. I have three arrays with 3x200watt panels. I run each array into a seperate 100/30 mppt. This way shadows and shade don’t bother as much as I have done antenna that get in the way.

Mick

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