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muir-industries avatar image
muir-industries asked

Multiple MPPT's, One set of wires from panels?

Hi there everyone, I've done some relatively large RV solar installations for quite some time, but I'm now tasked with helping a friend make a poorly planned system work. What should have been a 24v system was done in 12v (150/100 mppt and dual 3000/12 inverter chargers with 10 battle born batteries).

The major limiting factor here is the 150/100 only being good to 1,450w of solar on a 12v system. I'll be upgrading that to more like 2,500w. The prior "installer" (they should be ashamed to even call themselves that) ran a set of 6awg wires from the roof of the RV into the power bay, the question I have is whether or not I can send all of the solar power down via this single pair of wires and connect them to both charge controllers?

I would expect they would hit a positive and negative bus bar, then split to two 150/100's. I don't know if there is any technical reason this couldn't be accomplished, but I also don't know how each controller would 'determine' how much of the available power it should be using. IE does one peak at 1,450w before the other starts to use whatever is left etc?

If I have to run another set of wires then so be it, but if it's a realistic plan to share the gigantic copper that already exists I'd certainly rather do so.


Thanks!

MPPT SmartSolar
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Mike Dorsett answered ·

Like you said, a poorly planed system! with 2 x 3kW inverters, I would have gone for 48V....

However: With the MPPT's you should NOT use one set of wires for 2 MPPT's (as one might try to take all the power and overload) Unless you can get them to operate in Master/Slave mode - this will take some programming - see the BlueSolar-HEX-protocol-MPPT.pdf for full details.

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muir-industries avatar image muir-industries commented ·
That link to the PDF isn't valid.


I will look into the master/slave, that's what I was expecting would be required, I just hadn't heard of it or seen it. One would think if they're linked together via VE bus they'd "be aware" etc. Maybe set each to have a charge limit on the batteries of something like "90 amps" as to ensure the controller never reached it's maximum or something to that effect.

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Mike Dorsett avatar image Mike Dorsett muir-industries commented ·
Sorry - it should not have been marked as a link, only as the name of the Victron document to look for. Warwick above has a good idea with calculating the input ratings of the MPPT's - they would only need to be programmed with the max battery charge current to give them protection (say 90% of the maximum & derate for ambient temperature). This would however be more expensive in the equipment cost.
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Warwick Bruce Chapman avatar image
Warwick Bruce Chapman answered ·

I think the risk here is that if one of the MPPTs goes off, eg. reboots during a firmware upgrade while the sun is shining, then the other will potentially be exposed to excessive current.

I would only do this if the PV input voltage and PV input current limitations of ONE 250/100 can be respected at 12V.

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muir-industries avatar image muir-industries commented ·

While I agree about the potential risks, I’m a bit confused. If we were able to “respect the limits of one controller at 12v” as you say, we simply wouldn’t have a need for two?

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