question

roger-gregory-allen avatar image
roger-gregory-allen asked

Two PV contollers with same input, get widely different output

Here is the context

  • RV rooftop install
  • 2x 400W panel arrays (4x100W)
  • 2x Bluesolar 100/50 MPPT controllers
  • a Multiplus, Cerbo, BMV-712, and 600AH Li battery bank round out the system

The 2 controllers and the 2 sets of panels are "identical" (same manufacturer and specs). When there is a very light load (input and output power are approximately equal), the "first" controller seems to always show much higher output (in Watts) than the second one. Under heavier load, the two controllers have nearly the same output.

I suspect this has something to do with demand, but I don't know how that would work. In a similar vein, if I'm on grid power and the batteries are full, then both controllers show little or no output. This is very consistent, and consistently favors the first controller in the light load scenario.

For what it's worth, these two panel arrays are on a completely flat surface (the RV roof), within ~10ft/3M of each other, and completely unshaded.

I think this is just a gap in my understanding, but I'd like to understand what is happening.


Here is a snippet from the VRM portal with little load:

1687192084722.png

I have seen the difference much higher than that (more like 150W on MPPT-1 and 5W on MPPT-2).

A few minutes after the first screenshot, I turned on an air conditioner to tax the system, and the view changes to something like:

1687193594773.png

I think this second view argues against any problems with panels, cabling, or connections for the second array.

Can anyone explain what is happening?

Thanks!
--Roger

MPPT Controllers
1687192084722.png (23.9 KiB)
1687193594773.png (23.5 KiB)
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5 Answers
seb71 avatar image
seb71 answered ·

In a similar vein, if I'm on grid power and the batteries are full, then both controllers show little or no output.

That is completely normal. The MPPTs will bring the output power down to what the battery can take in addition to the existing load. The power has to go somewhere. No (or less) demand means no (or less) production.

-------

I also have two PV arrays (with two identical MPPTs), but in my case they are not oriented the same: the two PV arrays are on the East and West sides of a roof, respectively (it's not exactly East and West, but very close).

Nevertheless, at astronomical noon (when theoretically both arrays have similar or at least very close Sun exposure), if the load and power into the battery are less than what the PV arrays could produce at that moment, I noticed that one array is "favored" over the other (in clear sky/no shadow conditions).

Both are set the same. Same PV panels. Arranged and connected the same way. Same combiner boxes. Same disconnecter switches. Same fuses. A SmartShunt is set as the Battery Monitor. DVCC is on etc.

Same length and section for the battery wires (from the MPPTs to the busbars).

There is a slight difference in the length of the PV wires between East and West array.

From the combiner box to the MPPT: 10.2m (East PV array) vs. 7.8m (West PV array) or something like that.

The one with shorter PV wires (West side array) is the "favored" one (again, at noon). Coincidence or not? I can't say.

Maybe the East side PV array is at higher temperature at noon than the West side PV array (and higher PV temperature leads to lower voltage) and that's a factor, too.

As soon as I increase the load, the "dormant" array/MPPT "wakes up".

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seb71 avatar image seb71 commented ·

A graph from today, illustrating this:


east-west.png

Astronomical noon was at 13:36.

You can see that even before noon, the West side array "took over" completely (the power demand was low), but as soon as the load increased, the East side array started producing power. Around noon, with high(er) loads, both arrays output practically the same power.

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kevgermany avatar image
kevgermany answered ·

Would be good to check that both controllers have identical settings. If this is the case, try swapping the controllers over.

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roger-gregory-allen avatar image roger-gregory-allen commented ·

As far as I can tell, the two controllers are configured the same, and they weren't altered from the "out-of-the-box" state.

Beyond that, given that they seem to give equivalent output under the higher load condition, I don't think there is any setup or configuration difference between the two halves of the PV system.

I'm thinking that the "issue" is with my understanding and not some actual problem with the system, but it sounds like you don't think this is normal, is that right?

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derrick thomas avatar image
derrick thomas answered ·

Do you have dvcc enabled with SVS and SCS active?

And ve.smart networking disabled?

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nesswill avatar image
nesswill answered ·

I agree with seb71 this is normal and expected behaviour, i have x3 RS450/100 and they behave exactly as sep71 describes.

As mentions above you can swap them around to give you piece of mind if you like/can.

Good luck.

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roger-gregory-allen avatar image
roger-gregory-allen answered ·

OK, thanks for the input. As I said, I mainly was trying to figure out if this was normal, and what might explain it. I suppose the slightly different cable length part could be the cause, though I think the difference is less than 10% in my case. In any event, as long as others see the same behavior, I'll not worry about it.

@Seb71 , thanks for the great write-up!

--Roger

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