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Solar Newbie - MPPT getting power but not charging batteries

"Finished" my solar install 11 days ago and have been frustrated ever since... somehow seem to be generating a lot of power but my batteries do not see it. I checked each of the six panels prior to install, each was within Renogy specs. I then checked the voltage of each of the series legs, again, within spec. I then checked the parallel connection prior to the roof penetration, within spec. The MPPT shows the watts and amps being generated, so everything upstream from the MPPT seems to be working OK. However, for some reason the batteries aren't actually getting charged despite vastly less power being drawn. The fuse between the MPPT and the power distribution block is 50A and in good condition.


This seems like a 'really' simple leg of the overall system and yet I'm not figuring it out somehow. For example, right now I'm using about 150 watts per the BMV-712 and generating 450 watts per the MPPT. According to everything I'm seeing online, I should be seeing a positive wattage on the BMV as well as "infinite" life remaining, and the SOC going up over time. However, that is not the case. It also doesn't matter what stage the MPPT is in, the BMV doesn't 'see' a change whether the MPPT is in BULK, ABSORPTION, or FLOAT.


Any ideas? Thanks in advance.


A bit about my system overall:
2003 Airstream
960W renogy panels, 6 x 160W each hooked up in Series-parallel configuration (two sets of 3 panels in series, then paralleled into the MPPT)
800 Ah BattleBorn batteries, 8 x 100 Ah each
Multiplus 12/3000/120-50 120v VE Bus

Victron SmartSolar Charge Controller (150V 45A Tr)

BMV-712

Go Power! TS-30 30 Amp Automatic Transfer Switch (shore power & battery charge primary, secondary is from the inverter)

MPPT ControllersBMV Battery Monitor
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2 Answers
Matthias Lange - DE avatar image
Matthias Lange - DE answered ·

I think the MPPT is connected to the wrong side of the BMV shunt.
The only think connected to the "battery side" of the shunt is the minus of the battery.
(and there also should nothing else connected directly to the battery minus)
Anything else (loads and chargers) must be connected to the "load side" of the shunt.

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olesimo avatar image olesimo commented ·

Matthias: picture 2094 that I just posted has the shunt in it. This was also one of my thoughts, maybe I hadn't hooked the shunt up properly, so the BMV wasn't "seeing" the solar power but it was there nonetheless. However, if this was the case then the battery voltage itself would still be steady/climbing during sunny times, correct?


FYI - the shunt has two negatives on the output side, one going to the surge limiter, the other going to the OEM negative bus for the Airstream DC wiring.

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Matthias Lange - DE avatar image Matthias Lange - DE ♦ olesimo commented ·

Is "1" the same cable in both pictures?
If yes then I can see at least 3 other cables(2, 3, 4) at the wrong place.
They have to be on the other side of the shunt.


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olesimo avatar image olesimo Matthias Lange - DE ♦ commented ·

You're correct, Wire 1 as you notated in the photos is the same wire, and now I see what you're saying. The distribution block should ONLY have the battery negatives, then the single wire to the battery side of the shunt, then perhaps I need to add a secondary negative bus to the load side of the shunt to handle the other negatives?


If I'm now understanding correctly, then right now I'm essentially driving a car with no fuel gauge...solar power is going directly to the batteries, but the BMV isn't useful for showing SOC. Now that I'm trying to cross-check this, the battery voltage on the BMV and the MPPT is also different - the one shown on the MPPT is significantly higher though not 'full'. This would make sense, we've had a couple cloudy days and today is the first day of really strong sunlight we've had so far this week.

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Matthias Lange - DE avatar image Matthias Lange - DE ♦ olesimo commented ·

Yes, you on the right way now. :)

The problem with the voltage difference can be caused by too thin wires.
What cable cross section do you have between the MPPT and the batteries?
For the 45A (that can you get from MPPT) it should be at least 25mm² (AWG 3 ?).

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olesimo avatar image olesimo Matthias Lange - DE ♦ commented ·

I gotcha, thanks again for the detailed responses. Clearly I'm a mechanical engineer, not electrical :)


You're right about the wire size, I'm way too small. The main wire from the MPPT is 10AWG, the in-line fuse is even smaller, maybe 12AWG. Will get on upgrading that ASAP, stupid mistake for sure.


While I'm in there fixing stuff, do you see any other obvious issues? Am I OK using the positive side of the distribution block for the various systems? I think the shunt is the only subsystem that would need clearly delineated paths like this, correct?

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olesimo avatar image olesimo olesimo commented ·

Seems like my reply to Justin was deleted? Here is the picture I referred to with the shunt wiring.


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Justin Cook avatar image
Justin Cook answered ·

@OleSimo, the BMV shows net activity from/to the batteries... so if your MPPT is generating ~450w and your BMV is showing ~150w coming out of the batteries still, that means that you're actually using ~600w, with 450 coming straight from solar and the remaining 150 from your batteries (assuming that all of the components are connected and programmed properly)

From your description, this is what I believe to be occurring; try shutting off all of your consumers (including inverters and all accessories) and then see if your BMV starts showing current into your batteries.

If that doesn't work, please post a few pictures of your system and connections so that we can examine the installation and make sure that everything's connected properly and/or post a system diagram and screenshots of your BMV readings and settings, for the same reason. There are certainly quite a number of common connection mistakes that can be made in connection of these systems that would explain inaccurate readings, but again, in this case I suspect that your readings are actually entirely accurate but just not what you were expecting to see.

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olesimo avatar image olesimo commented ·

Justin: I wrote a pretty detailed reply, then tried to convert it to a "comment" on your reply so that the thread made more sense, but the whole thing disappeared instead. Appreciate the time you took on your response, but Matthias has me on the right track now. All the best.

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olesimo avatar image olesimo commented ·

Thanks for the quick reply Justin. Totally understandable that this is the most common answer, but not the correct one in this case. Average power usage prior to installing the solar array is about the same - this is a qualitative judgment as opposed to quantitative as I haven't done detailed analysis. We were able to boondock for ~5 days before installing solar, and we can still boondock for ~5 days, even with 960W of solar and no other significant changes (usage, climate, sunlight, etc).


As a further check, I fully disconnected the solar array to see the change in the BMV wattage. If you were correct, then the BMV reading should have gone from -150 watts to -600 watts, but simply stayed at -150 watts.


I'm always a little bit hesitant to show pictures of my set up. Some of it is gleaming and new, but other portions of it are the original 2003 Airstream wiring. We rebuilt the interior completely, but stopped short of full rewiring. That said, here are a few pictures. One of these days I'll start building cable trays and whatnot so that lines are more easily traced. Note: I originally had the MPPT on the floor, but have since moved it vertically as instructed in the owner's manual. Note 2: I cheaped out on buying these little inline fuses which was dumb - fuses are annoying, plus the wire is smaller than everything else so I could understand a bottleneck here, but not zero transmission. Will be ordering some Blue Sea circuit breakers in the near future as an easy drop-in upgrade.


Any other ideas? I've dealt with various other small issues along the way to get the new system up and running, removing various redundancies from OEM Airstream build as I've upgraded the components, etc. Just this one has me stumped - 4 wires to an MPPT seems almost foolproof, yet here I am :)




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