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nmeyrick avatar image
nmeyrick asked

100/30 PV voltage drops to same as battery voltage

I have a recently installed smart Solar 100/30 connected to 4 of 100w 12v panels wired as two series pairs to give 24v.


The controller then feeds a 700Ah 24v bank made up of 12 US Battery 2200 6v batteries wired up as 3 groups of four in series to make 24v


Most of the time the controller has been working great and is showing PV voltage in the range 30-35v. But on a few occasions it has suddenly dropped to show zero PV power and PV voltage of around 25v, very close to the battery voltage. Pv power showed as zero and no amps were being delivered to the battery. This is different to when the batteries are fully charged, when it shows zero PV power but PV voltage of 35-40v.


Last time this happened I tested the voltage across the Pv terminals of the controller with a multimeter and it showed approx 25v in line with the Victron app. But when I disconnected one terminal of the Solar from the controller to test the voltage on the wires coming from the panels it instead showed in the 35v range, as I would expect. On reconnecting the wire to the controller it showed PV power of 35v again, and power being produced.


Apologies if this has been asked before but I haven’t been able to find this by searching. Any insight into why this happens and anything I can do to stop it would be very much appreciated.

MPPT Controllers
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4 Answers
klim8skeptic avatar image
klim8skeptic answered ·

Last time this happened I tested the voltage across the Pv terminals of the controller with a multimeter and it showed approx 25v in line with the Victron app.

You probably have a bad (high resistance) connection between the mppt and solar array.

Check each joint, fuse, breaker and connection for soundness.

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zaf avatar image zaf commented ·
Thanks for the hint,I had the same problem and with your advise I solved the problem.Yes it was a.bad connection despite it was reading correct voltage before entering mppt
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mariner avatar image
mariner answered ·

Manual references this a few times, might be what you’re looking for-

“PV voltage must exceed Vbat + 5V for the controller to start. Thereafter minimum PV voltage is Vbat + 1V.”

PV input needs some buffer room above the battery voltage to charge.

You should be able to wire the 4 panels in series with your voltage being around 70, well under your 100v limit. Still verify your open circuit voltage of 4 panels together wouldn’t exceed (with some 5% buffer or so).

Only reason not to do this would be if your panels have different shading.

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nmeyrick avatar image
nmeyrick answered ·

Thanks for both of those although I don’t think either of these quite fit what I’m seeing.

I understand that pv needs to be 5v above bv to start charging, but these were instances where the panels were already charging and the of was in the region of 10v above bv. Incidentally thanks for the tip to put all 4 panels in series, I did consider that but I’m wary as the install is on a sailing boat and my understanding is that would make it far more impacted by shading issues.

it also doesn’t feel like a problem with connections as when I see the issue I am still measuring at least 35v on the end of the wire from the panels if I remove it from the regulator and attach a multimeter. It’s only while connected to the regulator that it is sho a lower voltage which makes me suspect some that is happening inside the regulator itself?


apologie sir I’ve misunderstood what was said but any more suggestions would be much appreciated


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thshh avatar image thshh commented ·
got the same issue on a mppt 75/15. which firmware is running on your 100/30?
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kevgermany avatar image kevgermany ♦♦ thshh commented ·
As said earlier, probably a cabling/connection problem. Disconnecting a PV cable and reconnecting will disturb the cables and mask the cause
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simon-martin avatar image simon-martin thshh commented ·
I too have this exact problem with my 75/15 Did you manage to resolve it?
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klim8skeptic avatar image klim8skeptic ♦ simon-martin commented ·

@Simon Martin You probably have a bad (high resistance) connection between the mppt and solar array, or perhaps a bad panel.

Watch the test your solar panel video, and perform the test on the panel, then at the mppt's pv input terminals. Check each joint, fuse, breaker and connection for soundness.


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simon-martin avatar image simon-martin klim8skeptic ♦ commented ·
Many thanks for your reply. My panel is on the roof of my caravan. I tested the panel through the mc4 connectors on the roof as these are the the closest ones to the panel. They read 20v but when I measure the amps (on the 10a setting on the multi meter) it reads 0A.

Does this mean a faulty panel? I'm struggling to understand how there can be the correct voltage but no amps - if a fuse or something in the actual panel had gone presumably there would be no voltage either?


I guess I'll have to take the panel off and check the connectors or fuse (if there is one).

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kevgermany avatar image kevgermany ♦♦ simon-martin commented ·
You will only get current if you connect across the panel cables.

Suggest you check all the cables /joins.



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habonlacika avatar image
habonlacika answered ·

I am also having the same issue with a 75/15. The PV voltage is constantly 0.1V lower than the battery. Interestingly if I disconnect the battery the mppt turns off. and sometimes rarely it charges normally. Interestingly the PV voltage reported in the app also stays at battery voltage when the panels are in comlpete darkness.

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klim8skeptic avatar image klim8skeptic ♦ commented ·

@habonlacika As above, test your solar panel to rule out a faulty panel. If the panel reads ok, perform the same test at the mppt's pv input cables, to test cable between the mppt and panel.


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habonlacika avatar image habonlacika klim8skeptic ♦ commented ·
Thanks for the hint, indeed, I disassembled all wiring and there was a weak connection.
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