PV Voltage Drop when connected to MPPT

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kokonuts asked Oct 6, '22

PV Voltage Drop when connected to MPPT

Hi,

I have a solar system that has consistently worked with no issues but recently, I am finding that my battery has drained and that my MPPT will no longer turn on. The primary culprit is the MPPT (BlueSolar 100/30) because I see that my PV voltage drops from 35V when measured open circuit (or disconnected from MPPT) to ~2V once connected to the MPPT PV inputs.

My System: I have two NewPowa 160W solar panels in series that measures 35V open circuit while in sunlight. It is connected to a BlueSolar 100/30 MPPT which is then connected to two 100Ah 12V Renogy Smart Lithium Iron batteries in parallel.

My batteries have been drained and both are sitting at 1.8V measured at their terminals. I have disconnected the loads to the system to rule out the rest of my electrical loads as a variable in this situation. None of the LEDs to my MPPT will turn on since the voltage difference from the PV input and batteries are only 0.2V.

I have verified that the PV polarities are properly connected with my multimeter, and now, I am stumped. I bought a brand new MPPT (same model) and replaced it into my system and am still seeing that my PV voltage is dropping to 2V. I figure the odds of having two defective charge controllers is unlikely. Please, any suggestions or ideas will be greatly appreciated.

MPPT Controllers

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ludo answered · Oct 06, 2022 at 03:52 PM

I guess you'll have to "prime" your renogy cells with a lead-acid battery and some jumpstart cables.

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klim8skeptic answered · Oct 06, 2022 at 08:03 PM

l Bought a brand new MPPT (same model) and replaced it into my system and am still seeing that my PV voltage is dropping to 2V. I figure the odds of having two defective charge controllers is unlikely.

It sounds like you have a bad panel, or dogy wiring / fuse. Time to test your solar panels. and array wiring.

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ludo commented · Oct 06, 2022 at 08:14 PM

You seem to ignore the battery voltage ?

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klim8skeptic ♦ ludo commented · Oct 06, 2022 at 09:03 PM

You seem to ignore the battery voltage ?

Battery flat? > Charge them

Battery BMS cut out? > Charge them

See where we are going?

Mppy not working? Find out what's wrong > fix it.

The mppt will charge a flat battery, and should reactivate the battery BMS. Hopefully.

But first the OP needs a working mppt. > Priorities.

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kokonuts commented · Oct 22, 2022 at 05:08 PM

It turns out that you were both correct! My batteries were deeply discharged because my panels were not outputting current. Once the batteries were charged with a separate charger, I could see my MPPT try to work and at least come alive. However, the batteries were not able to maintain charge because my panels were not outputting current.

I'm still not 100% clear as to why my panels are able to output voltage and not current. I isolated the cables out of my current measurement and measured straight at the diodes and saw only 2 mA of current. I also verified that my diodes were functioning correctly. I saw burn marks on the cathode side on one of the diodes and looking at the connections, it is possible that there may have been an arc(?).

My solar panel setup is similar to the following image from electronics-tutorials.ws except that I do not have blocking diodes. I measured the current across each of the 4 bypass diodes and saw that they all carried some voltage but no current. I am wondering if this means that all 4 independent strings have at least one damaged cell in them.

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Alexandra ♦ kokonuts commented · Oct 22, 2022 at 05:43 PM

@kokonuts

Shading can cause bypass diodes to blow. You will have to check them all.

Actually there are few causes.

Lightning can do so, not even a direct strike.

But the cause will be that something exceeded its peak inverse current.

Glad your batteries are ok though.

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Modifications Space

Node Red Space

Answer 1 · 2022-10-06T16:15:46Z

Alexandra answered · Oct 06, 2022 at 04:15 PM

@kokonuts you do need to perform the bms wake up method in your battery manual as suggested by ludo.

In the troubleshooting section of the mppt manual there are some tips on what to do to work out if it is panels or mppt or something else.

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Answer 2 · 2022-10-06T21:05:16Z

ludo answered · Oct 06, 2022 at 09:05 PM

No, the mppt will definitely NOT charge a battery on deep discharge.

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klim8skeptic ♦ commented · Oct 06, 2022 at 10:48 PM

Are you sure about that?

Will a Victron MPPT start up a Lithium Superpack from a low voltage cut off state? - Victron Community (victronenergy.com)

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Answer 3 · 2022-10-06T23:00:10Z

ludo answered · Oct 06, 2022 at 11:00 PM

It sure didn't work for me with two mppt's trying their best.

And my multiplus 12/2000 didn't start charging.

Also check Alexandra's post.

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Answer 4 · 2022-10-10T01:26:57Z

kokonuts answered · Oct 10, 2022 at 01:26 AM

"Priming" the batteries worked! Huge relief. Thanks!

I found an old analog battery charger and since my battery monitor had completely reset, I am not sure how much I had to charge the battery for the MPPT to start working again. I would love to understand more what "priming" means here or why an MPPT will not work with deeply discharged batteries if you have any more knowledge you can share with me.

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