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Jay Kote avatar image
Jay Kote asked

Need expert assistance in trying to find cause of sub-optimal mppt performance.

I have been tracking two mppts which each connect to 375W solar panels. They are barely producing 200 - 250W. That's around 60% - 70%. A solar panel tester finds the proper voltages and current. But the mppt is still delivering far less power output.

Does anyone have tip/tricks on how to figure out line loss? The ohmic losses are so low, I don't know how to measure. But I'm using the largest wire sizes available for the application -- even 8AWG for the 8 - 10 meters of solar wire (100% copper).

Any other suggestions on cause/solution?

Responses to (good) Questions:

The test setup is as follows: The battery chemistry is lithium. The tests are being conducted when the MPPTs are in BULK mode. The reference is the results from an MPPT Solar Tester which reports nominal of 330W at mid-day when the testing is being done. The battery voltage range is between 22 - 24v and around 75 - 90% SoC. The MPPTs are SmartSolar (100/20) x 2.

The wire gauge used are 8AWG run from solar to solar breaker/disconnect. 12AWG from breakers to MPPT (necessary because I cannot get 8AWG or 10AWG) to fit in the MPPT (100/20 - SmartSolar). The wires to battery are 4/0. The wires to the inverter are also 4/0.

Useful Comment ( See Mike D's comment below) on optimum range needed for MPPT

The difference in solar voltage and battery voltage should be at least +5v or higher and minimum of at least +1V.


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5 Answers
nickdb avatar image
nickdb answered ·

Panels won't produce their rated power constantly. It depends on many factors including irradiance, temperature, orientation etc.

It needs absolutely optimal conditions to hit its rated power and those rarely exist.

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Jay Kote avatar image Jay Kote commented ·
I agree. But we are talking mid-day sun. And I'm comparing the power out to not rated, per se, but rather what an MPPT Tester says I should be getting.


Here is my test setup. At the two wires that go to the MPPT, I instead connect the MPPT solar tester. At mid-day, it says I should be getting around 330W. However, the MPPT is only putting out around 200 - 230. I've on occasion seen 250. So I'm still down 30%. (Remember the MPPT Solar tester is taking into account the irradiance).

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gazza avatar image
gazza answered ·

A little more info please. What battery voltage system, battery chemistry, PV configuration and which model of MPPTs. ?

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Jay Kote avatar image Jay Kote commented ·
The battery chemistry is Lithium. The wire gauge used from MPPT to the bus (forced to do this is 12AWG). But the bus wires are 2/0 to the shunt and a positive block which uses 4/0 to connect to the inverter and then 4/0 to connect to the battery.


The battery test voltage range is typically between 21 - 24 volts. Testing occurs at time when SoC is around 70% and the MPPT's report state of charge as "BULK". I don't test during ABSORPTION time because I know the MPPT is regulating to ensure at the voltage it's not over driving the batter for it's state of charge/voltage.
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gazza avatar image gazza Jay Kote commented ·

So, with that panel voltage this is a 12V system? What MPPT model ? Is it One 375W panel on each MPPT..?


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gazza avatar image
gazza answered ·

Ok I have just seen you have added the MPPT model as an edit in your first post.

So, for a 12V system at best you will get 290W out of the MPPT(100/20)
....for a 24V system at best you will get 580W from the (100/20)
This is from the spec sheet for that MPPT model.

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gazza avatar image
gazza answered ·

Can you post your battery charge settings from Victron Connect? Is it a single 24V battery or 2x 12V in series?

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Jay Kote avatar image Jay Kote commented ·

Will do. My batteries are custom cells lithium pack and able to charge at max of 200A. It is a 6S configuration. I have tested it using a charger to 30A and it does fine. I have drawn load of 2500W with Amp above 100A. Done both repeatedly to stress test the battery. Battery seems to hold balance during load tests.


I have also tried running both mppts at the same time or at one time. The performance is almost the same whether they are both on or both off as long as the SoC is bulk mode. I will take screen shots of the VC connect BT and attach as soon as possible.


Update (10/10/2023 @ 10:00 PM)

I could not figure out how to download the settings. I think somewhere I have seen an "export" setting, but could not remember where that was. So, I recorded them manually. They are on the enclosed PDF.

DS1 and PS1 are the two MPPTS (SmartSolar 100/20) - I don't recall why Amps was set to 16A. I can adjust this tomorrow and see what happens to 20A.

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Jay Kote avatar image Jay Kote commented ·

MPPT-SMARTSOLAR-100.20-DS1-PS1-CONFIG.pdf

DS1 and PS1 are the two MPPTs. I noticed 16A. I will change this to 20A tomorrow.

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mppt-smartsolar-10020-ds1-ps1-config.pdf (108.8 KiB)
Mike Dorsett avatar image Mike Dorsett Jay Kote commented ·
If the MPPT current is limited to 10A, this will limit you to ~240W.
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Jay Kote avatar image Jay Kote Mike Dorsett commented ·
It was limited to 16A for some reason. I don't recall doing that. But now it's been changed to 20A. I am at the mercy of the sun. It's bee overcast/raining here for last two days. So don't know until sun comes out.
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gazza avatar image gazza Jay Kote commented ·

Using the full 20A of the charger would definitely help!! I also suggest that 24.2V is a little low as an absorption voltage? What is the recommended max voltage for your 6S battery?

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Jay Kote avatar image Jay Kote gazza commented ·

24.2v is consider safe range. If you go higher, it's not deemed safe although for brief periods it can go higher but not recommended.

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Mike Dorsett avatar image
Mike Dorsett answered ·

The setup is 2 x 375w Panels to 2 x 100/20 MPPTs. Each panel has a dedicated MPPT to avoid issue with shading which is very likely given that this is mobile application and the panels will not always have option to tilt and orient to max the solar production the way I have now.


One of the tests you need to do is to measure the module voltage into the MPPT unit.

As you only have one module per MPPT, a lot will depend on the module Voc, Vmp ratings.

edit: Voc 39.5V Vmp = 33.7V Imp 8.33A

For good operation of an MPPT the max power voltage from the module should be at least 5V higher than the maximum expected battery voltage.

Whilst the MPPT will start at Vbat+5, and operate down to Vbat+1, it may not be able to produce the maximum power with so little headroom.

edit: so with V bat at 24v, and module Vmp at 33.7V you should have nearly 10V headroom - sufficient for the MPPT to work properly. You DO need to measure the operating point (Vp at MPPT input).

Figuring out the volt drop on the cables is relatively easy: you can look up typical resistance figures for tinned copper wire of each gauge that you use, and with the length used. Typically in single module installations with relatively short runs these figures are quite low. You can also measure any voltage drop across your PV breakers, from the breakers to the MPPT and from the MPPT to the battery. If you have a clamp meter, you can also measure the currents in the different points of the system, and check to see if these are as expected.

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Jay Kote avatar image Jay Kote commented ·

When you say the module voltage, do you mean take volt meter and check the voltage on the terminal screws in the MMPT or do you mean check what the MPPT reads?

Now I am getting one reading with the an MPPT Solar tester. It claims I should be getting 330W. The same numbers are not being reported by the MPPT. It has locked in at a much lower voltage and lot lower currents -- around 8A.

(What I did was I took Solar Breaker wires (+/-) which feed the MPPT. I attached those to the MPPT Solar Tester. It reports a lot higher voltage and current then the MPPT is delivering.)

I have changed the max amps from 16A to 20A. (I'm not sure how/when that changed, although even 16A should be plenty for a 375W going into a 23V - 24V battery.

Now based on a freak situation I notice something odd. One panel was shaded and the other was on. The shaded panel had no amps, but 37v. The panel producing power had a lower voltage but around 3A. I calculated that I had 1/2 ohm drop. It seems very high to me.


Update: 10/12/2023

For last two days we have had no sun. My battery has dropped to about 60%. At 20%, I will have to turn on the charger.

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