mrgreen2020 avatar image

BlueSolar MPPT 75/15 not charging


I have issues with my MPPT as it is not charging the battery. On the attached screenshot you can see that the AGM battery is supplying the load but the charger is not starting. It was days with pure sunshine and it was only charging the first two days. This is now present from more than two weeks.

I know, that the panel needs to be +5V compared to the battery to start charging but to me it seems, that there is a voltage drop and the panal voltage is following the battery voltage.

I already checked the fuse and it is ok. Also the input voltage at the controller is the same as measured in the app.

It would be great if you could tell me, what I am doing worng or what am I missing here.

Many thanks in advance!

MPPT Controllerscharger
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6 Answers
Trina avatar image
Trina answered ·

1. Do not use your batteries right now for anything! They are getting too low!

2. To me right now looks like you have NO solar input.

: Solar panels might not be working or are VERY undersized? Your MPPT has never gotten out of "bulk" mode!

-is there plenty of sun?

-is wires right sizes?

-is panel big enough (details)?

-double check panel (PV) voltage output?

-double check wires from panel to mppt?

-double check mppt is in "charge" mode under battery settings in app?

-what are battery details?

Also double check 12v vs 24v (etc) in battery settings at top match your battery (possible issue for MPPT not "turning on")? It does look like on current day you have 16.49v input (it needs 17.5+ volts or more!) but still very little current (bad/too small/undersized PV and/or wires). Add another screenshot after 1 more day of good sunlight and checking the things below ONCE you have 17.5+ volts.

Screen shot of (a) battery settings, (b) mppt wiring and (c) any shunt and other wiring would be helpful.

Might also be the battery settings set incorrectly, however with system not coming out of charging "bulk" mode with plenty of sun the problem is likely either PV's or between solar panels to MPPT side of things.

Don't use batteries until solved though -they are getting quite low. For what you are drawing you need at least a 200ah deep cell FLA or AGM deep cell battery (unless using lithium batteries ) and 200w solar (wire in series if you can).

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Hi @Justin Cook - Bay Marine Supply USA @TrinasGarden

Thanks you for your very extensive feedback and taking the time for the answer. I will try go through it step by step:

- Last days have been plenty of sun

- Wires are for PV usage and have 4mm²

- Panel details as shown in attached picture: Peak Power: 100W, Max P current 5,75A, Max P voltage 17,8V, Open circuit voltage 21,8V

- I measured today PV output voltage with disconnected MPPT of 18,90V. With connected MPPT this drops below battery voltage. I also measured with connecting only plus pole to MPPT and measured between MPPT and minus cable and had also 19,42V

- MPPT is in charge mode (see screenshot)

- Battery is a 128Ah AGM

- In regards to the wiring, the panel is directly connected to the PV+/- of the MPPT

What I currently don't understand is why the voltage drops that much when connecting the MPPT.

Previously, the MPPT seemed to work normal

I hope this information is useful. If not, just let me know if you need additional information.

Again, Thank you very much for the support.

hi @Mr.Green2020 Try a different fuse as it could have weakened. Maybe also reset the mppt and ensure the firmware is up to date. Is the load output turned off. Just some other options to explore.

Justin Cook - Bay Marine Supply USA avatar image
Justin Cook - Bay Marine Supply USA answered ·

@Mr.Green2020, your PV voltage is too low for the MPPT to turn on and start charging; per the user manual, the MPPT requires at least vbatt+5v from the PV side to start charging - so, if your battery is at 12.5, your PV needs to be at 17.5v or better.

You may want to consider wiring your panels in series to increase the voltage on the PV side (careful to stay well under the controller's 75v input voltage limit), or perhaps upgrading your panels to a higher-voltage output.

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Trina avatar image
Trina answered ·


Off hand if all fuses good a drop on voltage normally means-if charging, that it's converting volts to amps.

However a drop below voltage would say to me

1. Disconnect everything but MPPT on battery

2. Check polarity again from panels and any fuses from PV to MPPT to Battery

3. Bad PV

4. Faulty MPPT (if there is no other loads or devices of any kind attached to battery)

Reverse current or shunt on positive side or something similar might do this too.

Can you show all wiring around mppt and battery?

The pics show that PV is NOT putting out enough power. If you can try changing it or replacing it just to see if that immediately solves the issue.

Sometimes the victron dealer will have a panel you can plug in to test before going towards a RMA.

My knee-jerk reaction though is bad solar panel or solar panel wiring IF nothing else but MPPT is attached to battery.

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Only queation I then have is why there is a drop in voltage if its not charging and no load is connected. Unfortunately i have no additional panel to check

rickp avatar image
rickp answered ·

PV drop of several volts from open circuit is normal, it’s the result of a circuit load. According to your screenshot, your panels aren’t making enough voltage as wired. Looking at the spec decal, I’d guess they are wired in parallel. Try putting them into two series-wired pairs, then in parallel to the SCC (2S2P configuration). That should double the incoming voltage and may get you going.

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mrgreen2020 avatar image
mrgreen2020 answered ·

Hi @RickP, @TrinasGarden

I replaced again the fuse on the MPPT and measured the voltage on the panel, this time with plus not connected to the MPPT and had again 18,95V.

I also check the wiring and it is like indicated in the drawing.

Unfortunately I have no other Panel around to test if the panel is faulty.

Two days ago, the panel as able to produce 60Wh with Pmax 42W. But yesterday again no charging with same weather conditions.

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I would have to concur with RickP post below.

The problem most likely is either the solar panel or it's wiring to the controller being too light gauge (will carry voltage but minimal current).

Also if you put any critical loads on the system it is possible that problems are happening even if panel could charge it.

For your loads from first screenshots you really need better solar input unless you are in a very sunny place and don't use system until it is in absorb charging stage.

Disconnect the fuse box/load outs and only have battery connected to mppt connected then connect PV and give it a day or three of watching what happens with charging.

Even if your battery is inadequate or has problems you should be able to narrow down if the panel and wiring from it to mppt to battery is adequate to give amps and turn on MPPT as should happen with good sunlight.

klim8skeptic avatar image
klim8skeptic answered ·

3 things of note about your solar panel.

Very thin, 3mm.

Very light weight, 1.9kg.

Dimensions are approx 2:1, suggesting a 8x4 (32) cell panel.

Please comfirm..

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Yes, its 8x4. But as it is that thin I dont think I will manage to chancethe wiring