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dwells asked

MPPT Bluesolar 75/15 different battery voltage when charging


screenshot-20210818-190313.jpg


screenshot-20210818-190327.jpg

I have a MMPT Bluesolar 75/15 connected to my 12v 110amp hr van leisure battery. The controller seems to be reading the battery voltage inconsistantly.

When the panels are charging the battery the voltage reads 13+V and the charger switches to float mode. When dark or the solar charging is turned off the battery voltage reading drops to around 2.5V. This seems to be stopping the battery properly. I've checked the fuse in the controller and it is fine.

Does anyone know what might be causing this?

Thanks

MPPT Controllers
screenshot-20210818-190313.jpg (248.9 KiB)
screenshot-20210818-190327.jpg (242.4 KiB)
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Alexandra avatar image Alexandra ♦ commented ·

@Dwells

Check it isnt the bms disconnecting randomly? You dont mention the battery type.

Have you verified at the mppt terminals that that is the voltage there? If it is then there has to be a bad connection somewhere.

Even with the voltage there during the day, does not mean it is 'seeing' the battery.

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dwells avatar image dwells Alexandra ♦ commented ·
Thanks for getting back to me @Alexandra . Sorry for the delay I was out buying a new multimeter! The voltage at the MPPT is reading the same as on the Bluetooth readout. Just checked again and the voltage measured at the battery terminal is 12.7 but measured at the MPPT terminal is 14.6. I've checked all the connections I could find and all seem secure? Can you think of why this measurement would be different.
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dwells avatar image dwells dwells commented ·

I've just tried disconnecting the split charger form the engine and the reading at the MPPT is different again. Somewhere in between the other two readings. Could the starter battery be confusing the solar controller?

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Alexandra avatar image Alexandra ♦ dwells commented ·

@Dwells

Looks like Mike Dorset has you from here on.

To trouble shoot you may need to disconnect it from everywhere and simplify it by putting panels directly in and directly onto the battery without any other connections.

Is the secondary battery ok?

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dwells avatar image dwells Alexandra ♦ commented ·
Thanks @Alexandra sounds like I've got some work on my hands!


Yes the second battery seems fine.

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3 Answers
Mike Dorsett avatar image
Mike Dorsett answered ·

@Dwells Firstly, all mppt units should be wired directly to a battery - they can be damaged if solar is applied and there is no battery connected. A diagram of your system would probably help diagnosis.

IF you need to disable an MPPT for any reason - if set properly with lead battery this should not be the case- then the disconnect relay should be in the PV input side, and adequately rated to take the short circuit current *1.25 and the open circuit voltage.

If your relay contacts open when the MPPT is charging, this is most likely to damage the MPPT by over voltage on the output. If you have the facilities, the MPPT unit can be tested using a bench power supply connected to the PV input, and a small battery connected to the battery terminals (i.e. a motorbike battery). I have also tested these with a dual bench supply, using one supply to provide the battery voltage, and the other the PV voltage. This allows testing in a controlled manner.

From your description of the MPPT behavior switching quickly through the various charge modes, it looks like there is too much resistance in the leads to the battery - allowing the voltage at the MPPT to rise above the absorption voltage during charging. This causes the unit to go into float mode and limit the output voltage.

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dwells avatar image dwells commented ·
Thanks @Mike Dorsett

Unfortunately I inherited this system when I bought the van so I'm not sure why it was set up that.

It sounds like I might need to reconfigure the system which is a bit outside of my comfort zone so I'll probably have to get hold of an auto electrician. I will test it as you suggest first though to make sure everything is working.

I appreciate all the help

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Mike Dorsett avatar image
Mike Dorsett answered ·

Measure the battery voltage at the MPPT Battery connections under both conditions with a multimeter. Check also that you don't have a blocking diode between the controller and the battery - e.g. in a charge splitter.

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dwells avatar image dwells commented ·

Thanks for getting back to me @Mike Dorsett.


The voltage at the MPPT is reading the same as on the Bluetooth readout. Just checked again and the voltage measured at the battery terminal is 12.7 but measured at the MPPT terminal is 14.6.

There could well be something interfering (I guess there must be) unfortunately it is beyond my knowledge to trace everything back to where is should be coming from although I have checked all the connections and fuses.

Any thoughts on why these measurements would be different?

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dwells avatar image dwells dwells commented ·
I've just tried disconnecting the split charger form the engine and the reading at the MPPT is different again. Somewhere in between the other two readings. Could the starter battery be confusing the solar controller?
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Mike Dorsett avatar image Mike Dorsett dwells commented ·
I'd be more concerned about why the voltage reported is 2.4 in the 'off' condition - is the unit wired directly to the battery, and what type of battery?


Also, what is the PV input voltage? both of your screenshots show a relatively low voltage, this should be ~20V Voc (sun, no load).


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dwells avatar image dwells Mike Dorsett commented ·

Hi @Mike Dorsett

I'm so sorry I missed you message.

The unit is wired through a Durite latching relay which appears to be connected to the alternator.

The pictures were taken in very low light conditions. The battery does charge in under brighter light but because the measurement at the MPPT is wrong it appears to switch through float, absorption and off too quickly.

The battery is a lead acid battery.

Any thoughts? I really appreciate your help with this.

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Mike Dorsett avatar image
Mike Dorsett answered ·

you also need the PV (solar ) voltage to be 5V more than the battery voltage to start charging. i.e. if battery voltage is 13.25V, solar needs to be >= 18.25V. This is not the case in either of your examples.

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