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jaydub asked

MPPT 75/15 misreading voltage and going into float too soon

I have been using a MPPT 75/15 with 200AH battery bank and 200 watts of solar for about four years. I am currenly on firmware version 1.50 and as of recent, having trouble with the charger going directly into absorbtion then float when the true bank voltage is only 12.4

Here are a few facts:

1. Battery bank seems fine as it charges normally with my Victron Bluesmart 12/25 charger. Voltages and currents all make sense in all three phases. After charging with the AC charger I get expected life based on readings from my BMV monitor. That is all good.

2. When on solar (and no AC) it does charge in bulk for a few hours in the morning, but then switches to Absorbtion, then float way too soon. The true voltage of battery bank (as measured by a voltmeter and my BMV) at this time ia only about 12.3 or 12.4 volts. The MPPT reports the battery to be much higher - between 13.8 and 14.7 volts. It's as if it is not reading the correct voltage while charging.

3. When I disable the charger via the settings in the app, the MPPT **does report the correct voltage** of about 12.4 volts, so this does not seem to be a wiring problem.

4. As soon as I re-enable the charger in the app, it reports 14.7 (and cuts to absorbtion) then 13.8 (and cuts over the float) within seconds. How can this be? The charger just reported 12.4 volts a few seconds prior when the charger toggle was disabled????

I have never seen this before. No configuration changes have been made and I am using the AGM spiral settings as they are what I need for my battery.


Has my unit gone bad?


Thank you



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2 Answers
JohnC avatar image
JohnC answered ·

Hi @jaydub

While your wiring may look fine, you need to determine exactly where this V-drop is occuring. It may be a bad crimp, dicky fuse/breaker or even a broken wire within the insulation. A cheap multimeter is invaluable to help find this.. either via V under load, or resistance.

The mppt can only work from what it sees, and a high resistance somewhere is likely to be causing this.

Most unlikely to be the mppt or it's firmware, and the behavior you're seeing is probably just it 'Rebulking'. There's settings within it to alter this, but you shouldn't need to once you sort your wiring. Actually, I think it's marvellous how well that Rebulking works, I love it..

I know you've said your wiring is ok, but keep an open mind. Then go find the fault. Good luck..

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jaydub avatar image jaydub commented ·

Thanks. I will check the wiring. Maybe I did not understand your answer but the thing is, I am not seeing a voltage drop. The MPPT is reading higher voltage than the real voltage measured at the battery terminals with a multi meter. That is why it goes into float early.

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spirou avatar image spirou jaydub commented ·

Voltage drop depends on current pushing through so you won't see it under idle conditions but can be substantial while charging.

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jaydub avatar image jaydub spirou commented ·
Certainly, but it seems I am having the opposite problem. During charging the MPPT reports higher than true battery voltage, not lower.


Maybe resistance in wire manifests itself like this....it just seems backward to me. I would expect the MPPT to report lower not higher voltage. I will check wiring and probably just replace it to rule it out.

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JohnC avatar image JohnC ♦ jaydub commented ·

That is indeed how high resistance shows up. The mppt has a V target, it tries to push Amps into a faulty circuit, then the V rises. So it thinks it's met the target, but hasn't.

Don't overlook the negative side of the circuit either. Nor just look at fuses, etc. Measure them..

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Vance Mitchell avatar image Vance Mitchell jaydub commented ·

Are you measuring the voltage at the battery terminals on the battery or on the MPPT terminals?

It sounds like the voltage drop is between the battery and the MPPT, but if you are measuring at the MPPT then it can either be the terminals themselves or internal.

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jaydub avatar image jaydub Vance Mitchell commented ·

I will measure at the mppt terminals next. I have been measuring at the battery posts. Good idea. Thanks.

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jaydub avatar image
jaydub answered ·

Hello,

I'm sorry for the delay in following up. Today I had time to fully test the 10 AWG wiring between the battery bank and the MPPT 75/15. Everthing tested OK. Normal resistance.

I then pulled the MPPT 75/15 to inspect it, and saw visible signs of overheating on the bottom green plastic wire guide. Warped and discolored plastic. The 20A fuse was still intact.

Needless to say, I ordered a new one today. It's not all a loss as I will move the bluetooth dongle to my 12/500 inverter and purchase a new one with integrated bluetooth. I also plan to replace the wiring just to be safe.

Thanks for the comments.

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