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damon-1 avatar image
damon-1 asked

MPPT 75/15 V Max

Hi Guys


This may seem like an odd question, but it has left me baffled.


Have a 75/15 MPPT connected to a 12V lithium battery, unit worked perfectly for a while. Here is where it gets strange for me, perhaps not for more experienced.


The yield remained on 0.00 as well as the P max, however the V max shot up to 40V+, with that, my battery's max voltage shot up to 40V+ as well, with the min remaining where it should be. The difference in voltage between my V max and battery's max volage would remain about 0.5v higher on the V max.


What would cause something like this? could it potentially be the charger that had fed through the battery back to the mppt? or perhaps faulty panels? or is the MPPT faulty.


Since then i have replaced the components, but out of curiosity I would just like to get to the bottom of the problem. Any help/answers would be greatly appreciated.

MPPT SmartSolar
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2 Answers
Alistair Warburton avatar image
Alistair Warburton answered ·

The charge controller is essentially a buck converter, possibly buck boost, (even potentially, although not likely, PWM with a huge smoothing cap) Either way the DC is 'chopped' so that the output voltage is kept under control.

When the PV capacity, W, exceeds the max output W, at the rated voltage, limited either by the charger or the battery the panel voltage is allowed to float up towards the open circuit voltage.

When MPPT is in play then the limit is input power and the charger will, well should, reduce tio output voltage to reduce the load on the entire system, whilst actively searching for the PV string voltage and current that yields the best overall power for the system.

The parts/modules are fairly basic wrapped up in a bunch of clever that manages the buck converter setpoint to achieve the desired output voltage at any given point, weather that be charge profile, MPPT requirement or max output current.

At no point should you ever see more than the maximum charge voltage, for the given phase of the charge algorithm... It sounds to me like something failed an connected the panels to the battery directly, I am guessing you have 2 panels in series in the/each string or your panels are nominally 24V

12V panels typically have an open circuit voltage of 20-22V with MPPT at circa 18V and 24 V panels are typically about twice that... 42V! ... Not the answer to anything useful in this case :-) (HHG)

I will be interested to see if anyone has a better suggestion but it sounds like a dead controller to me and I don't expect you battery BMS was best pleased either, it would likely have been pretty stressed.

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kevgermany avatar image
kevgermany answered ·

Not enough info to go on.

MPPT records the battery voltage it measures, I presume this is your Vmax.

If panel voltage goes via MPPT to battery, it's an internal fault in the MPPT, will be covered by warranty.

You mention another charger, this could also cause over voltage on the battery side. You'd need to eliminate by testing. MPPT cannot distinguish between external voltage overshoot and internal problem.

Possible wiring fault.

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