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ajax-md asked

75/15 MPPT output not what is expected



  • Controller power output consistently reads lower than expected for prevailing solar conditions. (Perfect alignment, perfectly cloudless days)
  • Voltage output from panel to controller (indicated): 14 to high 15's
  • Amperage output from panel to controller (indicated): 1.5 to 3 amps
  • Total power from panel to controller (indicated) usually in the 20 - 30 watt range, highest ever recorded was 56 watts for a 125 watt panel
  • Values recorded with battery at various depths of discharge, various electrical loads applied. This doesn't seem to affect the output of the controller.
  • Indicated outputs from controller to battery are similar, voltages usually a bit lower than what the panel is shoving into the controller.

Tests conducted so far:

  • Triple checked all wiring connections for tightness and correctness in accordance with the manuals of the panel and the controller
  • Disconnected panel from wiring run and controller. Multi-meter indicates 127.75 watts output (17.5 volts out, 7.5 amps out=127.75) Panel seems healthy.
  • Connected panel to wiring run and controller. Meter indicates 93 watts at the end of the run, measured at the controller terminals.
    • Note: I do have an excess length of wire because I wanted the ability to move the panel higher, later if I desired. Possible voltage drop due to lengthy cable run?
  • In spite of the fact that I measure 93 watts at the end of the cables, the controller still indicates only in the 20's to low 50's wattage input from the solar panel. Why?

I've had this panel and controller for a couple of years. It's always output this low. Yes, I'm an idiot and never questioned why until recently. Let's skip past that and get to the possible causes.

The only causes that spring to mind are faulty controller or wiring run is too long, causing an excessive voltage drop. I figure the total run is 30-40 feet of cable with MC-4 connectors on the panel itself.

Lastly, there's one little statement in the Victron manual that gives me pause:

12V battery and mono- or polycristalline panels connected to a 75V controller ● Minimum number of cells in series: 36 (12V panel).● Recommended number of cells for highest controller efficiency: 72(2x 12V panel in series or 1x 24V panel).● Maximum: 108 cells (3x 12V panel in series)

Notice that the manual states that the "minimum" number of cells is 36. My Solbian panel only has 32 cells. Could this be the problem?

MPPT Controllers
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2 Answers
Guy Stewart (Victron Community Manager) avatar image
Guy Stewart (Victron Community Manager) answered ·

That is your problem. A 36 cell panel is required. A 60 cell panel is even better.

You would get better results with a PWM regulator.

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ajax-md avatar image
ajax-md answered ·

Thank you, Guy.

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