Sizing a MPPT controller.

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cgogs asked Mar 21, '22

Sizing a MPPT controller.

Good evening all.

I am about to purchase 3 panels of the below spec:

Pmax Watts80 Voc(V) 21.4 ISc(A) 4.85 Vpm(V) 17.7 Ipm(A) 4.56 I intend to wire in parallel to avoid shading issues but could be persuaded otherwise

I have tried used the Victron MPPT calculator which suggests the Solar Smart MPPT 75/15 however some calculations are in red i.e

Min. PV voltage @ max. temperature:

16.9 V'

I've played with the settings to get the tool to suggest the MPPT 100/20 but I still get similar results. If I hover over the red text I get the message ' Minimum PV voltage @ max temp. < minimum input voltage at MPP. Does this mean that due to the voltage difference between batteries and panels needing to be 5 volts to start, that at maximum specified temp (70.C STC) the panel output voltage would be too low to start the controller? If so I don't think I really have much to worry about with regards to the panel ever getting that hot.

Any help much appreciated.

Kind regards

Chris

MPPT Controllers Solar Panel

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snoobler answered · Mar 21, 2022 at 06:37 PM

It's essentially a caution. MPPT requires +5V above battery to initiate charging, and then I believe it's +2V to maintain. This is from the perspective of Voc, i.e., 21.4V

Check the NOCT rating of your panels. If the Voc is > 18.6V, then there are no concerns with the MPPT being able to start and continue charging.

Worst case, most panels have bypass diodes such that even if a panel is shaded in series, the underperforming panel is essentially bypassed allowing the unshaded panels

Lastly, while panels flat on a roof will almost never perform to specification, 75/15A charger be limited to 220W maximum even though the panels are rated for 240W. A 100/20 would maximize it and allow for the installation of an additional panel. A 100/30 would allow even more.

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Kevin Windrem answered · Mar 21, 2022 at 06:40 PM

Panels in the sun could get that hot so I'd be worried about them restarting after a cloud passes.

Snading with panels connected in series should not be an issue as long as the panels have reverse current protection diodes (aka bypass diode). These are diodes across the panel or sections of the panel to prevent a shaded panel from damage from panels in the same string in full sun.

https://www.electricaltechnology.org/2019/10/blocking-bypass-diode-solar-panel-junction-box.html

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Kevin Windrem answered · Mar 21, 2022 at 08:11 PM

3 panels in series will most likely exceed the 75 volt input rating at cold temperatures. (Remember open circuit voltage goes up as temperature goes down.) Best go with a 100 volt MPPT.

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snoobler commented · Mar 21, 2022 at 09:05 PM

Assuming -0.33%/°C, which is conservative, 3S should be good down to -25°C.

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Answer 1 · 2022-03-21T19:07:51Z

cgogs answered · Mar 21, 2022 at 07:07 PM

Thank you for the informative replies. Especially about getting a larger charger to allow for more panels.

The 3 panels are going onto a boat, I had intended parallel for shading reasons, but series could help by reducing wire sizes. Do you have any input?

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