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MPPT Derating not consistent

@mvader (Victron Energy) said the following about the temperature derating of the 100/30 MPPT:

"Its a linear derating from full power at 40 degr C to zero at 70 degr C"

Is it really as simple as that, or is there some dependency on the operating point?.

I have tested two different systems, one of them shows very clear derating with temperature and the other shows no observed derating at all.

System #1 960W, Vmp:33.6V, Imp:28.9A, MPPT:150/35

System #2 720W, Vmp:43.6V, Imp:16.5A, MPPT:100/30

When operating in good sun with the controllers kept under 40 degreesC by forced cooling then System #1 delivers in excess of 900W and System #2 delivers around 650W.

When the MPPT temperatures (at the heatsink) are allowed to rise to 50 degC, System #1 output derates by almost 300W whereas System #2 does not derate at all. There is no change in the temp of the panels, only the MPPT, and in both cases the LiFeSO4 batteries are accepting ALL of the power that can be delivered to them.

Why do these two systems behave so differently if the MPPT derating is a simple linear 3.3% per deg above 40 degrees?

MPPT Controllerstemperature
2 |3000

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