rin67630
Solar Enthusiast
Yeah! Real-Life(TM) can be quite different than lab conditions.In real world, the higher wattage peak would have more heating so more current leaking through the photodiodes, which could shift Vmp to a lower voltage than what 25 degrees C gives (but lower illumination could be at still lower voltage).
Vmp is heavily dependent on temperature, which is not a constant.
When you have a succession of sunshines / clouds the Vmp is really dancing: the cells will cool-down then warm up following the irradiance with a few minutes delay. Any MPPT algorithm will have a real hard time to deal with that.
I have decided to catch this situation and switch over to 90% of Voc, which is a good approximation of Vmp for 5 minutes before returning to MPPT.
After 5 minutes I might get another collapse, but one collapse for 15 seconds every 5 minutes is not really impacting the day harvesting.
When you have a hard shade on a few cells across sub-strings, the span between Voc and Vcollapse gets very narrow.
Then it depends again on whether the remaining sub-strings are generating more voltage than the battery or not.
If the shade comes from a bird, the situation changes that fast that a sweep will return wrong results anyhow.
A hard time for MPPT algorithms again...
The modern half cell panels with 2 strings in parallel within a single panel are really superior with respect to bird shading.
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