Almot wrote:
HiTech wrote:
Measuring Voc requires a disconnect of the PVs from charging and is generally a sign of a crude MPPT strategy. Who still uses this?
I think Salvo meant that controller "can" measure battery voltage via power cables accurately when there is no charging current. Which is correct. The problem is that there is always some current.
Well - since users complained on "inconvenience" of accurate voltage sensing with separate cables, those who don't care about under-voltage, should be happy now :) ... If the battery is sitting next to controller, there is not much error, but then, there is not much inconvenience with extra sensing wires either. Whatever...
Correction to my previous post - it has to be ~6ft from battery to controller, to get 1% error at 30A. 12ft total run.
There is an ultra crude form of MPPT that really is not tracking at all but a SWAG, that is based on fully disconnecting panels from the load, measuring Voc of the array, and hoping that the aggregate system Vmpp is .80 of the aggregate Voc, and just fudging it down 4 percent more and using that voltage. It does not track power point at all. It tracks Voc, and makes SWAG at Vmpp. And it interrupts solar power generation to do it. Most of the time it is too low (for the many panels in the .80-.85 Vmpp of Voc range), and for panels like my Unisolar it is too high, potentially yielding less output than PWM would.
If a controller were close to the price of a PWM controller plus a buck converter, I could see using such a crude control strategy. But for anything high end it's yield is too low. It's basically never going to find the max power point, and if you are lucky it might be closeish for your specific panels.
You really need to to a sweep through all to local maximums to find the system max power point.
Jim
