Just witnessed MPPT voltage drop below 14V
Well this is interesting. Half sunny half cloudy day, 92 degrees out. Collected this data looking at the turnigy for the maximum panel output (not max amps to battery which was fun but not any real objective yet) for each line:
When it is sunny out, my Unisolar 68 watt strip stays between the theoretical Vmppt of 16.5v and a few tenths below that most of the time (16.35v or so average I think).
When moderate clouds come out, I saw it drop a hair below 14 volts multiple times (I got full data for one of them). That has interesting implications for serial vs parallel solar setups using 12 V panels on cloudy days.
On the flip side, when the battery does not need charging at over 14 volts, that makes PWM around the same efficiency as very good MPPT under those conditions. If the MPPT is not very good, the PWM could actually yield more watts to the battery.
It also means that on days with fast moving clouds moving through bright sunshine, a slow MPPT algorithm is going to be WAY off sometimes, such as for an optimization interval where it set Vmppt for sun and then there are clouds for most of the next interval. Output essentially falls of to near 0 because the IV solar output curve shifts radically left.
Fun stuff!
Jim
P.S. DSchmidt_2000 you were right on with the idea of using a constant voltage load. Once I created a good approximation of it, it was MUCH easier to adjust the pot to find the max power point.
When it is sunny out, my Unisolar 68 watt strip stays between the theoretical Vmppt of 16.5v and a few tenths below that most of the time (16.35v or so average I think).
When moderate clouds come out, I saw it drop a hair below 14 volts multiple times (I got full data for one of them). That has interesting implications for serial vs parallel solar setups using 12 V panels on cloudy days.
On the flip side, when the battery does not need charging at over 14 volts, that makes PWM around the same efficiency as very good MPPT under those conditions. If the MPPT is not very good, the PWM could actually yield more watts to the battery.
It also means that on days with fast moving clouds moving through bright sunshine, a slow MPPT algorithm is going to be WAY off sometimes, such as for an optimization interval where it set Vmppt for sun and then there are clouds for most of the next interval. Output essentially falls of to near 0 because the IV solar output curve shifts radically left.
Fun stuff!
Jim
P.S. DSchmidt_2000 you were right on with the idea of using a constant voltage load. Once I created a good approximation of it, it was MUCH easier to adjust the pot to find the max power point.
