I think we're looking at apples and oranges... er, coils vs. FETs. My toy-ish simulator only gives me a virtual DVM and virtual oscilloscope for analysis. Yes, if I put the leads across the coil, then I get results similar to fig.3.

I thought that we were going to measure the voltage across the FET? As I understand fig.3, the FET voltage isn't shown.... The plots I posted simulate the standard boost circuit with the o-scope ground on ground, channel one on gate and channel two on drain.

The duty cycle in this sim was 50% and is seen by the square waveform in red. The voltage on the drain-source channel (yellow) is at 10v/div when the FET is turned off and shows a quick voltage spike to 50 volts or more that trails off until the voltage drops below battery volts (about 30v), then it slams down to input volts (about 17 volts).

The second image shows the voltage across the FET when it is turned on. Channel two is set to 50mv/div to show the detail during this phase. ( The voltage looks flat in the first image but it's not, it's just lost in the larger voltage scale.)

If I understood your instructions, I am to measure the FET voltage just before the FET is turned off, that would represent the approx 150mV shown on channel two in the second image.

If I understood your instructions, the volts measured just before the FET is turned off would be multiplied by the voltage across the cap. Power = Vcap * Vfet/Rdson but R is constant for purposes of relative comparison so forget about it. Is that right?

That seems to represent maximizing power in from the solar panel. When you first wrote of measuring current by using Rdson as "the shunt", I assumed you meant on the output (battery) side. Based on that, I was suggesting that the "tall area" in the left hand part of the yellow waveform in the first image, starting just after when the FET gate is turned off until the voltage drops below battery volts represents the power that is transferred to battery. Now we're considering (Vds x Rdson) x Vbatt. Similar to the previous case, Rdson is fixed and can be dropped from the calculation but now Vbattery is constant for purposes of relative comparison so we can forget about it as well. All that leaves to consider is volts across the FET for as long as the diode is conducting. From what I've seen from a few different input-power setups, I'm suspecting the using the diode conduction time alone might be adequate to track the MPP... hm, "maximizing conduction time of diode with a pwm output" sounds like a good description for MPPT on a chip.

Do you have an analog solution? I know that many folk would prefer an analog solution but it seems hard for me to beat the functionality of a two dollar microprocessor chip.

Is this making any sense?