> I think I almost understand what I need to try ;)
>
> solar panel -> Cap -> LC pi filter -> boost (or buck) FET

I don't see any need for a filter between the cap and the switching
regulator.  The cap should be big enough so that each switcher pulse only
drops the voltage a small amount and keeps the panel essentially at a
constant operating point.  Make sure to consider the ESR of the cap
though.

> and inductor -> Cap -> shunt I sense -> battery
>
> 12V/1.26W Solar panel , OC V on a bright day = 18V, SC I 160mA
> 1st Capacitor ?? 47uF
> LC filter values ?? no idea ;)
> P-channel mosfet wired as a switch (NDT456)
> 2nd capacitor ?? 47uF
> Shunt I sense 1R (enough that I can measure current)
> Battery - 3.6V 600mAh

My gut reaction is that the caps are woefully inadequate.

Do the math.  Let's say the maximum panel power s 1.3W at 12V.  That makes
the current 1.3W / 12V = 108mA.  Let's say the switcher will operate at
10KHz, which means a pulse every 100uS.  For worst case voltage drop
analysis, let's assume these pulses are inifintely fast.  That means you
are suddenly removing 108mA x 100uS = 10.8uC from the cap.  10.8uC / 47uF
= 230mV, which is larger than I would like to see.  My knee jerk reaction
was a 1mF cap, which should work nicely given these numbers.  The larger
cap will also have a lower ESR.

> Basic software flow...
> FET off - check solar voltage
> If solar volt enough (ie daytime), start PWM at low level.
> Measure shunt V drop
> if lower than target V drop increase PWM
> repeat till target is met.

This is not MPPT, and will not work where the illumination is too low to
sustain the target current.  Instead, "hunt" around the current operating
point to adjust for maximum output current.  This is what a lot of the
discussion has been about the last few days, so I won't repeat it.


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