Today I had a nice sunny day, so I plugged the solar panel in and left it in the sun, then waited 15 minutes......

Still no good!!!! Something has to be wrong. So I removed the second LTC3105 so I could check that the first one is working. Back into to sun for 15 mins....

Success! The supercap voltage was just over 2.5v! So there must be an issue with the second LTC3105. I tried resoldering using the solder left on the pad.....Ba Bong...(don't do that) so I removed all the old solder using solder wick and then got some fresh solder paste and got to work with the hot air gun. I put a bit too much on, so a couple of traces were bridges next to the DFN package. A quick cleanup with the wick and try again....

Nope! No output from second LTC3105.

Back to the schematic to see what I've done wrong. I noticed a couple of things. Because I was using a supercap between the 2 ICs, I thought I wouldn't need a big 10uf cap on the input to the LTC3105, like the first one in the chain does. I used a 1uF. Maybe if I change that.

Nope. That didn't work.

The only other "extra" part on the board is a 33pF filtering cap between the output and the feedback pin. I may as well try removing that. Can't hurt right???

Right!!! It now works!!

After leaving it in the sun for 15 mins, the supercap is full and 3.3V on the output!

Let's add a load to the circuit. I have an Arduino Pro Mini 3.3v lying around with the default Blink sketch on it. I plugged it in to the Vin pin (not Raw) and the power light came on and the second led started blinking!

In full sun, the tiny solar panel could only just continue to charge with both LEDs on, but charge it did!

Even when it was moved into the shade, the supercap and the second LTC3105 still produced 3.3v output, with the supercap voltage slowly going down.

I let it fully discharge then put it back into the sun. The supercap voltage got back up to 0.4v.....then dropped back to .3..... The output was only .2v.

So looks like if there's a load connected that draws too much power on startup, the board gets stuck in a loop.

Methinks it's time for a voltage supervisor or similar that either enables the second LTC3105 only when the supercap voltage is over a certain threshold (1v maybe?). I also only had a 4.7uF cap on the output of the second LTC3105. Looking back at the datasheet, 10uF is recommended, so I'll replace that and try again.

Regardless, the board works! It could be used to power a small micro and sensor. Maybe even BTLE or a low power RF module?

To be continued ......