So I had intermittent clear skies here so I got outside with the actual solar panel to see how it all came together. I forgot to bring the iPhone cable that I could hack up so I didn't implement the .5A/1A mode switch yet, or test it out with a real phone but i used dummy loads purposefully made just under 5 and 10 ohms to make sure I reached full current draw.
So what do you want first, the good news or the bad news...
1A mode is not possible with this panel! At least least not with the sun this time of year. The sun was very string about 1:30PM today, but try as I might with angle and waiting patiently for zero cloud cover. I could not reach full 1A output! So I think that, as is way too common, the manufacturer is just a little optimistic. Looking closer at the powerfilmsolar website, it's supposed to output 15.4V at .3A, which is actually 4.62W and at a clean 90% efficiency of my converter in perfect sun, that's only 831mA. Now I didn't see just how much I could have pulled, but suffice to say, my circuit CAN handle 1 full Amp, the panel couldn't provide that much power in the sun today. (BTW, the open circuit voltage on these little panels is 19.4V!)
So I've decided to nix the 1A/.5A mode switch option. Oh well, there something to be said for simplicity
500mA output was fantastic. even with the sun behind a very descent cloud such that I could stare at it without getting fully blinded, it was still able to pump out the .5A! So yea, you need sun to charge, but I won't have to worry about perfect positioning and babysitting too much, which is good news for me, even if it's not quite as fast as a wall charger.
I also found a pretty cool little device to help with the low sunlight turn-off oscillation issue. I played with capacitors and such, but they didn't really fix the problem. So another Texas Instrument special (seriously, I'm not affiliated in any way, they just make great stuff), a TPS3808, come to the rescue. It's a supervisory circuit that draws VERY little power and can be set up to have a 10s delay on the Step-Down Converter On/Off pin. So I ordered that and I think it'll really help solve that issue! Basically if the sun goes behind a big enough cloud that I love panel power below some level that I can choose (8V), it'll shutdown the converter circuit for 10s, then try to power it back on again, and see if the sun is back. I'll keep you posted on that, but it looks very promising.
THE UGLY: well currently the state of the my breadboard...since it's all in test setup mode right now, but it's been a great tool in figuring out little details. I really want to have a custom PCB made for final version of this as a nice icing on the cake.
THE FUTURE: I'm running a 'drain' test right now to see if the circuit will kill my battery if accidentally let plugged into the solar panel over night or something. I really don't want to add diode to block back current and it looks like I won't have to. Preliminary measurements say that it is drawing no current backwards at all! but I'm going to leave it for a hour and half or so in airplane mode and see what happens.
So other than figuring out the detail on the low power shutdown circuit, all the testing that remains is to do a charge time test. I should be able to drain my phone enough by tomorrow to do a full charge through the circuit and I'll let you know.