Possibly the Final Update

A project log for miniSolar Charger

A solar powered, battery buffered, USB charger.

Peter ThompsonPeter Thompson 09/15/2018 at 13:230 Comments

The mini Solar charger has gone well and I think I'm ready to move on to my next project.

The past week I've been tackling problems with the battery draining too quickly when the device is not in use. Changing the code to run in sleep mode seems to have fixed this pretty well, and now I expect the battery to last a bit over a month in total darkness. The main cause of discharge now is backflow through the solar panel, which I measure at 0.6 mA. Adding a diode would fix this, but lob off a good chunk of efficiency (a good Shottkey diode would cut about 10% off).

Messy discharge measuring setup.

The circuit is working just as I expect it to, but the solar panel is underwhelming. I knew going in that 2 watts is pretty minimal, and you usually get much less than that. 

I also might put more thought into the enclosure. It fits pretty well; overall I'm happy with it. It's not totally sealed but could probably take a splash. The phone slides in and out okay, and the top flap keeps it tucked in. You can see in the picture that the sun has discolored the filament, but I haven't noticed any other property changes.

I do have a bit of a heating issue. When charging an iphone with it inside the pocket, the phone overheated to the point where it refused to charge. The solar panel gets pretty warm in the sun, and with this design, that heat transfers right through.

I might ditch the pocket idea altogether; the user probably already has a more secure place for their phone. 

That's all for now. I'm going to go back and update my documentation, get a few action shots, performance specs, etc.