Recently, I had to spend 4 days, walking in kind of desert and I need my smartphone for record the track.

No power supply was possible during that time and I should find a way to recharge my phone during the walk.

I didn t want to carry heavy power bank or buy an unknown quality cheap solar power bank.

So, I spent some time to find a suitable solution. the main issue is that the solar panel I'm using is max 500mA and even with a regulator, cannot be coeected directly to the USB.  A high current was requested at beginning and the voltage drop. Never work.

So, I develop a simple solution, light with a small old 650mAh battery:

- An IC (EUP 8054) connected to the solar panel charge the small battery

- A low consumption comparator, Max931, with hysteresis wait the battery to be >=4v

- When the battery is >=4v, the comparator enable (through an Tr inverter) a DC-DC switch LT1302 (Linear), providing 5v 600mA to the USB plug and to the smartphone

- When the small battery is empty, the Max931 switch off the DC-Dc and wait the sun to recharge again the 650mAh battery

- I add also a button to press when I want to force the recharge (forcing the comparator to commut)

It works well. I put it on the top of my back bag. Recharged several time my mobile during these 4 days, trecking  in Gobi desert...

Laurent