[Michael] took a battery charger meant to be connected to mains power and converted it to work with a solar panel. This was a traditional 4 cell charger which charges the batteries in pairs. He kept that functionality, but added USB charging with a special over-current feature. That’s because his Android phone has a fast and slow USB charging mode. The slow mode makes sure that it draws 500 mA or less to stay within USB specifications. But the fast mode draws more current when the phone detects that the USB connection is attached to a wall charger. [Michael] added a switch that patches a pull-up resistor to the data line, signaling to the phone that it’s okay to switch to fast charging mode.

As for the power supply itself, you can see that [Michael] snapped off the part of the circuit board that housed the original regulator. He’s added his own 5V switching regulator which offers a wide input voltage range. This is connected to two banana plug sockets which can be connected to the solar panel.