in this video you will see my latest usb portable charger. it is complete overkill, the solar outputs 25 volts regulated to 5 to charge anything usb, it can also recharge the double a batteries i use with a flick of a switch. there are two battery options, either two or one 9volt batteries or the 4 double a batteries. there is a SPDT switch that allows you to choose either the solar or a battery source, without accidentaly choosing both. so in short you can charge any usb device using solar power or battery power. you can recharge the double a batteries or pass the chagre to the usb device! the case i used is also water proof and durable as it can be.