Update: I replaced the Schottky diode with a MAX1555 Li-Ion cell charging IC. Thanks go to moldboy and Kohaku for their input on making the charging circuit safer.
What you need :
- An e-reader, I use a first gen Kobo, but I'm sure this is also possible with any other.
- Some solar panels, like these. Perfect size and voltage and decent power too.
A Schottky diode , you can get this at any shop that sells electronic parts. You need a diode to prevent discharge of the battery. The advantage of using a Schottky diode is the lower voltage drop.
- A MAX1551 or MAX1555 IC. They operate with no external FETs or diodes, and accept operating input voltages up to 7V, so very easy to implement. Here is a datasheet. I ordered a sample from the manufacturer.
- A SOT23 to DIP breakout board. You need this, because it's quite hard to solder wires directly to the MAX1551/1555, since it's so small. I got this from eBay.
- A small capacitor, to make life a little easier for our MAX1551/1555. I used a ceramic one I had laying around.
- Some wires, a soldering iron, solder, a dremel and padded double sided tape. A multi-meter can also come in handy.
Here are some minor technical details:
This e-reader (as most of them are) is powered by a 3.7V li-ion polymer battery. These kinds of batteries need pretty tricky charging curves: fast at first and trickle charge till full, then the current stops, since over charging can be dangerous. For this reason I replaced the Schottky diode I used before with a MAX1555 charging IC. A Schottky diode also works, but it can damage the battery in certain circumstances.
The specifications of these solar panels claim that they can do 80mA, but after some measurements I came to the conclusion that in reality they will only reach about 50mA. Adding the two panels together we come to 100mA. This means that they should be able to charge the battery from nothing to full in about 10 hours. However, the charging current is controlled by an IC and will be lower during the later half of the charging cycle.