I wanted a phone battery power bank with a built-in solar panel. This would not have worked, as my house does not get direct sunlight. However, I have a 7v solar panel that came with a BPL emergency light. I put the solar panel on the terrace and brought its wire down to the balcony.
I could not simply connect solar panel to a power bank, as most power banks work at 5 volts. So, I built a voltage regulator module using a LM7805 IC. This dropped the voltage down from 7v to 5v. The circuit is simple. Connect the solar panel to a 2.0 mm DC power socket. Connect the two leads of the socket to the first two legs of the voltage regulator. The first leg is connected to the positive of the socket. The second and middle leg is the ground, which is shared with the output. The middle and third legs are connected to the ground and positive of the USB port.
I only had a dual-USB connector and used it anyway. This was not a bad choice. The extra surface area give its legs better anchorage. The power bank is supposedly 20,000 mAH. After I took it out of the package, I was able to charge a BlackBerry Book and one-thirds of a Micromax FunBook, both of which require 3 or 3.5 hours of charging. Now, it is charging. I have only a couple of hours of sunlight today.
UPDATE (22 December 2013): I added a 1n4007 diode to prevent current flowing in the other direction when the voltage coming in from the solar panel falls down drastically. The power bank may already have the diode but I am not sure. Whether this diode would be appropriate is also something I don’t know. It is a diode and I used it. When I connect a 3.7v phone battery at the USB end, I can still measure 0.1v at the DC plug end to which the solar panel has been connected.