I finally dug out an old garden solar LED light (still in the bag from an X-Day a few years ago) and extracted the 3" square solar cell. The cheap plastic case had fogged up and was cracking.

In the sun, through a glass window, the cell gives approx 3.5V. Connecting it to the USB charge input does make the battery think that it's charging and the charge level LEDs do light up. Now, the question is: Is the solar cell gaining or losing on the drain from the charge LEDs?

I suppose the next step would be to take a battery with little charge, and leave it in the sun with the solar cell for a day and see what happens. Crude. Too bad I can't talk to the controller in the battery to get the exact charge level.

The solar cell is the same 3" size as the battery, which could be a nice package.

Speaking of packaging, I have a spare Raspberry Pi that I that I'm going to put in a portable case. One of the batteries would fit in nicely, but I'll have to check if it can keep up with a Pi running a wifi access point, maybe a USB camera.