So the potential needs to be at least as high as the thing you're trying to charge?

I scavanged another solar panel from an old radio (about 1" x 4"), it doesn't have any information on it, but my multimeter says 4.5 volts

The Grundig radio w/ handcrank is model FR-200 (AM/FM/SW1,2). There are two ports, one for headphones, one is a DC jack (4.5v) for external power. Does not appear to be designed to charge cell phones. I was thinking I might be able to open it up in a pinch and wire it to the cell phone battery, but I thought that might do some damage to the battery if there's no voltage regulator