I've got a small solar panel that came with a science kit (about 3" x 1") that says .5v - 300 mA on it.

Could this do anything useful (charge a 900 mAh AAA protected rechargable battery / perhaps a cell phone battery) or is it too small?

Also, I have a grundig radio with the hand crank on it. Would I be able to charge a cell phone battery using that? What about a small laptop battery (7.4v - 5200 mAh) (although I'm sure it would take forever....) would I need a voltage regulator?

The solar cell no, the radio not sure, some radios have a cell phone charger function with jack and cable/connectors. If it charges anything just cell phones due to the charge voltage.

Lee

I don't agree. 5V 300mA is more tan enough for 2 1.5V batteries. Just add a low drop regulator. I have 6 x 7.5V x 200mA film panels of slightly larger size each (still almost unnoticeable for EDC). They're recharging my 3600mAH LiIon powerbank, rated 12V-30V of charging voltage, in about 5 hours, just laying flat on the table. I'm recharging most of my EDC electronic gadgets from the powerbank (regulated output 1.5V-12V every 0.5V) after that, including 4 AA cells, but excluding my UMPC.

If it says .5V (point five, or half a volt) then it is not much use for anything. Sorry.

Regarding the radio, some hand-cranked radios are indeed designed to do what you suggest. Have a good look for a small socket on the radio which might indicate it could be used as a phone charger. Sometimes there is a socket for an external charger to top-up the radio's batteries. Sometimes there are two sockets- top-up and power output.

Alternatively please post the model number, or Google for it.

A

Yeah, something is wrong with my eyes again. 0.5V - is almost useless. My panels are ~4x2", so I've assumed this one is of the same kind.

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

It's better to avoid charging any gadgets batteries directly (even if they look like AA or AAA, but just not marked so). The device charging circuit could be very complex. The only 100% safe way is to use the dedicated charging port on the gadget.

Makes sense, thanks