Hand Crank Radio/Flashlights

Roto Glo hand crank flashlight

I've got a Roto Glo hand crank flashlight that I was messing with the other day. It's a pretty neat little gadget. You can use the on-off switch to select one or three led power. It's no searchlight, but it beats fumbling around in the dark. There's something about a renewable power source, even a dinky one, that is just very cool.

I've always wondered about the voltage and current capabilities of the little generator mechanism in it. I was checking it out yesterday and mulling over idea of taking the thing apart and seeing if I could tap the output of the generator and do some measurements.

While surveying how many screws hold the thing together, I noticed a little electrical connector jack that was hidden by the crank handle. Oooooh, that really piqued my interest. I rummaged around in my goodies and located a connector to fit it and was able to hook up a VOM to the output. I checked the voltage on it before cranking the generator and measured about 3.5 volts or so. Ok, so I could measure the battery voltage. That was a start. I left the VOM hooked up and started cranking the generator handle a little. The voltage rose to 3.8... crank a little faster.. 4.2... a little faster... 5.0. So started giving it a good cranking and got the voltage up to 6 volts. I was fairly impressed.

I dug out an old rechargeable 4.8 volt, 4 cell AA battery pack (Ni-cad cells) that had been sitting uncharged for a few years. The pack measured about 3 volts, so I put a 12V bulb across it, to drain it down, until the bulb barely glowed. I wanted the pack basically 'flat' to start. I got it down to just over 2 volts.

I hooked it up to the output of the little generator and did 100 moderate 'medium speed' cranks of the handle. I stopped and disconnected the battery (didn't want the internal flashlight battery to be part of my measurement) and measured 4 volts on the battery pack. I did another 100 moderate cranks and re-measured, 4.4 volts, another 100 cranks, and I was up to about 4.6 volts. I did another 100 and the pack was reading 4.8 volts. I was impressed again.

Now my disclaimer. Various rechargeable batteries all have their preferred charging parameters (voltages and rates), for both battery health and more importantly, safety reasons. Ni-cad cells are fairly tolerant and the pack I used for the experiment was just wasting away on the shelf. It was my 'lab rat'. NiMh cells are a little pickier about their charging. Lithiums even more so, to the point they can be dangerous. I've charged a lot of R/C packs (Ni-cad & NiMh) over the years and packs for my ham radio hobby. I have and use the right chargers for those jobs.

I just wanted to satisfy a curiosity, and see if this little hand crank generator possibly had the potential to get me out of a 'dead (rechargeable) battery pinch' if I really needed it to. The results were promising, (and fun) and I'm going to experiment with it a little further. I want to see if I can get some output current measurements while charging my 'lab rat' battery pack.

By the way, I haven't cranked up my little flashlight since my experiment almost 2 days ago. I turned it on (all 3 leds) and left it on the desk when I started typing this novel. I'm a terrible, two fingered typist and I've been pecking away on and off (and doing some other stuff) for over an hour and a half. The little Roto Glo light is just now reaching 'dead' status. Not too bad.

I'd like to do some real life testing on this small scale, generating experiment. Maybe charge up 4 NiMh cells for my 2 meter HT (the correct way) and drain it down with normal usage, and see if I can pack enough electrons back in the cells with the flashlight generator to make a few more transmissions. It's worth a try. Might be a useful trick that comes in handy some day. I'll post the results if anyone's interested. (We all have a little McGuyver in us, don't we?) ;-)

Doug