19 Screws! (OK, maybe 3 screws?)

A project log for Coin Cell Power Screwdriver

How many screws can you drive with a CR2477?

ted-yapoTed Yapo 01/07/2018 at 15:564 Comments

Not great, but something. If you count just getting them to start, it's 19, but really only 3 were properly driven.

The NiCd's were charged in about a day, so the current drain on the CR2477 was fairly high (for a coin cell), and not much energy was extracted to the NiCd's.

I have some others charged over longer periods, but I'm saving those for my #Coin Cell Jump Starter .


After this test, I decided to see how many screws the same cells could drive if charged fully in a normal charger.

Answer: at least 27! That's all the holes I had.  So, the coin cell charged the NiCd's to no more than 11%.  I suspect it was less than that, though, since the screwdriver still had enough juice to remove all those screws after I stopped the video!

If I end up with some extra cells after the jump starter experiments, I might try this again.

I always thought this screwdriver was lousy, but I never tried it with anything but alkaline cells.  With NiCd's, it's actually not that bad...


EricH wrote 01/08/2018 at 12:42 point
wow. Good demonstration, too, of what you've been saying about nicd's not being easy to detwrmine the charge-level... (flat discharge curve?) being that at 11% charge it was clearly still usable

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Ted Yapo wrote 01/08/2018 at 13:04 point

Yeah, and flat charge curve, too.  During charging, the NiCd voltage goes from 0 (after being shorted) to 1.2+ in a very short time, then stays there.  Really no way to know what's happening by the voltage.

From what I've read, the only way to really know the state-of-charge (for both charge and discharge) is Coulomb counting: keep a running tab of the current in/out of the cell.

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EricH wrote 01/09/2018 at 00:21 point
Wild. I've always had bad luck with nicds... thought it was from over-discharging, but if they're supposed to be stored shorted, I guess that fails. Maybe from turning the screwdrivers when empty, so reverse-charging... or memory-effect... but this stuff you're saying makes them almost seem useful, maybe even better'n supercaps!

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Ted Yapo wrote 01/09/2018 at 00:30 point

@EricH NiCds still power the bargain cordless tool industry.  Lithiums have taken over the high-end, but the NiCd is still used for many of the cheaper ones.  They're inexpensive, easy to charge, and the constant-voltage discharge means power lasts until they're dead.

Oh, I should say that you can detect a fully-charged cell while charging by a few different mechanisms, so you can tell 100% and 0%, but in between it gets hazy.

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