On 01/03/20 09:09, The Natural Philosopher wrote:
On 01/03/2020 08:53, Jeff Layman wrote:
Clearing out the shed I came across my old 4.8v Challenge Extreme
powered screwdriver, which I hadn't seen for at least 6 years (I use my
Hitachi drill/driver instead). When I opened the box and took out the
screwdriver I accidently touched the trigger and was amazed when the
motor turned. I though it might be the dregs of a charge in the cells,
but I tried again and it turned. In fact, it was not possible to stop it
turning by gripping the shaft.

I have no idea what brand NiMH cells were used in this cheap screwdriver
(Argos), but for the four of them to retain a good charge for 6 years
seems very unusual to me.

Probably Nicd then

They are. Nothing on the device itself, but I dug out the instructions
and they say they are NiCd. The instructions are dated 4/2006. I guess I
bought it within a year or two of that date; I'm surprised that NiCds
were still around them, but I guess they were still using them on
low-end products.

Sites vary in what they state is the self-discharge rate of a NiCd cell.
The best is 10%/month, but the most common is 15 - 20%. Not in my
experience - it's around 1.5% if those cells were last recharged about 6
years ago! :-)

--

Jeff