William Sommerwerck wrote:
Any nicad experts out there? This is an interesting one. (I'm not
looking for a "solution", just an explanation, out of curiosity.)

Many years ago I bought the Sony TCD-D3 DAT Walkman, which fits a fat
pocket. I made a few live recordings, as it was a lot less bulky than
a processor/VCR system.

The other day I pulled it out to confirm it was still working. I'd
bought two extra batteries with the unit, and put all three on
charge. They all charged up, but the third battery wouldn't power the
unit. Its end-of-charge voltage was nearly 7V (though these batteries
are nominally 6V), and the other two charged to about 6.4V.

When I put the third battery on charge again, it rose to about 7.5V,
even before charge was terminated. I let it sit overnight, and it
self-discharged (?) to about 5.5V. (The others //did not// show
significant self-discharge.) It still wouldn't power the unit.

"Obviously" this battery pack is defective. I'm just wondering if
anyone knows what's going on.

It's no big deal, as I have two good packs (which are 20+ years
old!), and the AC adapter. Also, the packs are "external", so I could
(if I wanted) saw open the pack and replace the cells. Or simply
throw together a homebrew pack to fit the DC jack.

Just askin'.


"We already know the answers -- we just haven't asked the right
questions." -- Edwin Land

--

Nicads have historically been plagued by "dendrites", or crystals of
conductive material (probably cadmium, but not sure) between the electrolyte
layers in the cells. This creates a low resistance leakage path inside the
cells, rendering them incapable of retaining a charge for any length of time
or putting out their rated voltage. Dendrites are responsible for the
so-called "memory effect" that nicads are famous for.

There have been several procedures published on the net as to how to remove
dendrites (search for "nicad dendrites"). One process involves charging a
large electrolytic capacitor (eg., 500uF) to a few hundred volts,
discharging it through the bad cell, then running the cell through a couple
of charge/discharge cycles. I built a little box for this process several
years ago, and it seems to work. It consists of a transformer, a voltage
doubler, and a large capacitor. A push-button switch triggers an SCR that
lets the capacitor discharge into the bad cell (sort of like a one-shot CD
ignition)

Dave M