On 1/9/2014 4:47 PM, William Sommerwerck wrote:
"Dave M" wrote in message
...

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 assume the discharge is + to +.

I'd be careful. Energy is proportional to the square of the voltage.
I use 12,000 uF and 10-20V, mostly because that's what was handy.
You want to hit the sweet spot where you instantly vaporize the dendrite,
but generate no more heat than necessary. Too little energy or too
much energy is worse than just right. That's why suggestions to
use a car battery are bad. Little chance of getting it just right.
Way too much chance of getting enough energy to explode something.

500uF AND 200V is WAY too much.

The process leaves the hole in the insulator/separator.
The field density is high and the dendrite wants to grow back.
And discharge thru the hole causes the high self-discharge rate.

I've had a lot of success restoring 40 year old F-NiCd's for
uses that don't require more than a few days storage life.
But, these days, you can get equivalent capacity in a pair of
AA NiMH cells.

Don't discount the safety issues with high voltage.
The "experts" who look at only part of the problem will call me an idiot,
because 200V probably won't kill you.
Two problems with that..."probably" is great for statistics, but
provides little solace to your family when you're the ONE who died.
And B, the shock isn't the only problem.
I've got scars where the muscle contractions that thru me across the
room caused me to rip my arm open on some sharp object in the path.
Luckily, I haven't fallen and cracked my head open on the furniture...
yet...

Eye Protection is a MUST. Sparks fly.

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).


I'll give this serious thought. The worst that can happen is that the
battery is ruined. (!!!) Thanks.

I'm still curious about the battery's unnaturally high voltage.