Al Shinn wrote:
> About NiCds - when a NiCd battery dies due to whiskers shorting out
> the cell, a common practice is to discharge a large high voltage cap
> into
> the battery - blows out the whiskers and restores function.
>
> Voltage-   about 300V with  about 100Mfd capacitor

That's obviously absurd!

1/2 (300V)**2 * 100MF = 4.5TJ = about 1 kiloton of TNT equivalent, or a
large fleet of B-52s fully loaded with conventional bombs, or about 1/15 the
energy that leveled Hiroshima in 1945.

Assuming you really meant millifarads instead of megafarads (additionally
assuming "fd" was supposed to mean Farads), that's 4.5KJ unleashed on the
battery in a fraction of a second.  That's still rediculous when you
consider that's enough energy to launch a normal 5 pound clay brick 300 feet
into the air.

Don't try this at home folks.  This is messed up advice on a number of
fronts and could get you seriously hurt.  This guy can't even get basic
units right, so anything else he says is best disregarded.  You don't want
the equivalent of a 5 pound brick falling 300 feet onto your battery, and
you don't want to be nearby if you did try it.

Even with a lower capacitance, there is no need for 300V.  A NiCd cell is
only 1.2V.  The point is to produce a short current pulse to blow out the
little whiskers, or dendrites, while minimizing any other damage.  It
doesn't take 10s of amps to do this.  Ideally the damaging high voltage is
gone just as the dendrites fuse open.  That's tricky to arrange since the
amount and strength of the dendrites is unknown.  It's best to start at
5-10V on 1mF and work your way to higher energies until the dendrites blow.
Increase the energy by 2x each time, but 15-20V is max.  Use a higher
capacitance if you get that far and they're still not blown.  If 20V and
10mF doesn't work there is little point trying to salvage the battery.


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