From: pjswan@engin.umich.edu (Peter Swanson)
Subject: Re: Renewing Nickel Cadmium batteries
Date: Sat, 12 Sep 92 14:01:32 EDT
Organization: University of Michigan Engineering, Ann Arbor

In article <w#wn#w=@lynx.unm.edu> jfreter@triton.unm.edu (The Myth) writes:
>In article <BuFFKG.42n@ngc.com> harrys@ngc.com (Harry Saal) writes:
>>PC Magazine (Sept. 29 1992) mentions a letter in The New Scientist  
>>(April 25, 1992) from Michael Bell (of Hitchin, Hertf.) that says:
>>"If you drop a 'tired' (nickel cadmium) battery about a foot onto a
>>concrete floor a few times it renews the battery wonderfully."
>>
>>I didn't see this in the FAQ; perhaps this is a thread which has been
>>followed before. The obvious questions:
>>
>>True? False? Does it work on other types of flooring? If so, from how
>>many feet? How many times is "a few"? What is the line between a battery
>>being "tired" and "dead"? Should I remove it from my laptop, or just drop
>>the whole computer in disgust?
>
>
>Definately false.  NiCads rely on an electro-chemical reaction between
>the nickel, an acid (or was it basic) gell, and tha cadnium.  The dropping
>will not do much at all except maybe rupture the cells.
>
>About the only way to renew batteries is to do a very slow charge (mili-amp
>current for a day), then discharge them completely using a resister that
>is sized to closely match your computer's load. That is, if you're supposed
>to get 3 hours of battery life, and your batteries are rated at 3000 
>mA-hrs at 10 volts, then you should use no smaller than a 10-ohm resister
>to discharge (I use a 75-ohm to deep-cycle mine).
>
>Repeat the above several times, but pause between each long enough to let
>the batteries completely cool off.
>
There must be a battery FAQ somewhere.

I was talking about this very subject with a friend of mine (who is an
RC enthusiast) last summer.  According to him:

You should not fully discharge a battery (series of cells), because
some of the cells will discharge faster than others.  The difference
in charges between cells will set up a reverse current which will
destroy some of the cells, wrecking the battery.  This does not affect
single-cell batteries like C and D cells, it only affects the larger 
multi-cell batteries like 7.2V ones.  This is also the reason you should
never mix old and new (partially discharged and charged) batteries.

Because Ni-Cads have a very flat voltage/charge curve, it is sufficient
to discharge them until their voltage drops about 10%.  This is a nearly
complete discharge but does not damage the battery.  Most sophisticated
equipment (computers, for example) automatically shut down when this
happens to avoid damaging the batteries.

Ni-Cads are killed when the gel inside them crystallizes, preventing
the movement of ions withing the cell.  The crystals begin at one
end of the cell and slowly grow towards the other end, reducing the
charge capacity of the cell (a "tired" battery). Breaking up these crystals
renews the battery's capability to store a charge.  Of course, eventually
the cells die anyway of other causes and there is nothing you can do 
about it.

Some expensive chargers use a very short, high voltage reversed current
to shatter the crystals without damaging the cells.  This can extend
battery life significantly.  

Dropping the batteries a short distance onto a hard surface might also
break up the crystals and renew the cells, as long as the shock is not
hard enough to rupture the casing.

Of course, I could be wrong.  If you have a better reference, post it.
No Ni-Cad religious flamewars, please.

Peter "What, me give references?" Swanson
-- 
| Peter J. Swanson           |  pjswan@caen.engin.umich.edu                  |
| Graduate Student           |  controls specialist                          |
| Electrical Engineering     |                                               |
| University of Michigan     |  Look! We have guests!                        |