Re: [NTLK] "Slapping your cells silly" {Battery treatment revisited}

From: Robert Masters (rdm_at_enfield.lympago.com)
Date: Thu Apr 29 2004 - 17:03:24 PDT


On Fri, Apr 30, 2004 at 08:41:36AM +0900, Nick M. wrote:
>
> On Apr 29, 2004, at 9:35 PM, MKow1234_at_aol.com wrote:
>
> > ...a battery cell is just a bunch of chemicals with two metal
> > electrodes in it ... The physical jarring of the battery probably
> > shakes some ions or
> > something from the surface of the electrodes, allowing for a bit more
> > electrical
> > storage capacity...
>
> My friend's father works for the prison system in the UK. He said that
> batteries are a commodity for prisoners, so they try to make them last
> as long as possible. When they start to get low, they take the
> batteries out and whack them (end wise) against something, thus
> bringing them back to life. Amazing. He said they can last up to 3
> times longer that way.
>
> I wouldn't have believed it, but he did a demo in front of me with his
> TV remote batteries. Purdy cool if you ask me 8^)

Whilst I cannot explain this for Manganese or Alkaline batteries, I
can explain why this would work (up to a point) on NiCds and (perhaps)
NiMH or LiI cells.

Let's start with the physical structure of a NiCd (or any other) cell:

+---shell-----------+
| electrolite | <--insulator
| ====core===========|
| |
+-------------------+

What you have is a metal shell, an electrolyte, a carbon or other
material core, and an insulated end-piece.

With a NiCd when the cell is charged and discharged repeatedly, you get
crystals forming in the electrolyte. These crystals are in the form of
thin whiskers of conductive material. As they grow, the resistance of
the cell gradually drops, as does its capacity to be recharged. The
recharge capacity drops because more and more energy leaks across the
electrolyte. This happens (slowly) even in a new cell. The whisker
crystals just make it happen faster. Eventually, though, one of those
crystals completes a direct circuit between the shell and the core.
At this point the cell is completely dead.

Now it is possible to recover a cell from this state - for a while.
What you have to do is to break the crystals that have formed. These
will re-form faster than before, because you still will have pieces
of crystal present, and the regrowth will have less distance to cover.

So how to break them? The traditional way is to warm the cell by putting
it in the charger for the normal time, and then freezing the cell, and
re-charging again. Yes, I said freezing it. Bung it into the freezer
in the kitchen for 24 hours. The abrupt thermal stress breaks the
crystals. A more modern approach is to over-volt the battery for 1/2
hour or so, and burn out the crystals like little fuses.

Based on this, I suppose you _could_ recover a NiCd by whacking it!
All you are trying to do, after all, is fracture the crystals.

Now as to if this problem happens with NiMH or LiI cells is another
matter.

-Rob.

-- 
Rob Masters  |  PLUG: http://www.cafeshops.com/aliciasmith      |Model       /\
"I spy, with my little eye, something that's about to be-"      |Rocketry    ||
"Dead in a minute" Mr C & Mr V - Neverwhere,Neil Gaiman         |in Perth    ||
"Reality might not get out of beta today." O.Timas - Bot,S.Gange|           /||\
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