On 12/15/2010 11:00 AM, Jan Steinman wrote:
> Although it's open on my desktop, I've not looked at the
> BatteryUniversity.com site yet, but my understanding of classic NiCd is that
> repeated incomplete discharging to about the same level causes a crystalline
> precipitate to form at a certain location related to the discharge depth,
> therefore causing the mis-named "memory effect." (It should really be called
> the "forget effect." :-)
>
> Memory effect is not a problem when the cell is repeatedly discharged to
> different levels. Classic case: a cop runs a walkie-talkie for eight hours a
> day and drops it in the charger overnight -- bad news for NiCd cell life. But
> memory effect is not likely if you're using a flash or motor drive for
> various periods of time and then topping off on a charger.
"Among rechargeable batteries, nickel-cadmium remains a popular choice for
two-way radios, "
Since what you say here doesn't agree with what's on that site, perhaps you
might read it and resolve the differences.
"Nickel-cadmium does not like to be pampered by sitting in chargers for days
and being used only occasionally for brief
periods. A periodic full discharge is so important that, if omitted, large
crystals will form on the cell plates (also
referred to as memory) and the nickel-cadmium will gradually lose its
performance. "
> Neither NiMH nor lead acid nor lithium secondary cells have this same
> mechanism, as far as I know.
Again, the linked site speaks to crystallization effects in NiMH, not as bad as
NiCAD, but still an issue. It's really a
useful seeming reference. I don't know why you chose to speak to its subjects
without reading it.
> As far as I've read, prolonged charging causes an entirely different failure
> mode: dehydration. The charging current is dissipated in the cell as heat,
> which can cause a certain amount of venting. This can happen on *any* sort of
> cell that is over-charged.
"Rather than venting, the internal gases generated during charge were
recombined. These advances led to the modern
sealed nickel-cadmium battery, which is in use today."
Obviously, dehydration issues will depend on battery chemistry, physical design
and charger characteristics. Batteries
in a couple of gadgets I have don't last long with the very simple internal
charging circuits, one being an automatic
emergency light - stupid design. On the other hand, battery packs for my older
Makita drills are quite happy sitting on
the charger unused for months. The difference is easy to see; the charge light
is on sometimes and off others. Obviously
a smart charger that takes care of the batteries.
D.C. Moose
Moose
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