Battery Memory Myth (was Re: [PLUG] Looking for a new-to-me laptop)

Phil Tomson ptkwt at aracnet.com
Fri Apr 25 17:40:02 UTC 2003


On 25 Apr 2003, Randal L. Schwartz wrote:

> >>>>> "Phil" == Phil Tomson <ptkwt at aracnet.com> writes:
>
> Phil> I don't think it's an issue of overcharging - the charging
> Phil> circuitry should prevent that anyway.  I believe that the issue
> Phil> Nickel metal hydride batteries is what's referred to as 'memory'
> Phil> which means that if you don't fully discharge them they begin to
> Phil> lose their ability to hold a charge.
>
> Ow!  Not the "memory myth" again. I wonder how long this bad meme
> will persist...
>
> <http://www.geocities.com/chitianzi/memoryeffect.txt>
>
> Repeat after me: "There is no such thing as NiCd battery memory fatigue."
> And also "even if there ever was, it was only about NiCds!"
>
> {sigh}
>

Actually this site: http://www.batteryuniversity.com/parttwo-33.htm
seems to indicate that there is something similar to the mythical 'memory'
problem with both NiCd and NiMH.

Here are some excerpts:

"The word 'memory' was originally derived from 'cyclic memory'; meaning
that a Nickel-cadmium battery could remember how much energy was drawn on
preceding discharges. On a longer than scheduled discharge, the voltage
would rapidly drop and the battery would lose power. Improvements in
battery technology have virtually eliminated this phenomenon.

The problem with Nickel-cadmium is not so much the cyclic memory but the
effects of crystalline formation. The active cadmium material is present
in finely divided crystals. In a good cell, these crystals remain small,
obtaining maximum surface area. With memory, the crystals grow and conceal
the active material from the electrolyte. In advanced stages, the sharp
edges of the crystals penetrate the separator, causing high self-discharge
or electrical short.

When introduced in the early 1990s, Nickel-metal-hydride was promoted as
being memory-free. Today, we know that this chemistry is also affected but
to a lesser degree than Nickel-cadmium. The nickel plate, a metal that is
shared by both chemistries, is partly to blame. While Nickel-metal-hydride
has only the nickel plate to worry about, Nickel-cadmium also includes the
memory-prone cadmium plate. This is a non-scientific explanation why
Nickel-cadmium is affected more than Nickel-metal-hydride.

...
Crystalline formation is most pronounced if a nickel-based battery is left
in the charger for days, or if repeatedly recharged without a periodic
full discharge. Since most applications do not use all energy before
recharge, a periodic discharge to 1 volt per cell (known as exercise) is
essential to prevent memory.

 ...

Simple Guidelines
* Do not leave a nickel-based battery in a charger for more than a few
days, even on trickle charge.
 * Exercise Nickel-cadmium every 1 to 2 months and Nickel-metal-hydride
every 3 months. Running the battery down in the equipment may do this
also.
 * Do not discharge the battery before each recharge. This puts undue
stress on the battery.
 * Avoid getting the battery too hot during charge. The temperature should
only rise for a short moment at full charge, then cool off. "


So the recommendation seems to be somewhat similar to the
old recommendations for avoiding the 'memory' problem with NiCd cells (but
apparently for different reasons than the old 'memory' theory.)

Phil






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