> From: Moose <olymoose@xxxxxxxxx>
>
> Some battery experts would refer to the "memory" effect in Ni-Cads and NiMhs
> as an over-charging effect, as it is not
> primarily a result of short, shallow discharge cycles, but of prolonged
> periods on a charger, particularly on the simple
> chargers often used on Ni-Cads.
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.
Neither NiMH nor lead acid nor lithium secondary cells have this same
mechanism, as far as I know.
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.
I've successfully resurrected dead lead-acid gel-cells by inserting a
hypodermic needle through the seal and repeatedly injecting distilled water.
This typically takes numerous injections over a few days, while the cell is
taking a constant-current charge a couple percent of capacity. (i.e. ~40 mA on
a 2AH cell.) You can then monitor your success by the terminal voltage, which
as the cell becomes hydrated, will drop from many volts to 2.45 volts, at which
point, you should drop the current to 2.3 volts per cell to avoid
re-dehydration.
----------------
Sell your cleverness, and purchase bewilderment. -- Rumi
:::: Jan Steinman, EcoReality Co-op ::::
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