Voltage depression can be minimized by proper battery care. The
more often you top off a charge the sooner voltage depression will
occur and, the more often discharge will be required to
re-condition the batteries. The more frequently batteries are
reconditioned, the shorter the life of the battery. For maximum
life, one should use the batteries in the camera until the low
battery indicator flashes. Then, they should be replaced with a
spare charged set and those removed from the camera, recharged.
Batteries should be discharged using a discharge capable charger
designed to accommodate the type of batteries being used, NiCd or
NiMh, only when the service time of freshly charged batteries
becomes noticeably shorter. This will maximize the life of the
batteries. Caution! Using a NiMh Fast charger on NiCd batteries
may cause overheating and possible a fire depending on the charger.
The following is an excerpt from
http://www.wppltd.demon.co.uk/WPP/Batteries/Memory/memory.html
describing the memory effect and voltage depression.
"Does the memory effect exist?
Yes and no. The "real" memory effect as reported in certain space
borne applications and caused by persistent under charging of a
partially discharged cell is almost certainly never seen in mobile
phone batteries or any others in terrestrial service except in
certain very unusual circumstances.
Much more commonly there is AN effect, properly called voltage
depression, which people now tend to refer to as "memory effect".
This is a bit unfortunate as the cause (and cure) is different
from the true memory effect. For details - read on:-
First of all, the term "memory effect" is quite unscientific.
People tend to attribute any failure of a NiCd to memory. Let us
define memory as the phenomenon where the discharge voltage for a
given load is lower than it should be.
This can give the appearance of a lowered capacity, while in
reality, it is more accurate to term it voltage depression. Memory
is also hard to reproduce, which makes it hard to study.
Originally, memory effect was seen in spacecraft batteries
subjected to a repeated discharge/charge cycle that was a fixed
percentage of total capacity (due to the earth's shadow). After
many cycles, when called upon to provide the full capacity, the
battery failed to do so. Since we aren't in space, the above is not
really relevant.
Voltage depression is more severe in NiCd batteries than in NiMH.
The explanation below concentrates on NiCd, the same principles
apply to both."
""To recap, we can say that true 'memory' is exceedingly rare. When
we see poor battery performance attributed to 'memory', it is
almost always certain to be a correctable application problem. Of
the...problems noted above, Voltage Depression is the one most
often mistaken for 'memory'.....Bob Myers"
This, too, supports John's position that a "memory effect" is not
an issue with NiMH batteries.
However, it was interesting to read the information about possible
life-shortening consequences from over-charging OR over-discharging
the batteries. I suppose this would argue for the wisdom of
"topping off" the batteries rather than forcing them to completely
discharge.
David
Found some info about NIMH batteries here:
http://data.energizer.com/batteryinfo/application_manuals/nickel_metal_hydride.htm
Here is the section about memory effect:
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Memory/Voltage Depression
The issue of "memory" or voltage depression has been a concern for
many designers of devices, using nickel-cadmium cells. In some
applications where nickel-cadmium cells are routinely partially
discharged, a depression in the discharge voltage profile of
approximately 150 mV per cell has been reported when the discharge
extends from the routinely discharged to rarely discharged zones.
While the severity of this problem in nickel-cadmium cells is open
to differing interpretations, the source of the effect is generally
agreed to be in the structure of the cadmium electrode. With the
elimination of cadmium in the nickel-metal hydride cell, memory is
no longer a concern.
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Also, check the LIFE section towards the end of the page.
There are apparently other concerns which will shorten the life of the
cell. Overcharging and Over-Discharging...
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Cell Reversal
Discharge of nickel-metal hydride batteries to the degree that some
or all of the cells go into reverse can shorten cell life,
especially if this overdischarge is repeated routinely.
Prolonged Storage under Load
Maintaining a load on a cell (or battery) past the point of full
discharge may eventually cause irreversible changes in the cell
chemistry and promote life-limiting phenomena such as creep leakage.
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