From: elmore@rap.ucar.edu (Kim Elmore)
Subject: NiCd Batteries, Part 1/3
Organization: NCAR, Research Application Program
Date: Fri, 18 Dec 1992 17:01:57 GMT

	Every so often, The Subject that Won't Die returns to life.
One of these is the "memory" effect that NiCd cells are supposed to be
subject to.  It turns out that there isn't such a thing except in very
rare cases.  Below is a short compendium from two knowledgeable
sources on just what goes on with NiCd cells.  Both outhors are
Amateur Radio operators; hams have tons of experience with NiCd
batterry packs, since almost everything we use in the field is powered
by them and they inevitably fail at the most inopportune moment if not
carefully cared for.  At the request of another netter, I'm posting
this to both rec.aviation.products and rec.video.  I hope it helps, or
at least provides you with references that you can chase down yourself.

		Merry Christmas!
					Kim Elmore
					N5OP, PP ASEL/Glider 2232456
					Technical Coordinator, ARRL, CO Sect.
*  _._. __._ _.. _.._ _.. . _. ..... ___ .__. _. ..... ___ .__. _.. _.._ _._  *
*    Said by NQ0I while working on his shack:                                 *
*		"All these *wires*!  Why do they call it `wireless'!?"        *
*  _._. __._ _.. _.._ _.. . _. ..... ___ .__. _. ..... ___ .__. _.. _.._ _._  *
Replies to: elmore@brightband.rap.ucar.edu (wires)
	    44.20.0.67 n5op@n5op.ampr.org (radio)
/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/


		NiCd Cells: A Short Data Compendium

*Chapter 1*

  A couple of years ago I had problems with NiCds and called GE (now
Gates Energy Products) and inquired about the "Memory Effects".  When
I mentioned "Memory", the applications engineer asked if I had their
manual and wanted to know which edition I was looking at.  I told him
I had their second edition (1975).  He said he would send me their
third edition (1986) and that it would explain the new thinking on
this subject, now referred to as "Voltage Depression"

>From Nickel-Cadmium Battery Application Handbook, Second Edition
1975...
______________________________
7.1.3.1 Memory effects

   Temporary effects on discharge voltage levels at any point during
the discharge period, or an apparent reduction in capacity to a
predetermined discharge voltage cutoff point, are developed in the
battery system from repetitive use patterns.  If the battery is
discharged to random depths of discharge, and overcharged for random
amounts of overcharge time, and subjected to various duty cycles,
these temporary effects will not manifest themselves.  The following
paragraphs will discuss the factors and causes of these temporary
memory effects.

   Sealed cells subjected to a repetitive depth of discharge under
certain cycling conditions may exhibit an apparent temporary loss of
capacity.  This phenomemon, sometimes referred to as "memory", was
first noticed in a satellite where the cell received a very precise
charge/discharge regime using only a small portion of the available
capacity over and over again.  If the cell experiences such a series
of repeated partial charge and discharge cycles of exact magnitudes,
the cell may become so conditioned that it will deliver to the normal
end-of-discharge voltage only slightly more capacity than has been
required of it during these preceding repetitive cycles.  Thus, if the
discharge cycles are short, the cell capacity may be temporarily
shortened to coincide with the discharge capacity previously
experienced during the repetitive discharge cycle.  The complete
discharge voltage profile of a "Memorized" cell may appear as
portrayed in Figure 7-3.  This is more likely to occur when the
overcharge coefficient is small, the rate of discharge is high, and/or
the temperature is elevated.  This effect is also more significant
when the cutoff voltage is above 1.0 volt at the C rate.  If the cell
is subjected to a deep discharge and then recharged, this "memory" is
erased and nearly all of the original cell discharge energy is
regained.

   It must be emphasized that the "memory" effect does not manifest
itself when the battery is discharged to random depths of discharge or
overcharged for random amounts of overcharge time as is typically the
case in most applications...
___________________________________

Compare this explaination with their third edition (1986):
___________________________________

4.5.3 Voltage Depression

   The effects of elevated charge temperature on the immediate cycle
capacity of the cell have been discussed in Section 3.2.1 and 4.3.3.1.
Cells exposed to overcharge for very extented periods of time,
particularly at elevated cell temperatures, may develop an additional
shortcoming called VOLTAGE DEPRESSION.  This phenomenon is one in
which the cell voltage is depressed approximately 150 mV below the
normally expected values which were calculated on Figure 4.19.  This
depression affects Eo and is independent of discharge rate.

   This depression effect initially appears on the discharge voltage
curve near the end of discharge.  With extension of the overcharge
time (non-discharge) of the cell, this depression progresses slowly
toward the mid-point and beyond.  Accompanying this effect of
depression in the voltage dimension of the curve is an actual slight
increase in the capacity dimension as illustrated in Figure 4.21.
This depressed voltage effect is an electrically reversible condition
and disappears when the cell is completely discharged and charged
(sometimes called conditioning).  It thus appears only on the
discharge following a very extended overcharge.  It will reappear if
the extended overcharge is repeated.

   The phenomenon which causes this depressed voltage is continuous
overcharging of the active material of the electrode.  The effect is
erased by discharging and recharging that portion of the active
material which has experienced the extensive overcharge.  For this
reason the depressed voltage effect in the discharged portion of the
curve is erased by the very act of observing it, when the discharge is
carried beyond the first knee of the depressed curve.  Complete
discharge, and subsequent full charge, essentially restores the curve
to its normal form.

   The reversibility of this effect is probably the very
characteristic that gives rise to the misnomer MEMORY.  When cells are
subjected to continuous charge/overcharge, with only modest discharges
(repetitive or otherwise), the revesibility of the effect actually
prevents the voltage depression from occuring in that portion of the
electrode active material which is cycled.  The voltage depression
phenomenon is, however, not erased from that portion of the electrode
material which has been subjected to continuous overcharge but NOT
discharged.  In this situation, whenever the cell is discharged deeper
than recent previous discharges and reaches the beginning of the
previously uncycled material, the voltage may decrease 150 mV per
cell.  This misleads the observer into believing that the discharge is
at the knee of the normal discharge curve and erroneously concluding
that the cell remembers and, thus, delivers only the amount of
capacity previously repetitively used.  Instead, the phenomenon is
actually related only to extended overcharging and incomplete
discharging, not repetitive shallow cycling.  This is because that
portion of the electrode material which has experienced overcharge and
not been discharged for an extended period of time slowly shifts to a
more inaccessible form.

   The depressed voltage effect can of course cause loss of useful
capacity in those application cases where a high cutoff voltage
prevents complete discharge of the minimum capacity cell in the
battery.  If voltage depression has occurred, complete discharge
requires continuation down through the depressed knee to that voltage
level which keeps all the electrode material
active...

_______________________________________

   Based on the above, make your own conclusion.  I have come across many 
written material based on the second edition but not many based on the third.  
The Panasonic material cited by KM4BA seem to be another example.

   Who do you believe?  Me?  I go with GE.  By the way, when these guys 
talk about overcharge, they talk about charging at C/10 for weeks or months.  
Using a standard 16 hour charger for a day or two do not excite them.  In this 
case, the only problem they see would be the elevated temperature that the 
cells would be subjected to.  Now, temperature... that's another problem.  
I talked to four applications engineers at Gates and each one stressed that 
temperature is the killer.  They say that each 10 degree C over the life of 
the cell reduces cell life by one half.  TEMPERATURE, THAT'S THE BIG THING!  

73, Norman  KH6R
(808) 668-3087