From: jimd@pequod.gvg.tek.com (Jim Delwiche)
Newsgroups: rec.bicycles.misc
Subject: More handwaving about the "myth of NiCad memory"
Date: 6 Nov 92 18:16:05 GMT
Organization: Grass Valley Group, Grass Valley, CA

In article <1992Oct30.075653.28973@intacc.uucp> robcruic@intacc.uucp (Robert Cruickshank) writes:
>
>Nickel-cadmium batteries "like" to be be fully discharged before recharging
>them, otherwise they will develop a "memory" and no longer hold a full charge.
>(you can charge it when it is partially discharged, just don't make a habit of
>it). 

<stuff deleted>

This is sort of true, and sort of not true.  As I understand it, the
behavior of NiCads is more like the following:

This shows a family of discharge curves for a given battery:

			     (etc.)
       	       	       	    /  	 After another shallow discharge
                           /    /    After a shallow discharge
			  /    /    /  New battery curve
	       	       	 /    /	   /  /
 V1 |-------------------/----/----/---
    |   \    \    \    \    \    \    \
 V2 |    ----------------------------- \
    |   			        \
    |   			         \
    |   			          \
    |   			           \
    |   			            \
 0  |----------------------------------------------
            <-Time->                         ^
  				          Fully discharged

                
Example of a single discharge cycle for an elderly NiCad:		
	      
 V1 |---
    |   \
 V2 |    -------------------------------
    |   			        \
    |   			         \
    |   			          \
    |   			           \
    |   			            \
 0  |----------------------------------------------
                                             ^
  				          Fully discharged
  				   
After a shallow discharge, the battery does loose the ability to
provide full V1 voltage for the entire discharge cycle.  As shown in
the little diagrams, at some point in the discharge cycle, the output
voltage will drop to the V2 level.  With repeated shallow
charge/discharge cycles, the point where this happens will recede back
along the discharge curve, until,with an old battery, the voltage will
drop to V2 early in the discharge curve.  The battery will continue to
deliver V2 for the full discharge cycle.  

If the battery is sized for the application such that V2 is
"acceptable", then the "memory" effect is not really noticed.  An
example of this is the handheld transceivers the local law enforcement
gentlemen use.  Since they are unhappy when the radio quits during the
shift, they sit in the charger whenever they're not in use.
Therefore, they almost always are in the shallow discharge mode.  If,
however, the system was designed so that V1 is required to make things
work, the user will notice a definite "memory".  An example of this
might be a poorly designed laptop which needs V1 to run the display.
As soon as V1 is no longer available, the system is unuseable.

The big enemy of NiCads is overcharging, which forces some kind of
outgassing to occur, which does reduce the capacity of the battery.
This process is identified by a warm battery during charging.  A smart
charger which does not overcharge does a lot to increase the lifetime
of a NiCad.  Some fast chargers just use a timer, so if you have a
partially discharged NiCad and drop it in the charger, it will be badly
overcharged.  Some, I think, actually monitor the temperature of the
battery during charging.  Other methods for detecting overcharging
exist.  Ham radio publications occasionally publish various clever
NiCad charging circuits.

Any chemists or experts who actually know what is going on feel free
to jump in at any time.  I'm just trying to remember what I've read
about NiCads in the past.

Jim

       	       	
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