From: elmore@rap.ucar.edu (Kim Elmore)
Subject: NiCd Batteries, Part 2/3
Organization: NCAR, Research Application Program
Date: Fri, 18 Dec 1992 17:03:10 GMT

*Chapter 2*		

Well, since this seems to be the Subject that Won't Die, here is a
more extensive posting (actually, a re-posting from some time back in
sci.electronics) concerning the topic of NiCd "memory":

My assertion that there is no "memory effect" (at least as thought of
by the general public) is based on several years as the component
engineer at this site responsible for batteries, and the technical
literature from several leading manufacturers of these cells.  In
particular, there was a very interesting study performed several years
back by General Electric, regarding the hypothesis that NiCad "memory"
had affected the performance in a satellite assembly for which they
were responsible.

GE found that while there may be some very infrequent cases of a true
"memory", the vast majority of problems seen with this type of cell
were due to improper selection of the cell for a given application
(improper choice of cell for expected discharge characteristics), and,
more importantly, improper charging of the cells.  NiCad cells, in
contrast to lead-acid types, are *not* suitable for constant "trickle
charging" (unless some provision is made for sensing cell
voltage/temp, or *very* low current is used), which is a mode seen
very often in consumer applications.  Improper charging
characteristics will do more than anything else to degrade overall
cell performance.

To quote from one technical reference which discusses the subject
(Varta "Sealed Ni-Cd-Batteries Product Range and Technical Handbook",
1987, section 2, "Characteristics of Sealed Ni-Cd Batteries"):

 "...Electrical stress and charging methods have a vital impact on
  service life...The best way to ensure long service life and
  trouble-free operation is to follow the charging instructions
  carefully.  Sealed Ni-Cd batteries may be stored for years regardless
  of the charge state they are in."

And, from the aforementioned GE note (General Electric Technical Marketing
Flash TMF 8517):

 "To the well-informed, however, `memory' is a term applied to a
  specific phenomenon encountered very infrequently in field
  applications...GE has not verified true memory in any field
  applications, with the single exception of the satellite application
  noted above.  Lack of empirical evidence notwithstanding, memory is
  still blamed regularly for poor battery performance that is caused by
  a number of simple, correctible application problems."

Gould Inc., Portable Battery Division, had this to say:

 "A nickel-cadmium cell which has been charged for an extended period
  of time exhibits a reduced operating voltage on subsequent discharge.
  The characteristic [has been referred to as] `voltage depression',
  `memory', or `stepped discharge voltage.'"


Thus, while there is a possibility of a true "memory" in NiCads, it is
*extremely* rare, as it manifests itself only under some very specific
and unlikely conditions, one being that the cell is repeatedly
discharged to the same level, *within no more than 2-3%* (which is
what happened in GE's one example - the cells in the satellite
assembly were being discharged under very precise computer control).
It may have been overstating the case slightly to say that there is
"no" memory effect possible, but I did so in the interests of
simplicity.  It is also far, far closer to the truth than the current
level of understanding, which has people actually wasting discharge
cycles for no good reason.

Bob Myers   KC0EW
Hewlett-Packard Co.
Ft. Collins, CO
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