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From: myers@hpfcdj.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: NiCd recharging
Message-ID: <17660109@hpfcdj.HP.COM>
Date: 13 Sep 90 16:39:44 GMT
References: <1990Sep7.132221.6202@ux1.cso.uiuc.edu>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 70


My posting regarding the lack of a "memory" in nicads has generated
a bit of e-mail, some of which I've answered directly.  But it looks
like there's enough interest to post a more complete response:

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", which is a mode seen very
often in consumer applications.  Improper charging characteristics will do
more than anything else to degrade overall cell performance. (Motorola's manual
writers may have written "memory effect" as the given reason for differences
between NiCad and lead-acid batteries, but it wouldn't be the first time that
manuals provided information which was less than completely correct from a
technical standpoint.)  [This was originally written in response to e-mail
I received, in which a person stated that his user's manual for a piece of
Motorola equipment specifically warned against "memory" in the NiCd battery.]

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 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   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
myers@fc.hp.com                          |  sentient life-form on this planet.