john@qip.UUCP (John Moore) (09/12/90)
In article <17660102@hpfcdj.HP.COM> myers@hpfcdj.HP.COM (Bob Myers) writes:
]>A while back (I believe it was Spring '90) there was a discussion
]>of how "fully" to discharge nickle-cadmium batteries before
]>recharging, so as to avoid the "memory" effect. Question:
]
]
]Once again, with feeling:
]
]There IS NO "memory effect" in nicads, at least none that has ever been
]documented by the battery manufacturers since the very early days of these
]cells. Cell degradation which has been attributed to "memory" is almost
]always due to improper charging practices, and there is absolutely NO need
]to deliberately discharge a cell for no other reason than to get ready to
]charge it up, IF your charger is properly designed to STOP WHAT IT'S DOING
]when the cell is fully charged.
]
]
Bob,
This is NOT a flame. However, in light of the conflicting claims,
I would appreciate you backing up your statements with references
or personal experimental experience or whatever. I would love
to believe you, but would like to see more details.
For example, what is the chemistry behind memory that its believers
claim? How do you rebut it? Is there any chemistry, or is it just
superstition? Did there use to be memory with oldcells, but no longer?
Thanks.
--
John Moore HAM:NJ7E/CAP:T-Bird 381 {ames!ncar!noao!asuvax,mcdphx}!anasaz!john
USnail: 7525 Clearwater Pkwy, Scottsdale,AZ 85253 anasaz!john@asuvax.eas.asu.edu
Voice: (602) 951-9326 Wishful Thinking: Long palladium, Short Petroleum
Opinion: Support ALL of the bill of rights, INCLUDING the 2nd amendment!gordonl@microsoft.UUCP (Gordon LETWIN) (09/13/90)
In article <17660102@hpfcdj.HP.COM>, myers@hpfcdj.HP.COM (Bob Myers) writes: > > Once again, with feeling: > > There IS NO "memory effect" in nicads, at least none that has ever been > documented by the battery manufacturers since the very early days of these > cells. You should take what Mr. Myers says with a grain of salt. Motorolla, a very major presense in portable radios and therefore a world expert on Nicad batterys, specifically says that there *is* memory effect on nicads. If you buy one of their MicroTacs they tell you to full discharge it's nicad battery periodically. They say not to do that with the lead acid battery also provided. Obviously, this isn't proof that Mr. Myers is wrong, but I'll place my bets on Motorolla - I expect they know quite a bit more about the subject then Mr. Myers. gordon letwin
jws@thumper.mlb.semi.harris.com (James W. Swonger) (09/13/90)
It seems that everybody -ExcepT- the battery manufacturers thinks there is a 'memory' effect. Hmmm... I have read the literature from Gates on their NiCd line. It denies that there is a memory effect but then goes on and on about the only proper way to use and charge them, blah blah or cell capacity will be reduced. Whether or not the 'memory effect' is due to shallow discharging should be simple enough to experiment with. Regardless of the cause, an awful lot of NiCds go bad early. I probably have 20 dead AA cells laying around myself. The stuff the manufacturers put out by way of denials seems like a bunch of doubletalk P.R. to me. Anybody who designs rechargeable products with NiCds seems to have to put in a lot of extra effort to get the rated life out of the batteries. No matter what you call it, the suckers crap out too easily.
elliott@optilink.UUCP (Paul Elliott x225) (09/13/90)
In article <57370@microsoft.UUCP>, gordonl@microsoft.UUCP (Gordon LETWIN) writes: > In article <17660102@hpfcdj.HP.COM>, myers@hpfcdj.HP.COM (Bob Myers) writes: > > Once again, with feeling: > > There IS NO "memory effect" in nicads, at least none that has ever been > > documented by the battery manufacturers since the very early days of these > > cells. > You should take what Mr. Myers says with a grain of salt. Motorolla, > a very major presense in portable radios and therefore a world expert > on Nicad batterys, specifically says that there *is* memory effect on > nicads. If you buy one of their MicroTacs they tell you to full discharge > it's nicad battery periodically. They say not to do that with the lead acid > battery also provided. > Obviously, this isn't proof that Mr. Myers is wrong, but I'll place my bets > on Motorolla - I expect they know quite a bit more about the subject then > Mr. Myers. *IS NOT*, and your mother wears army boots! Actually, I can't quote a definitive source, but from my circa-1980 conversations with several of General Electric's NiCd battery gurus, I recall that what is usually called the "memory effect" is more properly known as "Voltage depression". There *is* a "memory effect", but it is rarely, if ever seen. Here is the story on each of these: Memory Effect: This was seen on an early satellite (the orbiting type), and was caused by a charging system that would cycle the batteries so that they were discharged to *exactly* the same point evry time, then recharged back to *exactly* the same state of charge. After many repeated charge/ discharge cycles, it was found that the batteries would not supply power when discharge was attempted past the discharge setpoint. The solution was to vary the recharge threshold in some manner. The conditions that cause the memory effect are just *not* present in typical use, even highly cyclical use. Probably the best way to avoid the memory effect is to take the batteries into *overcharge* when recharging. This is the typical recharging method, by the way. Voltage depression: This is often referred to as the "memory effect", but due to prior claim the term "memory effect" is properly reserved for the condition described in the above paragraphs. Voltage depression is a state where the NiCd cell voltage under load drops below the nominal while the cell still retains a significant charge. In equipment where the required operating voltage is set for nominal cell voltage, this will result in drastically reduced operating time between charges. The typical cause of voltage depression is long-term trickle charging at low charge rates (less than C/20). My understanding is that under these conditions, a surface barrier is formed on the cells that increases the output resistance (I could be all wet here...) Below, I will attempt to construct a graph showing typical discharge curves for these conditions. Cell Voltage 1.3Vx+ @x+ 1.2V @x+@x+@x++@x+++x+++x++x++++++++++++++++++++++++ @ x + 1.1V @@@@@@@@@@@@@@@@@x@@@@@@@@@@@@@@@@ + x @ + 1.0V x @ + x @ + 0.9V x @ + x @ + 0.8V x @ + x @ + 0.7V x @+ x @+ 0.6V x @+ | | | | | | | | | | | | 0 1 2 3 4 5 6 7 8 9 10 11 Hours Legend: '+': Nominal NiCd at C/10 discharge rate '@': Voltage depression case "x": Memory effect case There! That wasn't so hard, after all! Please consider the above graph an artist's conception; don't rely on the absolute numbers. If anyone out there can provise a definitive source, or refutation of this, please post it. -- Paul M. Elliott Optilink Corporation (707) 795-9444 {uunet, pyramid, tekbspa}!optilink!elliott "an archetypal entity..., superimposed on our culture by a cosmic template."
myers@hpfcdj.HP.COM (Bob Myers) (09/13/90)
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.myers@hpfcdj.HP.COM (Bob Myers) (09/18/90)
>> Once again, with feeling: >> >> There IS NO "memory effect" in nicads, at least none that has ever been >> documented by the battery manufacturers since the very early days of these >> cells. >You should take what Mr. Myers says with a grain of salt. Motorolla, >a very major presense in portable radios and therefore a world expert >on Nicad batterys, specifically says that there *is* memory effect on >nicads. If you buy one of their MicroTacs they tell you to full discharge >it's nicad battery periodically. They say not to do that with the lead acid >battery also provided. > >Obviously, this isn't proof that Mr. Myers is wrong, but I'll place my bets >on Motorolla - I expect they know quite a bit more about the subject then >Mr. Myers. Actually, you should likely take what *Motorola* (and anybody else's user's manuals, so as not to look like I'm picking on Mot) with a grain a salt; it wouldn't be the first time that the information distributed in user's guides, etc., was less than completely technically accurate. Or are you saying that Motorola had one of their battery engineers write the manual? I've already posted my complete explanation behind my comments - with references, so I'm assuming 'nuff said. Right? 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.
myers@hpfcdj.HP.COM (Bob Myers) (09/18/90)
>The conditions that cause the memory effect are just *not* present in >typical use, even highly cyclical use. Probably the best way to avoid >the memory effect is to take the batteries into *overcharge* when recharging. >This is the typical recharging method, by the way. That's one way, but unfortunately, repeated high levels of overcharging (over-overcharging? :-)) can also reduce cell capacity and shorten service life (esp. if operating/charging at elevated temperatures). The *best* way is to employ a charging system which, through monitoring cell voltage, temperature, or both, knows when to quit. Unfortunately, this is an added expense that few manufacturers of NiCd-using consumer equipment are willing to put up with. There's another design problem involved, one which I was reminded of by some e-mail on this subject. NiCds have a lower nominal cell voltage than carbon-zincs or "alkaline" cells to begin with, but a fairly flat discharge curve. Some designers, though, don't correctly account for the somewhat lower voltage, and the equipment winds up with too high a "cutoff voltage" (that voltage at which the product stops working properly, or signals "low battery") - meaning that the user will think the battery is low even when plenty of capacity remains. This has the added "benefit" of making it more likely that the battery will be put back in the charger, encouraging further overcharging and further shortening the battery's life. A vicious circle. :-( (And of course, after just a few times around this loop, the user winds up with a dead pack, curses yet another "example" of "nicad memory", and starts the process of killing yet another battery.) NiCds *are* an extremely reliable power source, but you can't just slap 'em into a design in place of carbon-zincs, add a cheap charger, and expect 'em to be happy. 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.