Discussion of the MYTH of "charge memory" in NiCd batteries
(from a discussion on the BikeCurrent mailing list)
From GE Tech Notes ....
"Among the many users of batteries in both the industrial and consumer sectors, the idea of a memory phenomenon in nickel-cadmium batteries has been widely misused and understood. The term 'memory' has become a catch-all 'buzzword' that is used to describe a raft of application problems, being most often confused with simple voltage depression.
To the well informed, however, 'memory' is a term applied to a specific phenomenon encountered very infrequently in field applications. Specifically, the term 'memory' came from an aerospace nickel-cadmium application in which the cells were repeatedly discharged to 25% of available capacity (plus or minus 1%) by exacting computer control, then recharged to 100% capacity WITHOUT OVERCHARGE [emphasis in the original]. This long term, repetitive cycle regime, with no provisions for overcharge, resulted in a loss of capacity beyond the 25% discharge point. Hence the birth of a "memory" phenomenon, whereby nickel-cadmium batteries purportedly lose capacity if repeatedly discharged to a specific level of capacity.
The 'memory' phenomenon observed in this original aerospace application was eliminated by simply reprogramming the computer to allow for overcharging. [Note that no mention is made of adding an intentional *discharge* to clear the problem - RLM] In fact, 'memory' is always a completely reversible condition; even in those rare cases where 'memory' cannot be avoided, it can easily be erased. Unfortunately, the idea of memory-related loss of capacity has been with us since. Realistically, however, ' memory' cannot exist if any one of the following conditions holds:
1.Batteries achieve full overcharge. 2.Discharge is not exactly the same each cycle - plus or minus 2-3% 3.Discharge is to less than 1.0 volt per cell.
Remember, the existence of any ONE of these conditions eliminates the possibility of 'memory'. GE has not verified true 'memory' in any field application 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, correctable application problems."
End of quote ...
Basically memory (loss of capacity) due to discharge is a myth.
Reduction of your NiCads / NiMH capacity due to overcharging (heating) and, cell reversal in voltage depressed battery packs kill your batteries.
Have a nice day!
Steve
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Peter Ludwig adds:
Date: Tue, 14 Jan 1997 12:09:44 +0002
Steve wrote about the memory effect:
> Specifically, the term 'memory' came from an aerospace nickel-cadmium
To be correct this appeared in a satellite which cycles around the earth,
so charging & discharging is very precisely the same in every cycle.
Temperature is very constant through each cycle, and there are
absolutely no mechanical shocks or vibrations and and and so on.
> This long term, repetitive cycle regime, with
Also to be correct: The memory effect causes a lower voltage from
that point than otherwise expected. This resulted in an early cut off
by the software. The cells where by no means empty at this stage, but
as we know, when a NiCd cell's voltage starts to drop, there is
almost no remaining capacity. So the real 'loss' was caused by
cutoff.
Anyway, there is AFAIK no other confirmed case where the memory
effect had any influence in an field application.
greetings from Austria.
Peter
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Marty Goodman in Oct of 1997 adds:
I believe what Peter (who is rather fluent in English, tho it's not
his native language) was trying to say in the last big paragraph is that
NORMALLY, in the absense of a development of a "voltage knee", when a
NiCd cell's voltage starts to drop significantly, it's going to die VERY
quickly. And this is why control software in those satellites was
written to shut them down when the voltage dropped below such and such.
HOWEVER, if a voltage knee has developed (as was the case) you have a
situation where the voltage goes rather low relatively quickly, BUT it
will STAY at that low level with power drain for a long time... the
total power output (amps time volts = watts) you can get from a cell
with a voltage knee is NOT decreased, just the voltage at which it will
put out that power. Thus, the control software was, sort of,
NEEDLESSLY and IMPROPERLY shutting down the satellite (because it didn't
"know" about voltage knees, and thought that these were normally
discharging batteries that were about to be drained so deeply that they
might hurt themselves permanantly due to cell reversal).
---marty
From: Peter Ludwig
Subject: Re: Charge Memory post (again! arrgh!)
> application in which the cells were repeatedly discharged to 25% of
> available capacity (plus or minus 1%) by exacting computer control, then
> recharged to 100% capacity WITHOUT OVERCHARGE
> [emphasis in the original].
> no provisions for overcharge, resulted in a loss of capacity beyond the 25%
> discharge point.