The myth of the memory effect- was: NMIH batteries - was: Re: HP 25 and HP55

From: Tom Peters <>
Date: Thu Oct 21 23:39:27 2004

1. NiMH batteries never were subject to any sort of memory effect; the
chemistry doesn't do that. But they are harder to charge properly than a
NiCd. Now that dual chemistry or even triple chem chargers are commonplace,
people forget that-- all the newer chargers have a recent chip in them that
figures out from the charge curve what the chemistry is and watches for the
little negative delta that signals "full up." Not charging to that point
wastes capacity and makes you think the cell can't hold a full charge.
Going just slightly beyond shortens cell life a little. Going more than
slightly beyond shortens cell life a lot.

2. NiCD batteries never had a memory effect either! Read the literature
from the vendors, not the opinions of someone regarded as an expert because
he says he's one. Yes, NiCd's wear out, yes, their discharge curve gets
steeper as they age, but memory effect? Want to know where that whole thing
came from? NASA. Satellites. Batteries charged by a mil-spec charging
circuit that cost millions to develop and build, that routinely allowed a
set of satellite batteries to discharge to EXACTLY the same point EVERY
time, and then charged them back up to EXACTLY the same point again. And
guess what? Those batteries DID develop a memory effect.

So how many of you have a multi-million dollar charging circuit? On the
other hand how many of you have spent a $30-40-50 or $90 on a fancy
charge-discharge battery conditioner? I have one, only because I couldn't
seem to buy a multichemistry versatile charger without that feature.

But I have never used it. I have never purposely discharged a NiCd battery
except in normal use.

What happens to a cell in a multi-cell battery when you put a heavy enough
load on the stack to discharge it in an hour or two, or 15 minutes like
some of them do? Well, since all cells in all batteries have perfectly
uniform internal resistance, all the cells discharge to the same voltage,
and sit there pretty, waiting for you to charge them back up so they can
lead fulfilled lives again.


I think what can happen is that some cells discharge a lot sooner than
others, since in the real world the internal R is lower for some cells than
others. Since you are now pulling current *backwards* through at least one
cell in the stack, odd are good that some day one of the cells is going to
reverse on you, and become a highly inefficient inverted battery, one
that's unlikely to ever take an appreciable charge again. This is a
real-world problem for discharge devices that don't have both a timer and a
voltage monitor, and some ever sense temperature too.

So why does every peddler of LiOn and NiMH batteries tout their resistance
to the dreaded "memory effect"? Because those technologies are newer, and
you need something to differentiate the new from the old. Makes a great
selling point, and the myth of the memory effect having something to do
with real life and the way normal people use batteries is something that
has been repeated so often, it's become indistinguishable from reality.

References: (Which I urge you to read!) which quotes the
GE tech notes and from which I quote:

"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
"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
Reduction of your NiCads / NiMH capacity due to overcharging (heating) and,
cell reversal in voltage depressed battery packs kill your batteries. Very good, well written, good
hints for using your batteries effectively.
Here's a battery maker saying No Memory Effect in currently produced product. which says, in

If you use NiCd batteries you should be aware that most of the problems
experienced by NiCd battery users are not due to a "memory effect" but are
due to overcharging or improper storage. Overcharging is usually caused by
poorly designed first generation battery chargers. These chargers continue
to deliver current to the batteries even after the batteries are full charged.



At 09:30 PM 10/21/2004 -0400, you wrote:
> >>> -- I took apart the 25's battery pack and replaced the nicads with
> >>> nickel metal hydride cells (the same ones I use in my camera)
> >>
> >> Not a good idea. Nickle metal hydrides need different types of
> >> charging. The won't last long with a NiCad charging circuit.
> >
> >I won't charge them in the calculator, they are regular AA cells so I
> >can remove them and put them in a proper charger. :^)
>Here's a question for battery guru's ... I have a gadget I built for
>preventing "memory effect" in various nicad powered devices - mainly
>ones like my Digicams which only discharge the battery 1/2 way (or
>less) before complaining that the battery is low.
>It is basically two springy metal bars which lets me put up to 10 AA or
>C cells in parallel to discharge (I've got portable _at_M and 70CM rigs powered
>by a C cell pack), an Ammeter, a low resistance, and a stud rectifier diode.
>Basically, it draws about an amp from the cell when they are near fully
>changed, and trickles down to nothing as they approach .7ish volts, and
>never lets them go to zero.
>I works REALLY well with nicads - I've got cells that I've been using in
>my radios for many years and they still work very well (Prior to using
>this, they would noticably lose capacity after a while)...
>But: It doesn't seem to be quite as effective with NMIH cells ... they
>still seem to be losing capacity ...
>Any explaination, comments etc.?
>dave04a (at) Dave Dunfield
>dunfield (dot) Firmware development services & tools:
>com Vintage computing equipment collector.

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Received on Thu Oct 21 2004 - 23:39:27 BST

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