NiCd Battery FAQ V1.00
Contents:
* Chapter 1) Some Ramblings About NiCd Batteries, by Ken A.
Nishimura
* 1.1) About the Author
* 1.2) Legal Stuff and Disclaimer
* 1.3) Credo #1: Charge control is the key to battery management
* 1.4) How does a Ni-Cd battery work?
* 1.5) OK, so what happens when cells are charged?
* 1.6) OK, so what's so tricky?
* 1.7) OK, so it looks like batteries are well protected. Now what?
* 1.8) Credo #2: Heat is the enemy of batteries
* 1.9) What are battery manufacturers doing to prevent damage from
overcharging?
* 1.10) How about those high-temp batteries?
* 1.11) What about those super-high capacity cells?
* 1.12) Does the memory effect exist?
* 1.13) Summary
* 1.14) OK, so I need to "properly" charge cells. How do I do that?
* 1.15) I've seen terminology like "C/n" where n is a number. What
gives?
* 1.16) So, what's the right charge current?
* 1.17) You mentioned cell reversal. What is that, and why is it so
bad?
* 1.18) Credo #3: Take care of your cells, and they will take care
of you
* 1.19) OK, my cells died. I was nice to them, what happened?
* 1.20) I've seen people talk about zapping cells to revive them
* 1.21) How about self-discharge?
* 1.22) What about NiMH cells?
* 1.23) What if I want to learn even more about NiCds and other
batteries?
* Chapter 2) General Electric's tech. note regarding memory, by Bob
Myers KC0EW
* Chapter 3) More on NiCd dealings, by Paul Timmerman
* 3.1) About the Author
* 3.2) NiCd 'memory' and more
[Document Version: 1.01] [Last Updated: 5/95]
_________________________________________________________________
Chapter 1) Some Ramblings About NiCd Batteries, by Ken A. Nishimura
_________________________________________________________________
1.1) About the Author
Some Ramblings About NiCd Batteries
Author: Ken A. Nishimura (KO6AF)
I have decided to write this diatribe due to the continuing Ni-Cd
battery saga. Yes, batteries are LOW tech -- they can't compare to the
bells and whistles of our latest HTs, but... your new HT is but a
paperweight (albeit an expensive one) without power from your
batteries. This is not a response to any particular prior post, and is
unsolicited, so in short, I'm not flaming anyone.... But, I thought it
may be useful, so, without further ado, let us take a more careful
look into NiCd battery management.
Note:
1. The stuff below glosses over some of the more technical nitty
gritty details. I wanted it to be shorter than 50K Bytes!
2. I actively solicit corrections, both technical and editorial. If
you have information contrary to that presented below, please let
me know. Flames, on the other hand, are summarily disregarded, as
usual.
_________________________________________________________________
1.2) Legal Stuff and Disclaimer
Panasonic, Sanyo, and Millenium are registered trademarks of their
respective holders. This document is not intended to promote a
particular manufacturer or product. Material contain herein is
believed to be correct, but the author cannot be held responsible for
error, either editorial or material contained within. Use of the
information above is at the sole risk of the reader. Permission
granted to use any of the material above, provided this disclaimer is
preserved in its entirety.
_________________________________________________________________
1.3) Credo #1: Charge control is the key to battery management
More batteries are destroyed or damaged by bad charging techniques
than all other causes combined. Once a battery reaches full charge,
the charging current has to go somewhere -- most often, generating
heat and gases. Both are bad for batteries.
_________________________________________________________________
1.4) How does a Ni-Cd battery work?
Let us answer this with another question. How does a electrochemical
cell work?
First, a bit of nomenclature. A cell is a single electrochemical
device with a single anode and a single cathode. A battery is a
collection of cells, usually connected in series to obtain a higher
terminal voltage.
Batteries, whether they are primary (use once) or secondary
(rechargeable) are devices which convert chemical energy into
electrical energy. In the case of the latter, they can take electrical
energy and store it as chemical energy for later use.
The key to electrochemistry are the processes of oxidation and
reduction. Remember the phrase" "LEO (the lion) goes GER (grr??) " --
Lose Electrons Oxidation -- Gain Electrons Reduction. When one
oxidizes a material, it gives up electrons it becomes more positively
charged, or enters a higher oxidation state. Likewise, when one
reduces a material, one is adding electrons to it and either making it
negatively charged or reducing its oxidation state.
Now, one can make a cell using two materials, say A and B and
immersing them in a solution which can conduct ions, called an
electrolyte. (An ion is a charged atom or radical of a molecule
capable of transferring electrical charge). Now, let us say that
material A is easily oxidized -- it likes to lose electrons, while B
is a material that likes to be reduced. When these two materials are
immersed in an electrolyte, and a circuit is completed from A to B, A
is oxidized and electrons are released to flow to the circuit. After
performing electrical work, the electrons flow into B, where B is
reduced. The circuit from B to A is completed by the flow of ions in
the electrolyte. A secondary cell can be reversed by forcing electrons
into A, and reducing the oxidized A to regain unoxidized A for use
again.
This, of course, is an oversimplified view, as only certain
combinations of materials and electrolytes provides useful and
practical batteries.
Oh, one more bit of nomenclature: The cathode is where reduction takes
place, and the anode is where oxidation takes place. So, in a battery
which is producing current, the positive terminal is the cathode, and
the negative terminal is the anode. Yes, this is counterintuitive from
our understanding of diodes, where the cathode is negative with
respect to the anode...
Now, the NiCd system itself:
When the cell is fully charged:
The cathode is composed of Nickelic Hydroxide.
Now, nickel is one of those elements that has multiple oxidation
states -- it can lose a different number of electrons per atom,
depending on how hard it is coerced. Nickel is usually found with
oxidation states of 0 (free metal), +2, +3 and +4. The +2 state is
referred with a -ous suffix, while the +3 and +4 states are referred
with a -ic suffix. So, nickelic hydroxide is really NiOOH (the nickel
has a charge of +3) or
Ni(OH) (the nickel has a charge of +4)
4
The anode is composed of free cadmium metal (zero oxidation).
The electrolyte is usually a solution of potassium hydroxide (KOH).
When one connects a load to the cell, as explained earlier, the anode
is oxidized and the cathode is reduced. Electrons leave the anode
where the cadmium is oxidized and forms:
Cd(OH) , plus 2 free electrons.
2
These two electrons go to the cathode where they reduce the nickelIC
hydroxide to form nickelOUS hydroxide or:
Ni(OH) (where the nickel has
2
a charge of +2)
This reaction can take place until the materials are exhausted. In
theory, cells are manufactured so that both anode and cathode are
spent at roughly equal rates.
_________________________________________________________________
1.5) OK, so what happens when cells are charged?
Well, in a nutshell, the inverse of the discharge. To charge, one is
forcing current back into the cell (opposite of discharge current).
Here, electrons are being taken out of the positive terminal, and
forced into the negative terminal. This means that the material at the
positive terminal is being oxidized (hence is now the anode --
confusing, eh?) and material at the negative terminal is being reduced
(now the cathode).
In the NiCd system, the cadmium hydroxide is being re-converted into
cadmium, and the nickelous hydroxide is being re-converted to nickelic
hydroxide.
Note that the electrolyte in both charge and discharge is a means to
move the hydroxyl (OH-) ions around. Unlike the lead-acid system, the
electrolyte really doesn't change in composition too much between the
charged and discharged state.
_________________________________________________________________
1.6) OK, so what's so tricky?
The easy part of charging is reconverting the spent material on the
plates to the charged condition. The hard part is knowing when to
stop. Let us take a moment to think about what happens when we
overcharge the battery. Once all the nickelous hydroxide is converted
into nickelic hydroxide, and in theory all the cadmium hydroxide is
converted into cadmium, the charging current has to go somewhere. As
the energy of the charging current cannot go into more chemical
energy, it goes into splitting water (water is still the major
constituent of the electrolyte). Just like the age old chemistry
experiment of splitting water into hydrogen and oxygen, a fully
charged NiCd cell does the same thing. You are forcing oxidation at
the positive terminal and reduction at the negative. When one oxidizes
water (actually the OH-) ion, one produces oxygen. Likewise, at the
negative terminal (now the cathode), one produces hydrogen.
This of course is bad. Oxygen + hydrogen = BOOM. Cell manufacturers,
or at least their lawyers, frown on this from happening. So, they
cheat. During manufacture, they deliberately oversize the negative
plate, and they partially discharge it. That is, they put a fully
charged positive plate, but put a slightly discharged, but bigger
plate of cadmium in. The amount of free cadmium in the oversized plate
is matched to discharge in step with the amount of nickelic hydroxide
provided in the positive plate.
Now consider what happens as full charge is achieved. Oxidation of
water starts at the anode, but since the cathode is oversized, and has
excess hydroxide, the current continues to produce cadmium metal
instead of hydrogen. At the same time, the separator (the material
used to prevent the plates from shorting) is designed to allow oxygen
gas to diffuse through, from the positive to the negative plate. The
free oxygen then oxidizes the cadmium metal to form more cadmium
hydroxide to prevent hydrogen from being formed. Voila -- a safe
battery.
_________________________________________________________________
1.7) OK, so it looks like batteries are well protected. Now what?
Not so fast..... this scheme will work only as long as the
overcharging current is limited to a value such that the rate of
oxygen liberation at the anode is less than or equal to the rate of
diffusion across the separator. If the overcharging current is too
high, excess oxygen is produced at the anode, and since not enough
oxygen can diffuse across to make up for the reduction at the cathode,
the excess cadmium hydroxide is used up. Then, hydrogen is formed.
This leads to a dangerous situation, due to both fire and
overpressure. Cells are designed to vent when this condition occurs,
releasing the excess hydrogen and oxygen to the air before really bad
things happen. While this may keep one's cells from blowing up, it
does damage them, since one is losing material from one's cell. As one
loses water, it upsets the chemical balance inside the cell -- lose
enough water, and it stops working.
Another problem is that the process of generating oxygen, and
recombining it at the cathode generates heat. With a moderate amount
of current, the cell temperature can rise considerably, to 50 or 60
degrees C. If after charging, the batteries are hot, then you have
overcharged them -- slap yourself on your wrist...
_________________________________________________________________
1.8) Credo #2: Heat is the enemy of batteries
A NiCd stored, used, or charged under high temperature conditions will
die an early death. Heat causes the separator to weaken, the seals to
weaken, and greatly accelerates changes in the plate material, some of
which cause the dreaded memory effect.
So even though the cells may not vent, the heat by-product is wearing
down the cells. Specifically, hydrolysis or degradation of the
separator material, usually polyamide, is greatly accelerated at high
temperatures. This leads to premature cell failure (see below).
_________________________________________________________________
1.9) What are battery manufacturers doing to prevent damage from overcharging?
Quite a lot. The demand for rapid charging has lead to a great
increase in overcharging abuse. Most all NiCd cells can be rapid
charged. The trick is to stop charging when it is fully charged. The
so called "rapid charge" type of cells just incorporate protection
against overcharging at high currents. Most often, this is done with
activated carbon inserted in the cell to promote the collection of
oxygen and to deliver it to the cathode for recombination. By
increasing the rate of oxygen transport, one is increasing the ability
of the cell to resist venting. Note however, that heat is still
generated.
The price one pays for this is reduced capacity. Everything takes
space in the cell, and space for carbon means less space for active
material. Also, there have been some indications that carbon can cause
the cadmium metal to corrode, possibly leading to a shorter life.
_________________________________________________________________
1.10) How about those high-temp batteries?
There are ways to make NiCd cells more resistant to the damaging
effects of heat. Mainly, using polypropylene separators and changing
the electrolyte to sodium hydroxide makes the cells more durable under
high temperatures. However, the cost is higher, and the internal
resistance is raised, making high current discharge more difficult.
Unless one knows that cells will be used at high temperatures, don't
bother -- learn to take care of the cells to avoid overheating them.
_________________________________________________________________
1.11) What about those super-high capacity cells?
Yup, the manufacturers are in a numbers game. It used to be that AA
cells were 450 mAh. Then came 500, then 600 mAh. Now, 700, 800 and
even 900 mAh cells are available. Next year, Sanyo will introduce a
950 mAh cell. OK, so what's going on? Well, the highest capacity cells
use foamy or spongy backing material for their plates. This allows
packing more active material into the plates, but the cost is higher
resistance. Recall that one of the great virtues of NiCds is their low
internal resistance -- this allows large discharge currents for
transmitting, for example.
So far, the highest capacity sintered plate (best for low resistance)
cell I have seen is the Sanyo KR-800 cell, rated at 800 mAh.
The Panasonic 900 mAh cell is of the foam type, and may work for a
specific application, but expect higher resistance. I also suspect
(but am not sure) that the Millenium cells are also foam type. For
most consumer applications, the internal resistance isn't an issue --
for high power transmitting (e.g. more than 1A of current), it can be
a concern.
_________________________________________________________________
1.12) Does the memory effect exist?
YES
Just as everyone is running around and saying that the memory effect
is a myth, here I am, saying that it is true. OK, so, why is this?
First of all, the term memory effect is quite unscientific. People
tend to attribute any failure of a NiCd to memory.
Let us define memory as the phenomenon where the discharge voltage for
a given load is lower than it should be. This can give the appearance
of a lowered capacity, while in reality, it is more accurate to term
it voltage depression.
Memory is also hard to reproduce, which makes it hard to study.
Originally, memory effect was seen in spacecraft batteries subjected
to a repeated discharge/charge cycle that was a fixed percentage of
total capacity (due to the earth's shadow). After many cycles, when
called upon to provide the full capacity, the battery failed to do so.
Since we aren't in space, the above is not really relevant...
Let us look at various causes of "memory" or voltage depression.
Memory can be attributed to changes in the negative or cadmium plate.
Recall that charging involves converting
Cd(0H) to Cd metal.
2
Ordinarily, and under moderate charging currents, the cadmium that is
deposited is microcrystalline (i.e. very small crystals). Now,
metallurgical thermodynamics states that grain boundaries (boundaries
between the crystals) are high energy regions, and given time, the
tendency of metals is for the grains to coalesce and form larger
crystals. This is bad for the battery since it makes the cadmium
harder to dissolve during high current discharge, and leads to high
internal resistance and voltage depression.
The trick to avoiding memory is avoiding forming large crystal
cadmium. Very slow charging is bad, as slow growth aids large crystal
growth (recall growing rock candy). High temperatures are bad, since
the nucleation and growth of crystals is exponentially driven by
temperature. The problem is that given time, one will get growth of
cadmium crystals, and thus, one needs to reform the material. Partial
cycling of the cells means that the material deep with the plate never
gets reformed. This leads to a growth of the crystals. By a proper
execution of a discharge/charge cycle, one destroys the large crystal
cadmium and replace it with a microcrystalline form best for
discharge.
This does NOT mean that one needs to cycle one's battery each time it
is used. This does more harm than good, and unless it is done on a per
cell basis, one risks reversing the cells and that really kills them.
Perhaps once in a while, use the pack until it is 90% discharged, or
to a cell voltage of 1.0V under light load. Here, about 95% of the
cells capacity is used, and for all intensive purposes, is discharged.
At this point, recharge it properly, and that's it.
The more common "memory effect" isn't memory at all, but voltage
depression caused by overcharging. Positive plate electrochemistry is
very complicated, but overcharging changes the crystal structure of
the nickelic hydroxide from beta-Nickelic Hydroxide to gamma-Nickelic
hydroxide. The electrochemical potential of the gamma form is about 40
to 50 mV less than the beta form. This results in a lower discharge
voltage. In a six cell (7.2v) pack, this means a loss of 300 mV.
Trick? Don't overcharge. Leaving cells on a trickle charger encourages
formation of gamma nickelic hydroxide. Expect the cells to discharge
at a lower voltage.
_________________________________________________________________
1.13) Summary
* DON'T deliberately discharge the batteries to avoid memory
* DO let the cells discharge to 1.0V/cell on occasion through normal
use.
* DON'T leave the cells on trickle charge for long times, unless
voltage depression can be tolerated.
* DO protect the cells from high temperature both in charging and
storage.
* DON'T overcharge the cells. Use a good charging technique.
* DO choose cells wisely. Sponge/foam plates will not tolerate high
charge/discharge currents as well as sintered plate.
_________________________________________________________________
1.14) OK, so I need to "properly" charge cells. How do I do that?
There are many methods of charging. One is trickle or the old 15 hour
method. This involves using a current of about 50 mA (for AA cells)
and leaving them on charge for 15 hours. At this current level, oxygen
diffusion is more than enough to take care of the excess current once
full charge is achieved. Of course, one runs the risk of voltage
depression due to overcharge.
The best method is the so called delta-V method. If one plots the
terminal voltage of the cell during a charge with a constant voltage,
it will continue to rise slowly as charging progresses. At the point
of full charge, the cell voltage will drop in a fairly short time. The
amount of drop is small, about 10 mV/cell, but is distinctive. There
are circuits out there built specifically to look for this. The Maxim
MAX712 and 713 ICs are ones that come to mind now. This method is
expensive and tedious, but gives good reproducible results. There is a
danger in this though. In a battery with a bad cell this delta - V
method may not work, and one may end up destroying all the cells, so
one needs to be careful. If one ends up putting in more than double
the charge capacity of the cell, then something is wrong.
Another cheap way is to measure the cell temperature. The cell
temperature will rise steeply as full charge is reached. When the cell
temperature rises to 10 degrees C or so above ambient, stop charging,
or go into trickle mode.
Whatever method one chooses, a failsafe timer is a requirement with
high charge currents. Don't let more than double the cell capacity of
charge current flow, just in case. (i.e. for a 800 mAh cell, no more
than 1600 mAh of charge).
_________________________________________________________________
1.15) I've seen terminology like "C/n" where n is a number. What gives?
This is a method of expressing current as a fraction of the Ah rating
of a cell. Simply, a 100 mA current means much more to a small N cell
than to a large D cell. So, rather than use absolute units of amperes,
cell manufacturers often use fractions of cell capacity, or C. A
typical good AA cell has a capacity of 700 mAh, so C = 700 mA. A
current of C/10 is therefore 70 mA, while C/2 is 350 mA.
_________________________________________________________________
1.16) So, what's the right charge current?
Depends. If using an unregulated charger -- one that doesn't do any
detection of full charge, then one must restrict your charge current
to the overcharge capacity of your cell. All NiCd cells I have seen
can handle C/10 (approx. 50 mA for AA cell) indefinitely without
venting. This is not to say that one won't get voltage depression, but
rather that one won't destroy the cell(s).
If one wants to get a bit more aggressive, a C/3 charge will recharge
the cells in about 4 hours, and at this rate, most cells will handle a
bit of overcharge without too much trouble. That is, if one catches
the cells within an hour of full charge, things should be OK. No
overcharge is best of course.
Only with automatic means of full charge detection should one use
charge currents above C/2. At this current level and above, many cells
can be easily damaged by overcharging. Those that have oxygen
absorbers may not vent, but will still get quite hot.
With a good charge control circuit, charge currents in excess of C
have been used -- the problem here becomes reduced charge efficiency
and internal heating from ohmic losses. Unless one is in a great
hurry, avoid rates greater than C.
_________________________________________________________________
1.17) You mentioned cell reversal. What is that, and why is it so bad?
In a battery, not all cells are created equal. One will be weaker than
the others. So, as the battery is discharged, the weakest cell will
use up all its active material. Now, as discharge continues, the
current through the dead cell becomes a charging current, except that
it is reversed. So, now reduction is occurring at the positive
terminal. As there is no more nickelic hydroxide, it reduces the
water, and produces hydrogen. Cell pressure builds, and it vents. The
cell has lost water and the life of the cell has been shortened
This is the big danger of battery cycling to prevent memory.
Invariably, unless one is very careful, one ends up reversing a cell.
It does much more harm than the cycling does good. Also, keep in mind
that cells to have a finite life. Each cycle is a bit of life.
_________________________________________________________________
1.18) Credo #3: Take care of your cells, and they will take care of you
I have a set of cells from 1981 that are still working. Sintered
plate, 450 mAh old technology -- originally trickle charged, now rapid
charged many times using a delta-V technique.
_________________________________________________________________
1.19) OK, my cells died. I was nice to them, what happened?
All good things must come to an end. No matter how well one takes care
of the cells, they will eventually die. There are two main reasons
cells fail, other than abuse. One is separator failure, and the other
is degradation of the active material. The first is far more common,
and the result is a shorted cell. Every time a cell is charged, the
active material is redeposited on the plates. Ideally, this occurs
uniformly across the surface of the plate.
However, in reality, there will be bumps and valleys. When there are
bumps on both the positive and negative plates are adjacent, separated
only by the separator, the resistance between those two points is
slightly less than in other regions of the cell. So, the current
density there rises. This means that more material is deposited there,
contributing to even more "bumpiness". In reality, needles called
dendrites form, and given time, they can force themselves through the
separator to short the cell.
A cell that appears to self-discharge in a couple of days has dendrite
problems, and will soon completely short out. Plan to replace the
cell.
Degradation of the active plate material is just a normal aging
process of cycling. Both of these mechanisms are very good reasons to
avoid cycling the cells after each use. Cells should live to about
1000 cycles if treated properly. Anything over that is gravy.
_________________________________________________________________
1.20) I've seen people talk about zapping cells to revive them
Yup -- a quick fix it. When cells short due to dendrites, the piece of
material that is actually shorting the cell is very thin. So, by
forcing a huge impulse of current into the cell, one can vaporize the
dendrite -- sort of blowing a fuse.
This works, and can revive an otherwise shorted cell. However, it is a
stopgap measure at best. First, the fact that one dendrite has formed
means that another is not too far behind. Second, the material that
was vaporized has now permeated the separator material, forming a
resistor that shorts the plates. The cell may no longer be shorted,
but will still have a poor charge retention.
Besides, unless done properly, this can be dangerous as large currents
are necessary.
_________________________________________________________________
1.21) How about self-discharge?
Yes, NiCds have a bad habit of going dead when you just leave them.
Fortunately you can recharge them. The current cells discharge about 1
percent a day, maybe a bit less. Expect them to be mostly flat after 3
months. Unfortunately, the so called 15 hour trickle chargers more
than make up for self-discharge. In fact most of the current goes to
making oxygen, not making up for self-discharge. If you want to make
something to keep your cells from self-discharging, make a 1 to 2 mA
current source. That should more than overcome self-discharge.
_________________________________________________________________
1.22) What about NiMH cells?
NiMH or nickel metal-hydride cells are a promising alternative to
NiCds. They use hydrides (metals capable of storing hydrogen) as the
negative material in lieu of cadmium. They have higher capacity for
the same size cell, and don't use toxic cadmium. They also are
advertised as not suffering from memory.
The downside: They are expensive (all new technology is). They have a
horrible self-discharge rate (I have measured between 3 to 10 percent
per day -- useless after 1 month). They are trickier to charge. Delta
V works, but the voltage drop is very small (2.5 mV/cell). Better to
charge them to a point where the voltage stops rising. And, yes, the
same thing goes with hydrides as with cadmium. They can suffer from
memory, though it is much harder to see than in NiCds. Expect to see a
bit less touting of the "memory free" operation of NiMH cells in the
future.
_________________________________________________________________
1.23) What if I want to learn even more about NiCds and other batteries?
There are several good texts on batteries. One is quite new and
contains fairly contemporary material:
* Maintenance-Free Batteries by D. Berndt,
Research Studies Press, Ltd.,
Taunton, Somerset, England
in conjunction with J. Wiley & Sons,
New York, 1993. ISBN 0-86380-143-9.
_________________________________________________________________
Chapter 2) General Electric's tech. note regarding memory, by Bob Myers KC0EW
From General Electric's tech. note regarding memory
(posted without permission and any errors are my own):
"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 [emphasis
mine - RLM] 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 from GE tech. note]
This note goes on to list the following as the most common causes of
application problems wrongly attributed to 'memory':
1. Cutoff voltage too high - basically, since NiCds have such a flat
voltage vs. discharge characteristic, using voltage sensing to
determine when the battery is nearly empty can be tricky; an
improper setting coupled with a slight voltage depression can
cause many products to call a battery "dead" even when nearly the
full capacity remains usable (albeit at a slightly reduced
voltage).
2. High temperature conditions - NiCds suffer under high-temp
conditions; such environments reduce both the charge that will be
accepted by the cells when charging, and the voltage across the
battery when charged (and the latter, of course, ties back into
the above problem).
3. Voltage depression due to long-term overcharge - Self-explanatory.
NiCds can drop 0.1-0.15 V/cell if exposed to a long-term (i.e., a
period of months) overcharge. Such an overcharge is not unheard-of
in consumer gear, esp. if the user gets in the habit of leaving
the unit in a charger of simplistic design (but which was intended
to provide enough current for a relatively rapid charge). As a
precaution, I do NOT leave any of my NiCd gear on a charger longer
than the recommended time UNLESS the charger is specifically
designed for long-term "trickle charging", and explicitly
identified as such by the manufacturer.
4. There are a number of other possible causes listed in a
"miscellaneous" category; these include -
+ Operation below 0 deg. C
+ High discharge rates (above 5C) in a battery not specifically
designed for such use
+ Inadequate charging time or a defective charger
+ One or more defective or worn-out cells (NiCds DO have a
finite life; they won't keep charging and discharging FOREVER
no matter how well we baby them.)
To close with one more quote from the GE note:
"To recap, we can say that true 'memory' is exceedingly rare.
When we see poor battery performance attributed to 'memory', it
is almost always certain to be a correctable application
problem. Of the...problems noted above, Voltage Depression is
the one most often mistaken for 'memory'.....
_________________________________________________________________
Chapter 3) More on NiCd dealings, by Paul Timmerman
_________________________________________________________________
3.1) About the Author
Author: Paul Timmerman
Posted to: sci.chem.electrochem
E-Mail: ptimmerm@mashtun.jpl.nasa.gov or
paul.j.timmerman@jpl.nasa.gov
This has been an interesting thread, but one which took some slightly
wrong turns. I hate to sound like a self proclaimed expert, but I will
anyhow. NASA has paid me for the last 5 years to study and model
Nickel-Cadmium batteries. Check out my battery modeling home page if
you care to, at:
http://mashtun.jpl.nasa.gov/section342.html
_________________________________________________________________
3.2) NiCd 'memory' and more
[...] stated that the "memory" effect was not caused by shallow
discharges, but by poor charging practices. Well, this is correct in
that most commerical apps have cheap constant current chargers, and
people forget to unplug them. However, with the most sophisicated
chargers, and repetative cycling, a noticable voltage plateau does
indeed develope, and it occurs at both electrodes. (I have the data)
The cadmium electrode is said to develope a cadmium/nickel alloy with
a smaller reversible potential. The positive electrode is more
complex. The formation of the higher valence Ni+3 (gamma) phase
material seems to produce marked semicondutor phenonenon, which can
lead to depletion of the charge carrier (holes?). In this case the
positive electrode potential can drop down to the reversible potential
for oxygen. If this happens any oxygen in the cell, it can reconbine
electrochemcially at the positive, creating a mixed potenital which is
seen as such a plateau. Both oxygen build up and gamma phase
acculation are often the result of overcharging, but will happen too
with "good" charging paractices, but more slowely.
Getting back to the origninal question, is reconditioning useful. The
answer is definitely a yes, but marginally. If you were to completely
discharge your cells every time you used them, you would dramtically
shorten thier lives. If however, you need max capacity and have the $$
for new cells, go for it. I would recommend a recondition once every
three months for most applications. Too much hassle? then don't do it
and buy new packs sooner.
Recondition should be done to 1.0 volts per cell or there abouts, if
no individual cell let down resistors are present. The strong cells
tend to drive the weak into reversal, generating hydrogen and causing
some permanent dammage to the cell, although the hydrogen will
eventually recombine. Lots of hydrogen is a dangerous thing. If you
feel must go lower, you should switch to a low rate reistor for the
last bit. With individual cell monitoring you could go all the way to
0.0 v theoretically, but that never happens.
But I prefer to run my packs to various depths, and get a mild
"stealth" reconditioning that way. This reduces the total number of
cycles on the pack and should help prolong the useful life.
I was stated that with modern Ni-Cd's rigorous battery charge control
was nearly superfluous. I couldn't disagree more. The trend for
rechargable applicaions is to push them harder and harder, with
expectations of greater cycle life, faster recharge, better voltage
regulation, and more temperature tollerance. The basic commerical
design has not changed very much, although some manufacturers are
doing a very good job in tuning thier production to give consistent
cells. The desire for 15 minute chargers is very real, and you need
special equipment to do it.
Pulse charing is a hot topic that alot of people are trying stake out
patents. There seems to be some advantage at the highest rates 15 -30
minute charging to this technique for nicds. The Cristie charger was
designed for Lead-Acid systems, where liquid phase stratification was
a big problem. It is quite helpful in "stirring" the acid in the big
cells they typically market. There is also some evidence that the
pulse charging changes the morphology of the cadmium electode, in an
advantageous way. High rates and current reversals tend to give higher
areas and better utilization. This is good, expecially for cheap
plastic roll bonded cadmium electrodes found in your average
commerical cell.
The comments about storage shorted or trickle charged is right on.
What wasn't said is the benefits of keeping them in the fridge or
frezzer. Keep them above -20f and you'll be sure not to freeze the
electroltye.
Jonathon was asking about bubbles and charging and heat. It sounds
like you got some sales literature thrown at you. The bubbles are
oxygen, and you don't shake them loose, you electro- chemcially
recombine them. Bill, they are generated at the Nickel electrode. The
real solution is to not make so many of them. You do that by
controlling the cell potential, and thus the driving potential on the
positive electrode. Pulsing may do that by relaxing the proton
diffusion gradients in the postive electrode, or it may not! Like I
said, at high rates, good idea. The effect on dead cells can be to
burn out the cadmium shorts and give you a few more cycles. Jon, I
could tell you what to look for but then I'd have to kill ya! :-)
The circuit that Gerald posted seems to be working off 60 hz AC. I am
not sure, but I think that is a little fast for optimal results. (So
move to europe 50 hz :-) The problem is that at two high a frequency
you just access the doulbe layer capacitance in the cell and you don't
exercise the main reactions, or the overcharge ones. I seem to
remember 15 hz as being indicated by the work of McBreen on the Zinc
electrode.
I could go on, but have probably said to much already. I hope somebody
finds it useful. If somebody has a specific application or problem, I
can handle it by email at
_________________________________________________________________
Please check attribution for Author. Processed by
filipg@paranoia.com [Feedback Form] [mailto]. The most recent
version is available on the WWW server
http://www.paranoia.com/~filipg [Copyright] [Disclaimer]
(
geocities.com/siliconvalley/peaks) (
geocities.com/siliconvalley)