Some Ramblings about NiCd Batteries
From:
Ken A. Nishimura
Newsgroups: rec.radio.amateur.misc
Subject: Ramblings about NiCd Batteries (LONG - 26K
Bytes)
Date: 18 May 1994 23:29:48 GMT
Organization: University of California, Berkeley
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- Batteries for Amateur Use, Care and feeding. (NiCd section)
Greetings:
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.
-Ken
P.S.:
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.
Some Ramblings About NiCd Batteries Ken A. Nishimura (KO6AF)
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.
Q: How does a Ni-Cd battery work?
A: 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)
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. These two electrons go to the
cathode where they reduce the nickelIC hydroxide to form
nickelOUS hydroxide or Ni(OH) (where the nickel has 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.
Q: OK, so what happens when cells are charged?
A: 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
reconverted into cadmium, and the nickelous hydroxide is
being reconverted 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.
Q: OK, so what's so tricky?
A: 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.
Q: OK, so it looks like batteries are well
protected. Now what?
A: 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...
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).
Q: What are battery manufacturers doing to prevent
damage from overcharging?
A: 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.
Q: How about those high-temp batteries?
A: 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.
Q: What about those super-high capacity
cells?
A: 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.
Q: Does the memory effect exist?
A: 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. 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.
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.
Q: OK, so I need to "properly" charge cells. How do
I do that?
A: 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).
Q: I've seen terminology like "C/n" where n is a
number. What gives?
A: 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.
Q: So, what's the right charge current?
A: 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.
Q: You mentioned cell reversal. What is that, and
why is it so bad?
A: 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 is 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.
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.
Q: OK, my cells died. I was nice to them, what
happened?
A: 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.
Q: I've seen people talk about zapping cells to
revive them
A: 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.
Q: How about self-discharge?
A: 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.
Q: What about NiMH cells?
A: 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 downsides: 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.
Q: What if I want to learn even more about NiCds and
other batteries?
A: There are several good texts on
batteries. One is quite new and contains fairly
contemporary material:
Maintenence-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.
Note: 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.
I hope the above was useful...I enjoyed contributing to the
mythology of NiCd cells. :-)
Send the author an e-mail:
Ken A. Nishimura