Some Ramblings About NiCd Batteries …

                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 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.


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