|The following article was graciously provided to us by Red Scholefield,
long time professional in the battery industry. Here is a bit of his background:
C. L. "Red" Scholefield started modeling at the typical
age (pre-girl) for a depression kid. While the exact date cannot be established,
family photos reveal him holding models of the Megow, Comet and Gillow
Driven by a love for airplanes he enlisted in the US Army Airforce
upon graduating from High School only to learn that Privates were not
issued their own P-40. Discouraged when he had to turn in his Ike jacket
for a bus drivers uniform, he finished his hitch having picked up a fair
amount of electronic training, turned in his Sergeant stripes and he joined
General Electric as an apprentice designer.
As a journeyman designer he found himself working for engineers with
far less knowledge. This drove him to become one, an engineer. Four years
later he graduated from Iowa State with a BSEE and three daughters. Now
with "credentials" he went back to General Electric to apply
his trade in the aerospace divisions, designing and testing all kinds
of neat things, from gattling guns to space hardware.
As aerospace endeavors fell out of public favor he looked for some
cutting edge technology and ended up in General Electrics Battery Business
Department as Manager of Design and Application. He languished here through
hot wheels and sizzlers, electric spoons, power tools and tooth brushes
and all the batteries one could ever hope for his R/C models. He somehow
miraculously made it through downsizing, rightsizing, aquisitions and
divestitures, working at essentially the same desk for GE, Gates Energy
Products, and finally Energizer Power Systems, until faced with an increasing
threatening pile of unbuilt models, he decided to retire at age 65 and
During his working days his international lecture circuit was supplemented
by articles on batteries appearing in hobby and trade magazines. Such
proliferation of essential information to the hobby resulted in his present
position of Associate Editor at RCM Batteries included. (That,
and the fact that he will work trade shows for food and supplied Dick
Kidd with hearing aid batteries.)
While he did achieve national status by coming in 7th in rudder only
in the '58 Nats, his modeling is best described as "un-specialized",
some would say "unfocused", he would say "experimenter".
He spends nearly as much time setting up models, testing and replacing
batteries for local modelers, and as President/Newsletter Editor of The
Flying Gators MAC as he does building and flying wet, electric, gliders
and giant scale models. Favorite planes Eliminator and Predator.
Wants to fly off water having done it once or twice. Intimidated by big
engines. Prefers 40 size planes. Opinionated.
Ni-Cd BATTERY TRAINING SCRIPT
This is a session designed to be an overview of sealed battery technology.
We will cover the construction and design of nickel-cadmium and sealed
lead cells, their discharge and charge characteristics, life characteristics,
and then wrap this session up with a review of some myths and misconceptions
about secondary batteries.
The first practical battery, the silver zinc voltaic pile, was built by
Alessandro Volta nearly 200 years ago. For this distinguishing accomplishment
the unit of electrical force, the volt, was named after Volta. Shortly
after Volta's discovery the first rechargeable battery was constructed
by Johann Wilihelm Ritter. Unfortunately no practical means existed to
recharge it, except from a primary battery. The electric generator was
not to come along for another twenty years so the development of rechargeable
technology was essentially stalled for the lack of a charger.
The next significant step in battery development came 60 years latter
as George Leclanche introduced his carbon zinc "wet" battery,
a technology that paved the way for today's common flashlight battery.
EARLY LEAD ACID HISTORY
At the same time Plante began studies which lead to the development of
the lead acid cell. It is interesting to note that his early work involved
spiral wound cells similar to the Hawker Energy Products sealed lead battery.
In the next 20 year period, Fauare and others developed pasted lead oxide
for the positive electrode and freeing the way for the commercialization
of the lead acid battery in telephone exchanges and railway car lighting.
While not a major step in battery development, the selection of the
sealed lead acid system by Charles Kettering of General Motors to support
his automotive self starting invention was a major step in the mass production
of batteries. The Germans are credited with the development of gelled
electrolyte cells. This was a major step in broadening the application
base for the lead acid system which hear to fore had been limited to rather
stationary applications where the chance of acid spillage was minimized.
EARLY NI-CD HISTORY
The nickel electrode and the alkaline system lagged the lead acid development
by 30 years. Edison's experiments in 1890 resulted in the Nickel hydroxide
positive electrode working in conjunction with an iron negative electrode
in an alkaline electrolyte to form the first rechargeable alkaline system.
A commercial nickel iron battery targeting the electric car market
was demonstrated in 1910. At the same time, Waldmar Jungner, a Swedish
inventor, developed the Nickel Cadmium pocket plate battery. To support
the need for a light weight, high energy battery for their military effort
of WWII, the Germans perfected a sintered plate, flooded electrolyte nickel-cadmium
battery that is essentially identical to those used on today's jet military
and commercial jet aircraft. European experimenters designed the first
recombinant nickel-cadmium battery in the early 1950's that is the basis
for today's nickel-cadmium industry.
BASIC BATTERY TECHNOLOGY
A battery is a device to store electrical energy. It might be considered
analogous to a gas tank on a car except where the gas tank stores fossil
energy the battery stores electrical energy.
Let's start with some definitions. The term battery is generally used
to describe a single unit comprised of one or more cells. Cells are the
"building blocks" of a battery. A battery can be a single cell
which is provided with terminations and insulation and which is considered
ready for use. But usually a battery is a series combination of individual
cells assembled in a pack and provided with some means for connecting
to the device it serves.
THE BASIC CELL
Every battery has the three basic components. The anode or positive plate,
the cathode or negative plate, and an electrolyte system in which the
chemical reaction takes place. Some means for inputting and extracting
energy from the cell must be provided in the form of current collectors.
THE IDEAL BATTERY
If we were to define the attributes for the ideal battery we would come
up with the following:
* It should have a high energy density
* It would be rugged to withstand the rigors of portability
* It would have a long life
* It would be safe
* It would provide for application flexibility
* And it would be rechargeable.
Lets examine then, what the battery world has available for us.
TYPES OF BATTERIES
Batteries are generally classified as either primary or secondary. Primary
batteries are the type that may be used only one time because the active
chemicals are used up when the cell discharges. Once the primary battery
is discharged completely, it is discarded. Secondary batteries, on the
other hand, may be used repeatedly because the chemical reaction which
produces electrical energy can be reversed by recharging the battery.
TYPES OF PRIMARY CELLS
Primary Cells come in a number of commercial variations to address different
markets. The common zinc-carbon has, for years, formed the basis for the
primary battery market and still serves for the low end applications because
of its low cost.
The Alkaline-Manganese is rapidly replacing the zinc-carbon as the
cell of choice for today's advancing electronics market. It's higher energy
density makes it strong competitor when the hourly operating cost is considered.
Mercury-Zinc and Mercury-Cadmium have been popular in the miniature
battery arena where they have been called upon to serve a variety of low
power applications ranging from implantable heart pacers to cameras, hearing
aids and watches. Because of environmental implications and technology
developments they are being replaced by other systems.
The Air-Zinc battery is finding popularity in a number of low power
devices such as the modern hearing aids and other medical prosthetic devices.
Air-Zinc rechargeable batteries are in the development stage. The thermal
battery represents the other end of the primary battery spectrum and are
limited to military and scientific specialty applications.
While there are many other primary battery systems that are targeted
a specialty applications, none has seen the surge in popularity like the
various lithium systems. The lithium battery, in its many forms, is called
upon to power our microelectronic world for ever increasing applications.
PRIMARY CELL DISCHARGE
If we look at a typical primary cell, the popular alkaline manganese dioxide
cell, we see the there basic components that are found in all cells. The
anode, in this case zinc, the cathode, manganese dioxide and the electrolyte
system of potassium hydroxide. As current is drawn from the cell the zinc
anode oxidizes to form zinc oxide and the manganese dioxide is reduced
at the cathode to form Mn2O3. The reaction is on way since reversing the
current flow will not cause a reverse reaction required for recharge.
TYPES OF RECHARGEABLE CELLS
Rechargeable cells are manufactured in three basic types: The most common
is the open type which is typical of our standard automotive starting
battery. The battery is open to the atmosphere and during use gases are
emitted and occasional replenishments of the lost water from the electrolyte
is required. A variation, the ;maintenance free battery merely increases
the volume of electrolyte so the battery will not require maintenance
during is service life.
The second form is the semi-sealed which employs some form of electrolyte
immobilization scheme to reduce the possibility of acid leakage. These
cells are open to the atmosphere and also release gases during charge
and discharge. The so called "gel-cells" fall into this category.
The third type is the fully sealed cell. As the terms imply, during
normal operation, a sealed cell does not permit the venting of gas to
the atmosphere, while in an open or semi sealed (sometimes referred to
as vented cells), venting is part of the normal operation. The fully sealed
cell requires that the gasses generated when charging the cell be recombined
as part of the process. This recombinant technology is employed in all
sealed Ni-Cd and in some sealed lead cell types.
Well, so much for the preliminary definitions. Let's discuss some
of the chemistry and construction of the two systems, Ni-Cd and sealed
The voltage of any type cell is determined by wet materials are used in
its construction. The total cell voltage equals the sum of the oxidation
potential of the anode and the reduction potential of the cathode. The
anode is the positive electrode and the cathode is the negative. The use
of different materials for the anode and cathode yields different cell
voltages. Adding together the potentials for the cadmium anode and the
nickel cathode yields the predicted cell voltage for a nickel-cadmium
cell, 1.3 volts.
As it turns out, actual open circuit cell voltages are quite close
to the predicted values. The same is evident when we examine the Pb and
PbO electrodes of a sealed lead cell.
The amount of stored charge in a cell is determined by how much active
material is used. This amount of stored charge determines the capacity
and is expressed in ampere-hours which is the product of the discharge
current and the duration of the discharge. The ampere-hour rating of cells
can be used to compare the capability of cells, but this comparison is
really only valid for cells which have the same chemical system. Cells
that use different chemistry should be compared on a variety of factors
such as weight, or power delivery as well as capacity.
We will frequently use the term "C" or "C" rate
when discussing charge and discharge rates of batteries. This term "C"
is numerically equivalent to the rated capacity of a cell. A cell discharged
at the "C" rate will expend its minimum capacity in one hour.
Because nickel cadmium manufacturers establish their capacity ratings
as either the five hour or one hour, some manufacturers provide both ratings
for ease of comparison.
Sealed lead product line are rated at the ten hour or twenty hour
rate but as with nickel-cadmium, some provide ratings at the five, ten
and twenty hour rates for comparison with other sealed lead manufacturers.
Some Ni-Cd cells are rated at the one hour rate. At .25C discharge
rate, a cell's one hour rated capacity will be delivered in four hours,
and at the 4C discharge rate, the rated capacity will be delivered in
15 minutes. For example, the "C" rate of a 600 milliampere-hour
AA cell is 600 milliamperes. The .1C discharge or charge rate for this
cell would be 60 milliamperes. When discussing battery applications, the
use of "C" rate simplifies understanding the fundamentals of
the application and helps normalize data for easier comparison between
different operating conditions.
ELECTROCHEMISTRY OF THE NICKEL CADMIUM CELL
The nickel-cadmium cell is an electrochemical system in which the electrodes
containing the active materials undergo changes in oxidation state without
any change in physical state. This is because the active materials are
highly insoluble in the alkaline electrolyte. They remain as solids and
do not dissolve while undergoing changes in oxidation state. This is what
makes a nickel-cadmium cell long-lived, since no chemical mechanism exists
to cause the loss of the active materials.
An important cell characteristic which results from these chemical
and other properties is that the cell voltage is essentially constant
throughout nearly all of the discharge. In the nickel-cadmium cell, nickel
oxyhydroxide (NiOOH) is the charged active material in the positive plate.
During discharge, the charged nickel hydroxide goes to a lower valence
state, Ni(OH)2, by accepting electrons from the external circuit. Cadmium
metal (Cd), is the charged active material in the negative plate. During
discharge, it is oxidized to cadmium hydroxide Cd(OH)2, and releases electrons
to the external circuit. During charging of the battery, the reactions
are reversed, thus returning the cell to the original voltage and capacity.
The electrolyte in which the reaction occurs is potassium hyroxide (KOH)
solution in water at concentrations in the 32% range.
When a cell is overcharged, oxygen gas is generated at the positive
electrode, but the sealed nickel-cadmium cell is designed to accommodate
the excess oxygen during slow overcharge with no noticeable loss of performance.
This is accomplished by building the cell with a negative plate which
is not fully charged when the positive plate becomes fully charged. Inspection
of the plates will reveal that the negative plate is physically larger
than the positive as depicted on the slide. The excess oxygen quickly
passes through the porous separator, reaching the active sites on the
negative plate where it is recombined from the gaseous state forming hydroxyl
ions. These hydroxyl ions then move back to the positive plate completing
the circuit. In the unusual instance where a cell is overcharged at a
higher rate than can be handled by the cell design, a resealable safety
vent will open, letting the excess oxygen escape.
This is a cross-section of a sealed cylindrical Ni-Cd cell. G.E.P. uses
a cylindrical nickel-plated steel case as the negative terminal and a
cell cover as the positive terminal. The plates, which are wound to form
a compact roll, are isolated from each other by a porous separator, usually
nylon or in high temperature GEP cells, polypropylene. This separator
material in addition to isolating the plates, contains the electrolyte
through which the chemical reaction must take place. An insulating seal
ring, polysulphone, electrically insulates the positive cover from the
Note the resealable vent mechanism employs an elastomer gasket backed
by a steel disk and held in place by a helical spring to establish the
safety valve. Unlike some designs employing a rubber slug which deteriorates
with age in the caustic environment the spring backed elastomer coated
steel disk maintains its sealing and venting characteristics throughout
the useful life of the cell. Other designs have employed a diaphragm which
is pierced by a sharp protrusion in the cover when excess pressure conditions
occur in the cell. While this provides a satisfactory safety vent, there
is no resealing after the vent occurs and the cell rapidly dries out and
fails to function. Although most sealed cells currently available have
some form of a vent mechanism, they are still referred to as "sealed"
cells. Most manufactures provide this high pressure vent on its sealed
cylindrical cells as a safety measure.
Nickel-cadmium cells are charged by applying direct current with the proper
polarity to the cell. The charge current can be pure direct current, full
or half-wave rectified alternating current, or some other pulsating d/c
wave form. A nickel-cadmium cell will charge at rates as low as 0.02C,
but the minimum charge rates used in commercial practice are in the range
of 0.05. Charge rates as high as 20C have been used successfully but,
as you will see, there must be a means for terminating the high rate charge
before an overcharged state is reached.
By industry convention, a charger that fully charges a battery in
one hour or less is called a "fast" charger while one that requires
longer than one hour but less than 14 to 16 hours is called a "quick"
charger. Slow chargers require 14 to 16 hours to fully charge, so they
are commonly called "overnight" chargers. These charge times
translate to charge rates ranging 0.05C to 0.1C for slow charge, 0.2C
to 0.5C for quick charge, and C or greater for fast charge.
Slow and quick charge regimes are popular because of the relatively
low cost and simplicity; of implementation. The charger does not require
any special circuitry to switch from a high rate to a low rate as the
battery is capable of accepting a continuous overcharge at the slow or
quick charge rates. Most sealed Ni-Cd cell designs today have built-in
overcharge protection due to the capability for the negative plate to
absorb the excess oxygen generated at the positive plate during overcharge.
While the cell may be able to recombine the excess oxygen at higher
charge rates, the temperature build up can become a significant factor.
Then what happens during higher charge rates? To answer this, let's look
at a graph of what happens to voltage, temperature, and pressure as a
cell is charged at 0.1 or 0.3C rate. The cell pressure stays low during
most of the charge time and rises as the cell approaches full charge.
The higher pressure is the result of the oxygen generation. The higher
the overcharge rate the higher the rate of oxygen generation. Likewise,
as the oxygen is recombined on the negative, the cell temperature increases
due to what is termed "the heat of recombination".
Since we must adhere to the laws of physics, all the energy going
into the cell must be accounted for. This energy (charge current times
the cell voltage) either goes into the chemical conversion of the active
materials (which is an endothermic reaction) or as this conversion nears
completion (the cell reaches a full state of charge) the energy goes into
the generation and recombination of oxygen with the resulting temperature
Pressure and temperature curves look far more dramatic a the C rate
chart. Notice how now the pressure and temperature do not level off at
an elevated level as seen in the slow and quick charge situations. Shortly
after the cell is fully charged at C rate, the temperature reaches a level
that can cause damage to the cell separator system if this charge current
(energy input to the battery) is not reduced. Sustained high rate overcharge
can be accompanied by venting of the cell causing further damage. Fast
chargers (and in some cases, quick chargers where the temperature build
up can become significant) incorporate special circuitry that reduces
the charge current automatically as the battery approaches the fully charged
state. While there are many variations, these types of chargers generally
employ some scheme that monitors the battery temperature or voltage profile
or a combination of both. The most basic systems employ a simple thermostat
or thermistor that measures the absolute temperature of the battery pack
and terminates the charge around 45 C. More elegant temperature termination
systems use a scheme that detects a rise above some ambient temperature.
This temperature rise is usually set at 10 degrees C above ambient. More
recently, microprocessor technology has been successfully employed to
monitor the rather complex voltage functions coincident with charging.
All of these systems should provide a sustaining charge once the fast
charge has been terminated. It is not uncommon to employ both voltage
and temperature monitoring with one serving as a back up for the other
to limit the risk of uncontrolled high rate charge. Charge acceptance
is a measure of how efficiently a cell will charge. The measurement of
the acceptance of the inputted energy is the amount of capacity that can
be delivered to a load at a specified temperature as a result of a given
amount of charge input to the cell. If the charge acceptance were 100
percent, then all of the input energy would be available as output. As
with most things in nature nothing is 100%. Ni-Cd cells are no exception.
The actual charge acceptance curve typically looks like this, with excellent
efficiency in the 10% to 90% state of charge range. Three areas of the
graph display distinct behavior that reflect different sets of mechanisms
causing losses of charge input energy. In area 1, losses are caused by
the input energy being used to convert some of the active material into
capacity that will be inaccessible when the cell is discharged. However,
this area of inefficiency gradually disappears as the cell is cycled and
this "inaccessible" capacity becomes stabilized. Area 2 shows
near 100% charge acceptance. Any inefficiency is caused by parasitic side
reactions that take place inside the cell. Area 3 represents the onset
of full charge and overcharge where the cell no longer can accept charge
and starts generating oxygen. The cell once fully charged cannot accept
additional charge and the acceptance essentially drops to zero. Charge
acceptance is reduced by slower charge rates. Optimum efficiency is a
1C or 2C rate. These are the plots for charge acceptance at 0.1C and 0.05C.
These lower curves show that the slower charge rates reduce attainable
capacity. Higher temperatures also reduce charge efficiency. Although
overall charge efficiencies are never 100%, the optimum charging efficiencies
are at room temperature or below. The key points about charging are:
1. Charging is accomplished by applying direct current with the proper
2. Charge rates are categorized into three types: slow, quick, and fast.
3. At charge rates higher than 0.3C, it is important for the charge rate
to be reduced or stopped automatically when the cell becomes fully charged.
4. Charge acceptance is a measure of a cell's ability to charge. The efficiency
of charging is affected by charge rate and temperature.
The discharge characteristics of a Ni-Cd cell typically look like this.
Notice how the cell voltage remains relatively constant at about 1.2 volts
until near the end of discharge. The steep voltage drop at the end of
discharge is typical for a nickel-cadmium cell. Under conditions of actual
use, certain variables cause differences in the discharge characteristics
of a cell. This means that these variables need to be considered when
estimating actual cell capacity for a certain application. These operating
* Discharge rate
* Discharge time
* Depth of discharge
* Cell temperature during charge, at rest, and during discharge
* Charge rate and overcharge rate
* Charge time, Rest time after charge
* Previous cycling history
Every nickel-cadmium cell or battery has a specific rated capacity,
discharge voltage, and effective resistance. Individual cells are rated
at 1.2 volts and voltage for batteries are multiples of the individual
cell nominal voltage of 1.2 volts. Five cells connected in series would
result in a 6 volt battery. As you can see, however, the discharge voltage
will probably exceed 1.2 volts for some portion of the discharge period.
Most manufacturers rate cell capacity by stating a conservative estimate
of the amount of capacity which can be discharged from a relatively new,
fully charged cell. The accepted rating practice is to state a cell rating
in ampere-hours (or milliampere-hours) to a cutoff voltage of 0.9 volts
at its one-hour discharge rate. Some manufacturers choose to rate at the
5 hour rate since it allows them to quote a "higher" capacity.
This graph shows that when rates of discharge are reduced the available
capacity becomes less dependent on the discharge rate. When rates of discharge
increase, the available capacity decreases as the discharge rate increases.
The transition from dependent to independent is generally in the C/2 area.
Note the "apparent" advantage that can be gained rating at the
5 hour rate vs. The 1 hour rate. Of interest to the product designer is
the real capacity to the cell at the application discharge rate. Other
numbers become academic in this light.
EFFECT OF TEMPERATURE ON MID POINT VOLTAGE
Let's look at the effects of temperature on cell capacity. See how at
higher temperatures the voltage is lower for the same discharge rate.
When you combine higher discharge rates with higher temperature the voltage
profile drops even lower, lowering the apparent capacity of the cell.
GEP rates cell capacity at 23 degrees C (room temperature) to serve as
a guideline for estimating actual performance.
The effective internal resistance, "Re" of the cell is the third
characteristic which is usually included in the rating of a nickel-cadmium
cell. This resistance has a dramatic impact on the voltage delivery and
hence the capacity to a given cutoff voltage. Some manufacturers choose
to rate their "internal resistance" as the AC impedance at 1000
Hz, which results in a more "attractive number" rather than
employ the ANSI standard that gives a true DC impedance that is of real
interest to the designer. There are a limited number of "AC"
applications running from batteries! AC impedance measurements do provide
for a number of diagnostic measurements of the batteries condition but
have little to do with predicting the performance in the application.
A cell discharge circuit can be represented as a voltage source in series
with the effective internal resistance. The amount of resistance varies
depending upon what portion of the discharge curve the cell is on. This
is reflected in the typical voltage profile. Toward the end of discharge
the voltage drops because of the increase in the internal cell resistance.
Chemical self discharge causes a cell to lose its energy during storage.
This discharge is not harmful to the overall life of the cell. Full capacity
can be restored with a normal charge. These are typical self-discharge
rates for nickel-cadmium cells. Notice the effect of increased temperature
on self-discharge rates. The rate of self discharge is about 1% per day
at room temperture and doubles for every 10 degrees above room temperature.
It is important to consider the voltage cutoff of a device that uses Ni-Cd
batteries. Let's look again at the typical voltage profile. If the voltage
cutoff is too high, the battery is underutilized. If the voltage cutoff
is too low, or if there is no voltage cutoff at all, individual cells
in the battery can be driven in reverse, which if done repeatedly, can
reduce the available capacity and ultimately shorten the useful life of
the battery. The recommended voltage cutoff depends upon the discharge
rate and whether the battery is used in "float" applications
or cyclic applications. Choosing the right voltage cutoff provides the
maximum capacity utilization and the maximum reliability. As the discharge
rate decreases the need for a voltage cutoff becomes more important. Use
of tapped cell packs for speed control in power tools and appliances is
acceptable because of the relatively high discharge rates involved. At
these high discharge rates the cells exhibit an electrical reversal (due
to the rapidly increasing internal resistance at their end of discharge),
which at the high rate precedes the potentially damaging electrochemical
reversal, and there is no significant degradation of cell performance.
The key points to remember about discharge characteristics are that nickel-cadmium
cells are rated at 1C or 5C discharge to 0.9 volts at 23 degrees C and
have a nominal voltage of 1.2 volts. Cell ratings are conservative design
minimums and actual performance is affected by the conditions under which
the cell is charged and discharged. The voltage profile of a Ni-Cd cell
is very flat during most of its usable capacity and it drops off very
rapidly when the cell approaches the end of its usable capacity. Also,
it is important that the device to be powered with a nickel-cadmium battery,
where voltage cutoff is employed, has the proper voltage cutoff in order
to get the maximum capacity and the maximum reliability.
Battery life can be described in terms of years of service or number of
charge/discharge cycles. Under controlled conditions, a Ni-Cd cell can
last up to 10 years with minimum cycling. On the other hand, cells have
been cycled up to ten thousand times, again under controlled conditions.
Notice how we say, "under controlled conditions". Ni-Cd cells
employing the same basic technology, are powering satellites for nearly
two decades. The key elements that determine cell life are temperature,
overcharging conditions, and, to a smaller extent, type and depth of discharge.
These key factors are tied to the number of cycles and the age of the
cells, so both time and cycling indirectly become elements in determining
cell life. We generally define failure as the point where the cell fails
to yield 80% of its rated capacity. The primary failure mode is the loss
of separator integrity which manifests itself in a cell short. This short
may be what is termed as a hard short or low resistance or as is typical
in onset of failure, the short is of some finite resistance or "soft"
short. This soft short causes the battery to "self discharge"
in a very short period as well as shunt some of the charging energy during
the charge cycle all of which result in what is perceived as a low capacity
cell. A functional failure occurs when the cell or battery causes the
end-use device to fail to function. In this case, the cell still has the
ability to accept charge and be discharged, but the performance level
is below that necessary to properly run the device. In some cases this
type of failure is the result of an improperly designed piece of equipment
or a misapplication of a particular type of battery. This is a graph of
expected capacity vs. cell cycle life. This shows that under rated conditions,
namely C rate charge/discharge at 23 degrees C, cell capacity drops within
the ranges shown. The effect of increasing temperature causes life expectancy
to be correspondingly reduced. This graph shows this effect. The percent
of rated life decreases dramatically for standard cells and less dramatically
with cells that are designed to be used in higher temperature applications.
In overcharge, all energy delivered to cells is converted to heat. Looking
at this graph, we can estimate the temperature rise per cell for every
100 milliamperes of overcharge current. For example, a Cs cell rated at
1.5 ampere-hours will increase in the range of 1.5 degrees C for every
150 milliamperes of overcharge. As you can see, it is important to not
overcharge cells at rates sufficient to cause a significant temperature
rise over extended periods of time because the excess heat will cause
a reduction in cell life. A shorted external circuit causes tremendous
current flows through a cell's internal path that can destroy the current
collecting tabs and cause the cell to become an open circuit or as a minimum,
damage the cell seal ring causing it to leak. Now let's discuss two common
concerns, myths, and misconceptions about Ni-Cd cells: reversal and memory.
What is cell reversal? Batteries made up of more than one cell have the
potential for cell reversal problems when the discharge is deep enough
to bring one or more of the cells in a battery to zero voltage. If discharge
continues beyond this point, the voltage on the depleted cell will reverse
polarity. Here is the general voltage curve as has been shown before,
except now it is continued into the area of overdischarge. The positive
electrode is usually the first to run out of capacity. Continuation of
the discharge will cause the reversal of the negative electrode and the
voltage will be further reduced to about -1.4 volts. The problem that
occurs is the generation of hydrogen gas. As the electrodes change polarity
they will generate hydrogen. Since the hydrogen will not recombine, the
internal cell pressure will build up to a level that causes the cell to
vent if the reverse charge current is maintained for a significant period
of time. The solution to cell reversal is to avoid design applications
where the cells will be reversed repetitively or deeply. This is done
by selecting a sufficiently high cutoff voltage to assure that cells will
not be reversed. Cell reversal is more damaging at lower rates since the
electrochemical reversal occurs at nearly the same time as the electrical
reversal caused by the increase in internal resistance as the cell capacity
is depleted. At higher discharge rates, such as we find in power tools,
the electrical reversal occurs before the electrochemical reversal with
a significant fall off in performance of the product. The use of tapped
cell packs for speed control in power tools depends upon this principle
and has been tested to verify that there is no damage to the cell pack.
Cells received several hundred cycles of 40% reversal at a 10 C rate,
that is the cells were charged in reverse at 12 amps for 40% of their
capacity, with no detrimental effects noted.
MEMORY OR VOLTAGE DEPRESSION
Contrary to popular belief, the memory effect is not a loss of cell capacity.
Memory is a step in the discharge curve of a cell. See how the voltage
of the lower curve is depressed compared with the normal discharge curve
that we have seen before? The end result of the step is significant only
if a device is designed with too high a cutoff voltage. Most designers
take this effect into account and allow for their devices to be run at
low enough voltage to avoid this problem. What causes memory? Actually
there are two ways to create a step in the voltage profile. One is a precisely
repetitive partial discharge followed by a slow full charge. The discharge
must be to exactly the same point every discharge in order for this effect
to appear. The second and more frequently encountered effect is voltage
depression which is also called memory. This is caused by continuous overcharge
at the overnight rate. If a battery is left on slow charge for long periods,
the crystals of active material in the plates grow larger. As the crystals
grow, the surface area of active material in contact with the electrolyte
decreases and this phenomenon manifests itself as a very slight increase
in internal resistance, plus a decrease in the open circuit voltage. The
voltage step will occur at different times, depending upon how long the
overcharge occurs and the temperature of the battery in overcharge. As
the overcharge continues, the area of voltage depression will occur earlier
in the discharge curve. The area of depression can be removed by one or
more discharge/charge cycles, thereby returning the cell's voltage profile
to normal. Today's cell designs have improved to the point where this
condition is seldom exhibited.
Thomas Edison never knew what he got started over 100 years ago when he
invented the first rechargeable alkaline storage battery. Today, in many
areas of our lives, battery powered devices are at work making things
more convenient, safer, more productive, and even more entertaining. Just
about any place you can imagine there is a battery system getting things
started, or keeping things going. They're used for very down to earth
reasons and for purposes that are literally out of this world. It may
have started with Edison, but even he couldn't possibly have imagined
how far battery technology would go. This concludes this session on sealed
nickel-cadmium cells. We hope this has helped you to become more familiar
with some of the technical aspects of sealed nickel-cadmium battery technology
and that you now have a better understanding of items that should be considered
when you are developing applications.
Red Scholefield Associates
4238 NW 33rd Place
Gainesville, FL 32606
Phone (352) 373-8856
Fax: (352) 335-9715