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 vintage.

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 two months.

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.

BATTERY HISTORY
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.
  

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 lead.

VOLTAGE
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.

STORED CHARGE
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.

CONSTRUCTION
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 negative can.

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.

CHARGING
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 rise.

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 polarity.
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.

DISCHARGING
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 variables are:

• 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