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.
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 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
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
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
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
THE IDEAL BATTERY
TYPES OF BATTERIES
TYPES OF PRIMARY CELLS
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
TYPES OF RECHARGEABLE CELLS
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.
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.
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
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.
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.
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:
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
"The above information is developed by Red's R/C Battery Clinic and may be reproduced and disseminated within the R/C community as long as source credit is given."