Modern Batteries
October 1963 Electronics World

October 1963 Electronics World   Table of Contents  Wax nostalgic about and learn from the history of early electronics. See articles from Electronics World, published May 1959 - December 1971. All copyrights hereby acknowledged.

As with most things of consumer, commercial, and industrial nature, the battery - more correctly 'cell' - science has come a long way in a relatively short time. Alessandro Volta invented the eponymous voltaic pile in 1799; it consisted of zinc and copper electrodes immersed in a sulfuric acid electrolyte, thereby being a wet cell. The first dry cell was the zinc-carbon type invented by Guiseppe Zamboni (not the guy who invented the ice rink resurfacer) in 1812. Rechargeable dry cells of the NiCad variety hit the scene in 1899. Then, it wasn't until 1991 - a century later - that Sony commercialized the Li-Ion cell (and varieties thereof) that now dominates portable electronic markets. By 1963 when this article was written, Ni-Cads ruled the portable electronics realm, but they are not environmentally friendly because of the heavy metals involved, and are heavier and less energy dense than Li-Ion and Li-Poly cells. Lithium has its own unique issues, like tending to eat holes in aluminum aircraft skin and dangerous raw material harvesting methods. Aluminum-Ion (Al-Ion) is the latest wonder chemistry in the works. Look for it to hit the markets soon.

Modern Batteries

By John R. Collins

The right battery in a particular application can last up to ten times longer than one chosen at random. Here is an explanation of the many types of batteries currently available, how they work, and where they can be used to the best advantage.

If you have not followed recent battery developments, you may be surprised at the variety now available and the jobs they can do. The differences are not simply a matter of size, voltage, or capacity. Batteries are specially tailored for constant or intermittent use, for light or heavy current drain, for long-term stability, or for dependable performance under rugged conditions. The right battery for a particular application may last ten times as long and represent a significant savings over a battery selected at random. Conversely, it is uneconomic to use a more expensive battery where a conventional type will do as well.

Fig. 1. - At present, the carbon-zinc cell is the most popular type of primary cell, accounting for bulk of dry cell sales.

Much of the current interest in batteries stems from military and space needs. In addition, the low power consumption of transistors has given impetus to the production of small batteries needed to operate portable radios, television sets, and communications equipment. Also, the normal human desire to avoid extension cords and to use self-contained power sources wherever possible has led to cordless power tools, electric shavers, model automobiles and boats, portable dictating machines, electronic photoflash equipment, and many other battery-operated devices in both industrial and consumer fields.

It is interesting to note that despite the high level of battery performance already achieved, there is room for much improvement. Many electrode materials are capable of far greater efficiency than those now used. The difficulty is finding practical combinations of anode, cathode, and electrolyte materials which will work well without introducing unwanted side effects. Lithium, sodium, and potassium, for example, have theoretically high ampere-hour capacity and e.m.f., but cannot be used with aqueous electrolytes. Other anode materials are limited by excessive polarization and corrosion. The search continues, however, and experimental batteries presently in the laboratory stage may one day replace some of the types now in general use.

Definitions

Strictly speaking, a battery is a combination of two or more individual cells, connected in series, parallel, or series-parallel to give the desired voltage and current capacity. The term is applied loosely, however, to single cells. Dry cells have four major components: the anode, the cathode, an electrolyte, and a depolarizing agent. The last is a chemical added to the cell to counteract undesired effects of chemical changes that occur when current is drawn. Polarization usually involves hydrogen accumulation at an electrode, which increases the internal resistance of the cell and causes a drop in working voltage unless it is removed.

Fig. 2. - Typical flat cell sealed in a plastic bag can be stacked to make various battery voltages and currents.

Batteries are divided into two broad classes: primary and secondary. Primary batteries convert chemical energy to electrical energy through a process that is not easily reversed. They are therefore discarded when useful amounts of energy can no longer be drawn from them. Secondary batteries use reversible chemical reactions, permitting them to be re-charged and re-used many times. Recharging is accomplished by passing current through the battery in the direction opposite to the discharge current, thus restoring the chemicals to their original state.

It is also common to classify batteries according to their intended use. "A" batteries are low-voltage types designed to supply current for the filaments of electron tubes, "B" batteries are high-voltage types intended as plate voltage sources, while "C" batteries are used for grid voltage supplies or in instruments such as ohmmeters.

The voltage of a cell is independent of its size, and is determined by the electrode materials and the electrolyte. Working or closed-circuit voltage will normally be somewhat less than open-circuit voltage due to IR drop in the internal resistance. Capacity is a measure of the stored charge and is usually stated in ampere-hours.

Battery efficiency is a measure of total power in relation to size and it is usual expressed in watt-hours per pound (wh/lb.) Theoretical efficiency is often much greater than that actually achieved, because voltage and current may decline to unsatisfactory levels before the chemicals in the cell are completely used.

Carbon-Zinc Cells

The carbon-zinc or Leclanche cell (Fig. 1) is by far the most popular type of primary battery, accounting for about 90 percent of U.S. dry cell production. Originally invented in crude form almost a century ago, it has been vastly improved but still retains the same essential elements.

The anode or negative electrode is composed of high-purity zinc that also serves as the container for the cell. Purity is important, since small particles of other metals result in the formation of many small cells on the inside surface of the zinc can. When this happens, the zinc is eaten away whether the cell is in use or not, and the small currents thus produced weaken the cell and waste the chemicals. This process is called "local action."

Fig. 3. - Discharge curves of penlight-size Leclanche and mercury cells through 150-ohm resistors show that mercury cells maintain a more stable voltage over their active lifetime.

The cathode (positive) electrode is a carbon rod made by mixing coke or graphite with pitch and heat-treating the mixture to make the electrode conductive. A metal cap which serves as the external contact is fitted to the rod. The depolarizing agent, called "bobbin," is a homogeneous mixture of about 90 percent manganese dioxide and 10 percent carbon black (which is added to increase conductivity) moistened with ammonium chloride. The electrolyte is a jelly consisting of ammonium chloride and zinc chloride, usually mixed with wheat flour or cornstarch. Inhibitors such as chromic salts are added to prevent corrosion of the zinc can.

The initial open-circuit voltage of a carbon-zinc cell is about 1.5 volts. Where higher voltages are needed, cells are connected in series to form batteries. For this purpose, it is customary to construct individual flat cells, as shown in Fig. 2, that contain the same elements as the cylindrical cells. They are sealed in plastic envelopes and are arranged in stacks. The entire assembly is then enclosed in a container with terminals connected to the top and bottom of the stack. Common ranges are 22 1/2, 45, and 90 volt batteries, made up of 15, 30, and 60 cells respectively.

Since the zinc can forms one of the electrodes and the zinc itself takes part in the chemical reaction, it is possible for a puncture to develop in the container. To prevent this, some manufacturers place the entire cell in a steel tube insulated from the zinc electrode.

Mercury Batteries

The mercury battery was developed for practical use during World War II and has since undergone continued improvement. It consists essentially of an amalgamated zinc anode, usually in the form of either a finely divided powder or a pressed shape, a cathode consisting of a mixture of red mercuric oxide with about 5 percent graphite which is molded under pressure into a shaped structure, and an electrolyte of potassium hydroxide containing zinc oxide. The electrodes are separated by two materials - a cellulose membrane to immobilize the electrolyte, and a porous plastic membrane between the cellulose and the cathode.

Fig. 4. - Comparison of efficiencies of various types of cells shows that modern ones have double the capacity of the old.

Like eclanche types, mercury batteries are made both in tubular-shaped single cells and in stacks of flat cells. The voltage of each cell is about 1.35 volts, and as many as 72 cells may be stacked to form a 97.2-volt battery for a portable transmitter.

A mercury battery has far greater capacity than a Le­clanche battery of the same size, since there is complete utilization of 80 to 90 percent of the active materials. More­over, it will maintain a much more stable voltage over its life span, as illustrated in Fig. 3. For this reason, they are sometimes used as a voltage reference instead of a standard cell, and are often employed in pH meters and test instru­ments where stability is important. Because of their long life and constant output, they are widely used for hearing aids, transistor radios, and portable communications equip­ment such as walkie-talkies.

Care should be taken never to exceed the recommended maximum current drain of a mercury battery. If current is drawn too rapidly, the depolarizer cannot take care of the hydrogen, causing a reduction in battery efficiency and a drop in the over-all working voltage.

Alkaline Batteries

Where especially high current drains are encountered, the alkaline-manganese dioxide-zinc battery is an excellent choice. It differs from the Leclanche cell primarily in the highly alkaline electrolyte used. The two principal features are a manganese dioxide cathode of high density in conjunction with a steel can that serves as a cathode current collector, and a zinc anode of extra-high surface area in contact with the electrolyte. These features, coupled with the use of a highly conductive potassium hydroxide electrolyte, give these cells very low internal resistance and high service capacity. They are usually hermetically sealed.

Fig. 5. - Various sizes of nickel-cadmium cells can be packaged to make higher battery voltages. The cells are rechargeable.

The cells are rated at a nominal 1.5 volts. The ampere-hour capacity is relatively constant over a wide range of current drains and a large number of discharge schedules.

Alkaline batteries are cheaper than mercury batteries although more expensive than Leclanche types. However, heavy current drain impairs the efficiency of Leclanche batteries to such an extent that only a fraction of their energy can be utilized. The primary advantage of the alkaline battery is its high efficiency despite continuous or heavy current drain. Under such conditions, it will provide more than ten times the service of the standard dry cell of equal size. It also works well under conditions of light current drain and intermittent use, but since the cost is greater it is cheaper to use Leclanche cells for normal service.

Alkaline batteries have made possible battery-operated devices that were previously considered impractical because of lack of suitable power source. They are ideal for cranking motion picture cameras, for operating portable tape recorders, and for powering the motors of models and toys. When used for electronic photoflash units using transistor or vibrator circuits to step up low-voltage d.c. to the high voltage needed to charge the flash capacitor, they will give two or three times as many flashes as either photoflash or general-purpose cells. They easily handle current drains that strain the capacity of other types of batteries.

Fig. 6. - Cutaway view of a typical nickel-cadmium button cell.

Silver-Oxide Batteries

The development of the silver oxide-alkaline-zinc (Ag₂0­KOH-Zn) battery represents a major advance in miniaturization. They provide higher voltage and greater milliwatt-hour ratings than any other commercially available batteries of equal size. The silver-oxide battery consists of a depolarizing silver-oxide cathode, a zinc anode of high surface area, and a highly alkaline electrolyte. In hearing-aid batteries, the electrolyte is potassium hydroxide, which provides maximum power at the required current drain. Sodium hydroxide is used in the tiny batteries used in electric watches.

The open-circuit voltage of the silver-oxide cell is 1.6 volts. At typical current drains, the operating voltage drops to 1.5 volts. The impedance is low and uniform, and do not change materially until the useful life of the cell is over. Such batteries can therefore be used as a reference voltage source in instruments.

Shelf life of silver-oxide cells is excellent. Even after storage for a year at 70 degrees F, they will maintain 90 percent of their service life. Their performance is also good at low temperatures. This is accounted for in part by the relatively large surface area of the anode, which permits many times the service capacity at low temperatures as other types of batteries of comparable size.

Experimental Cells

Anode materials are selected, as far as possible, on the basis of their theoretical ampere-hour per pound capacity. Hydrogen, rated at 12,000 ah/Ib. (compared to 372 ah/Ib. for zinc) is by far the best choice from this standpoint, but not used because of the practical difficulty of constructing a battery with a gaseous electrode. The limitations on using lithium, sodium, and potassium were mentioned before. Beryllium, rated at 2695 ah/lb., is toxic and expensive.

Because magnesium, rated at 999 ah/Ib. is inexpensive and easy to handle, it is receiving increased attention as an anode material. Various cathode materials have been employed with it, especially metallic oxides, including those of silver, mercury, and copper. These have permitted high discharge currents with little polarization and have been quite successful .

A very promising experimental cell has been constructed using a magnesium anode, an m-dinitrobenzene cathode, and a magnesium perchlorate electrolyte. It provides a useful discharge current, relatively constant voltage, and a very high efficiency of about 90 wh/lb. (Fig. 4). Preliminary tests indicate good shelf life and reliability.

Nickel-Cadmium Secondary Cells

Although lead-acid automotive storage batteries are by far the most common secondary batteries, they are far less important in electronics than the small, hermetically sealed nickel-cadmium cells (Fig. 5) used in equipment ranging from hearing aids to space vehicles. For space use, nickel cadmium cells are kept continuously charged by silicon solar cells that convert the sun's energy into electricity. For hearing aids, they are made in tiny button shapes and can be conveniently recharged.

In the charged condition, the positive electrode of a nickel-cadmium cell is nickelic hydroxide, while the negative electrode is metallic cadmium. The electrodes in both the button and cylindrical cells consist of molded screen-encased active materials (Fig. 6). The electrolyte is potassium hydroxide The operating voltage under normal discharge conditions is about 1.2 volts.

Nickel-cadmium cells in hermetically sealed units are especially desirable, since it is never necessary to add electrolyte and there is no danger of spilling or leaking. Their design is not easy because during the latter part of the charge cycle and during overcharge, nickel-cadmium batteries generate gas; oxygen at the nickel electrode, hydrogen at the cadmium electrode.

A conventional vented battery will liberate fumes through a valve. To hermetically seal a cell, it is necessary to develop a means of using up the internal gas. This is done by designing the cell with an excess ampere-hour capacity in the cadmium electrode and arranging the geometry of the cell so that oxygen can reach the cadmium electrode. When the battery is charged, the nickel electrode reaches full charge first and starts to generate oxygen. The oxygen thus formed reacts with the cadmium to form cadmium oxide.

During overcharge, the cadmium electrode is oxidized at a rate just sufficient to offset input energy, thus keeping the cell in equilibrium. The cadmium electrode never becomes overcharged, so no hydrogen gas is formed.

Silver-Cadmium Cells

Still in the developmental stage, silver-cadmium batteries promise to provide about double the efficiency of nickel-cadmium types. In working units thus far devised, 24 wh/lb. has been achieved, compared to about 12 wh/lb. for nickel-cadmium cells. Their theoretical efficiency, not yet approached in practice, is about 150 wh/Ib.

Finding a suitable separator for the silver-cadmium cells has been a difficult problem. Various materials were tried, ranging from cellophane to sausage casing. However, although these prevent silver migration, they become loaded with silver and become oxidized. Multi-layer membrane systems have been used, but these tend to increase resistance.

In view of the progress already made, it appears certain that silver-cadmium cells will find active use in the future. Their voltage during the first 40 percent of their discharge cycle remains constant at about 1.3 volts, but drops sharply to about 1.1 volts during the rest of the cycle. By cycling over only the first part of its capacity, good regulation can be maintained. Shelf life is also excellent, and the batteries can be made non-magnetic, an important feature for some applications.

 

 

Posted May 31, 2015