Common dry batteries -- the type used in flashlights, toys, and portable instruments -- come in three grades. The least expensive are called general purpose. Those labeled heavy duty cost somewhat more, and alkaline batteries cost the most.

Differing prices for the three grades primarily result from battery chemistry. General-purpose and heavy-duty batteries are both thought of as carbon zinc. But carbon-zinc batteries can use two different systems. General-purpose batteries typically are LeClanche cells. Heavy-duty types may use a high-performance LeClanche system, but they are more commonly based on zinc chloride.

Materials for the LeClanche system are cheaper than those for zinc-chloride cells which, in turn, cost less than alkaline components. Alkaline batteries generally cost less per hour of operation, however, than others where drain current is high and usage frequent. But LeClanche batteries are often the most economical way of obtaining light and infrequent current drain. Zinc-chloride batteries typically cost less per hour than the others where current drain and frequency of use are moderate.

Alkaline cells have higher maximum practical current rates than do zinc-chloride cells. And maximum current in zinc chloride exceeds that of LeClanche. The highest practical rate for typical D cells, for example, is 200 mA/in.2 of separator area for alkaline cells and 150 mA/in.2 and 100 mA/in.2 of zinc area for the other two.

Energy density for LeClanche cells run from 1 to 2 Wh/in.3 and from 2 to 2 Wh/in.3 for zinc chloride. Alkaline cells have about the same range of energy per pound as zinc chloride, but their active materials are heavier. So, energy per unit volume for alkaline cells (2.5 to 4.5 Wh/in.3) exceeds that of any other commonly available cylindrical cells.

Alkaline cells have lower internal resistance and perform better at low temperatures than zinc-chloride cells. And the latter is better in both categories than LeClanche.

The open-circuit voltage of fresh LeClanche batteries is typically about 1.55 V. Zinc-chloride batteries typically exhibit 1.6 V, and alkaline, 1.58 V. And in all the batteries, closed-circuit voltage declines gradually with depth of discharge.

Dry-cell batteries are not rated in ampere-hours because efficiency varies with current drain, operating frequency, temperature, and storage conditions prior to use. And acceptable levels of cutoff voltage vary from one application to another. The efficiency of all three batteries improves as current drain decreases. Thus, where battery efficiency is important, it is generally best to use the largest battery consistent with physical limitations.

Efficiency, moreover, varies from one type of battery to another. Under light current drain, the ampere-hour capacity of alkaline batteries is about twice that of corresponding zinc-chloride types. And capacities of zinc chloride are slightly above those for LeClanche. But at high current drain, the ampere-hour capacity of alkaline can be up to three times that for zinc chloride and 10 times that for LeClanche.

For alkaline cells under moderate current drains, duty cycle hardly affects capacity. The capacity of zinc-chloride cells depends only slightly on duty cycle. But up to a point, LeClanche cells experience a rapid drop off in capacity with increasing duty cycle.

LeClanche and zinc-chloride cells typically retain 90 to 95% of their fresh service life after one year of storage at 21°C. They retain 65 to 75% after four years. But storing the batteries for appreciable periods at temperatures over 21°C significantly reduces their service life.

A way to maintain better life, on the other hand, is by storing them at temperatures down to -20°C. For example, batteries stored at 10°C retain 90% of fresh service life for over three years.

LeClanche and zinc-chloride batteries are best kept at low temperatures in their original cartons or wrapped in plastic. When removed from storage, they should remain in the package until they reach room temperature. Otherwise, condensation can cause electrical leakage which reduces life and may destroy the battery case.

Alkaline batteries retain 95% of their fresh service when stored one year at 21°C and 80% after four years. Storage at higher temperatures reduces battery life, but the effect is less severe than in carbon-zinc cells. Storage temperatures below 21°C maximize battery life, but the percentage of life saved generally makes it uneconomical to provide special low-temperature storage.

Alkaline batteries in miniature packages are economical power sources for applications that do not call for the flat voltage discharge characteristic of mercuric and silver-oxide cells. The miniature alkaline cells operate over a wide temperature range, resist shock, vibration, and acceleration, and exhibit low, essentially constant resistance.

Sensitivity to discharge rate for miniature alkaline cells is comparable to that of silver oxide. Energy density is lower than for comparable mercuric-oxide and silver-oxide cells. Miniature alkalines have a slightly sloping discharge curve and they bulge slightly on discharge.

Lithium batteries:

Theoretically, lithium has the highest energy density of any metal. Batteries based on oxidation-reduction reactions between lithium anodes and cathodes exhibit exceptionally long life.

There are several different groups of lithium (Li) batteries. The type of cathode and electrolyte determines their classification. The two basic groups are solid and liquid electrolyte. These further divide into solid and liquid cathode. Additional classifications can be made by subdividing the batteries by chemistry (SO2, CuO, etc.) or construction (coin, bobbin, or spiral).

The reason for the multiple classifications is that the cathode, electrolyte, and construction determine battery output qualities. Two batteries constructed differently but with the same chemistry will have the same output voltage. However, they will have different discharge curves. Similarly, Li batteries with different chemistries may have similar voltages but different discharge curves.

For example, most commercial Li batteries use manganese dioxide (MnO2) as a cathode. These cells all have an operating voltage of about 2.8 V. But the type of construction used in batteries intended as backup power for computer memories differs from that of batteries used in cameras, though both batteries may be cylindrical in shape.

Memory backup cells must deliver a low current over long periods. A bobbin-type construction makes this sort of operation possible. The anode and cathode are made thick (to store a lot of energy), and the surface area between the two is relatively small. The small surface area limits the maximum current. Bobbin cells have energy densities as high as 700 Wh/L.

Unlike computer memory, cameras demand high peak currents. Therefore, the anode and cathode must have a large surface area. Cells designed either to deliver high pulse currents or for continuous operation typically use a spirally wound anode and cathode. This construction makes the surface area between the two about ten times greater than that in bobbin types. A porous polypropylene separator keeps the two electrodes from directly touching, which would cause a short.

Cell chemistry affects more than simply the output voltage. For instance, consider the discharge curves of two spiral-wound batteries with similar voltages but different chemistries, lithium polycarbonmonofluoride (Li-CFn) and Li-MnO2. At room temperature (70°F) with an 8-Ω load, the two batteries have similar voltage and about the same lifetime. But at low temperatures (-4°F), the battery made with CFn only lasts about one-fourth as long as the other.

Some new Li batteries use iron disulfide (FeS2) for the cathode. The cathode material determines cell voltage. Iron disulfide has two advantages over other cathodes: It is inexpensive and generates 1.5 V.

Other materials, such as copper oxide (CuO) can also be used to make 1.5-V batteries. But Li-CuO batteries exhibit a problem called "voltage up." This refers to the open-circuit voltage of a Li-CuO battery, which is 2.4 V. Once connected to the circuit, the voltage rapidly falls to 1.2 to 1.5 V. But this phenomenon may damage the connected electronics. The problem can be avoided by using FeS2 for the cathode.

Some batteries can withstand continuous use at a moderate to high current (500 mA or more). Therefore, it has spiral construction. To make the battery, FeS2 is powdered and mixed with a graphite paste to increase conductivity. To form the cathode, the mixture is applied to a metal carrier (film).

Batteries are assembled by placing a thin plastic separator between the Li and cathode. Before being rolled, the anode and cathode are about 1.5 in. wide and over a foot long.

Electrolyte is added after the electrodes are put in the can. Like the cathode, the electrolyte determines battery performance.

The electrolyte is aprotic, meaning that it has no reactive protons or hydrogen atoms. However, the electrolyte also has a high ionic conductivity, a quality usually associated with free hydrogen. The electrolyte is relatively nonreactive with Li and is liquid over a broad temperature range. This last attribute is needed to give the battery a wide (-40 to 70°C) operating range.

Mercuric oxide:

The zinc-mercuric oxide cell offers more energy density than either C-Zn or alkaline cells. During its service life, between 80 and 90% of the active electrochemical materials are consumed.

The anode is a zinc-mercury amalgam; the cathode, which acts as the depolarizer, is mercuric oxide. Potassium hydroxide is the electrolyte.

A flat voltage-discharge curve and higher than normal sustained voltage under load are other characteristics of the mercury cell. It has a relatively constant ampere-hour capacity regardless of current drain, excellent shelf life, good high-temperature characteristics, and resistance to shock, vibration, and acceleration.

Silver oxide:

Like the mercury cell, the silver-oxide primary battery has a very flat voltage-discharge curve. But unlike mercury, it operates at about 1.5 V (1.6-V open circuit). Both mercury and silver-oxide cells have about the same ampere-hour capacity.

The silver-oxide primary battery is best suited for electric watches. The positive electrode is a depolarizing mixture of silver oxide and manganese dioxide which can be varied to tailor the cell to the application. The anode is zinc. For hearing-aid cells (slow drain, long life), highly alkaline potassium hydroxide is the electrolyte, selected to boost energy density; for watches (very low drain, very long life), sodium hydroxide is chosen for longer term reliability at the expense of energy density.

Silver-oxide batteries have excellent shelf life and they operate well at elevated temperatures. At low temperatures, they perform much better than most mercury-zinc cells. Because of the high cost of materials, silver-oxide primary batteries are usually restricted to "button" cells and other small sizes.

Rechargeable batteries:

Rechargeable batteries are used to provide backup power for electrical systems ranging from circuit boards to emergency lighting. Most rechargeable batteries are either nickel-cadmium or sealed lead-acid.

Nickel-cadmium and lead-acid batteries are called "sealed" because they lose no electrolyte during normal charging or discharging. Gas generated within the battery is recombined within the cell rather than vented. Both batteries can be charged indefinitely and have low internal impedances (on the order of 10 mOmega Sign ) that let batteries deliver rapid high-energy pulses without degrading performance or cell life.

A major difference between the two batteries is cost. For a given energy capacity, nickel-cadmium cells are more expensive than lead-acid cells. But nickel-cadmium are often fabricated with special properties for applications where seal lead-acid cells are impractical. For example, special nickel-cadmium cells can operate at low-temperature extremes. Other nickel-cadmium cells can accept very high charge currents, allowing the battery to charge fully in several minutes rather than hours. Nickel-cadmium cells can also be stored for years in a state of full discharge.

Battery capacity must be evaluated on the basis of temperature extremes. Low temperatures increase internal battery resistance which, in turn, reduces discharge voltage and available discharge current.

Lead acid:

These batteries can be sealed only when they contain limited amounts of electrolyte. Consequently, gases normally generated during charging recombine because they are "starved" for electrolyte. These batteries produce 2 V per cell and lose their charge slowly during storage. Shelf lives of several years at room temperatures are common. However, sealed lead-acid batteries must be stored in the charged state. Otherwise, internal shorts may form in the battery during recharging. Sealed cells may vent electrolyte under extreme overcharging -- internal battery pressure builds up to between 30 and 50 psig. However, this condition is rare. In "dry" cylindrical sealed cells, essentially all of the electrolyte is absorbed in the separator so that electrolytic venting does not occur to any practical extent.

Another kind of sealed lead-acid cell uses gelled electrolyte. These rectangular batteries have flat rather than cylindrically wound plates, and they vent at pressures of only 2 to 5 psi, unlike cylindrical cells. Although these batteries can be operated in any position, manufacturers often recommend they be mounted upright so the electrolyte makes good contact with the plates and to minimize the chance that electrolyte is vented during charging (vents are on the battery top).

Although gelled-electrolyte batteries cost less than comparable cylindrical lead-acid batteries, working life is shorter. Moreover, most manufacturers recommend against fast-charging these cells. Gelled-electrolyte capacity at low temperatures is also somewhat less than that of cylindrical sealed lead-acid or nickel-cadmium cells.

Nickel-cadmium:

Nickel-cadmium rechargeable batteries are sealed, packaged in cylindrical cases, and incorporate a wound plate and separator structure. Nickel-cadmium cells can be recharged at high rates, in as little as one to three hours under certain conditions, and have inherently low internal resistance that allows them to power high-current or pulse-current applications. Standard cells can operate up to 120°F and premium cells to 158°F. Five to 10-year standby life is common over 300 to 1,000 charge-discharge cycles.

Nickel-cadmium batteries are also available in button cells. Button-cell devices are generally suitable for low-current applications where the chance of overcharging is minimal. Capacities range from 20 to 500 mA-h.

Nickel-cadmium battery voltage increases during charging, then decays slowly during discharge, providing a relatively flat voltage-discharge curve. Cell capacity depends on discharge rate, the voltage at which discharge is terminated, discharge temperature, and previous battery history. For example, the average battery voltage during discharge decreases as the discharge rate increases. Temperature extremes also tend to diminish capacity.

Nickel-cadmium batteries can sometimes go through cell reversal when cells are connected in series. Here, the lowest capacity may be driven into reversing polarity if it fully discharges first. The more cells in the battery, the greater the chance of cell reversal.

Typical nickel-cadmium positive and negative cell plates are made of sintered nickel powder. This process produces a porous structure that is about 80% open pores with surface area of about 0.2 m2/gm. The pores are partially impregnated with an active material, with some of the remaining space being left for the electrolyte.

The active material in an uncharged positive plate is nickel hydroxide. Upon charging, this material becomes nickelic hydroxide. An uncharged negative plate contains cadmium hydroxide, which is converted to metallic cadmium by charging. The polarity of a plate is, thus, determined by the chemical used for impregnation.

Battery holders:

A variety of battery holders, some suitable for PCB mounting, are available for carbon-zinc, alkaline, and lithium batteries. Included are types having pressure terminals for A, AA, AA, and C batteries and others with male and female snap fasteners for 9-V units. Batteries are also available with tab terminals suitable for soldering directly to PCBs.