Author: Pnina Dan, Ph.D.

Company: Tadiran Electronic Industries, Inc.

Tadiran Electronic Industries, Inc., 2 Seaview Blvd., Suite 102, Port Washington, NY 11050: (516) 621-4980

As portable systems gain market prominence, the reliability and energy capacity of batteries has become a critical issue. System power consumption and rechargeable-battery capacity--which translate into run time--are competitive factors that must be designed into a product at the outset. On top of that, today's six-to-nine month product life cycles and a feverishly competitive environment create heavy price pressures on computer, cell-phone and portable-instrumentation makers.

The bottom line is that system designers must seek out or develop the most cost-effective power-management, secondary-battery, and charging technologies available to differentiate their products and keep them competitive. There's also the increase of public and governmental concern regarding environmental issues. New legislation has focused the attention of battery makers and users on battery chemistries, user accessibility, and cell disposal.

One of the major challenges the battery industry continually faces is the disparity in the nature and rate of technological advance in batteries and in portable-equipment markets. New battery-operated devices come out every six months or so, but truly new rechargeable-battery technologies can take years to develop and perfect for commercial use.

While it can be said that "familiarity breeds contempt," in the case of secondary (rechargeable) battery technology choices for portable applications, familiarity often breeds reluctance to change. In the past, when designers of portable systems had few reasonable choices available, this wasn't much of an issue. However, there are now many choices, and the decision to go with one battery technology rather than another can have serious impact on a product's success.

In the cellular-phone market, for example, primary consumer care-abouts in terms of product characteristics are audio clarity, range, reliability, "features," size, weight, and talk/standby time between battery recharges. Of these, the latter three are directly affected by the choice of battery technology and they can be critical end-product differentiators. If one eliminates the communications-specific requirements from the above list, the remaining care-abouts, especially size, weight, ant time between recharges, apply to all portable electronic systems. Some polls have shown that longer battery life was the most desirable feature in future portable computers, with lower weight in third place.

In general, designers and end users want battery packs with the lowest possible weight and highest possible capacity crammed into the smallest possible volume at the lowest possible cost. These characteristics are especially important as the trend to miniaturization in portable products accelerates. While no single rechargeable battery type can optimize all of these factors, there are some obvious trade-offs among available technologies. In examining the secondary-battery technologies available for use in portable systems, it's useful to compare them with an eye toward the factors that affect size, weight, and time between recharges. This comparison can be easily translated to a bevy of end-use products (Table 1).

At the low end of the cost spectrum is the sealed lead-acid (SLA) battery. This chemistry is mature and reliable, but it's also at the low ends of the scales for volumetric efficiency (energy per unit volume) and energy density (energy per unit weight). You can expect an average of 85 Wh/I and 35 Wh/kg, respectively. Sealed lead-acid batteries were used in early hand-held systems but aren't widely used in smaller types. Most lead-acid batteries cannot be rapidly recharged (in, say, less than three hours) because of possible thermal damage, although special rapid-charge lead-acid types are available. Lead, of course, is a well-known pollutant and lead-acid batteries must be recycled.

Still near the lower end of the cost range but significantly better than lead acid in volumetric efficiency and energy density ((120 Wh/I and 36 Wh/kg) is the "standard" sintered-metal-electrode nickel cadmium (NiCd) cell. With the introduction a few years ago of sponge-metal electrode technology, the volumetric efficiency of NiCd cells jumped to 150 Wh/I and energy density rose to 50 Wh/kg.

An important parameter for rechargeable batteries is their self-discharge rate, which is the rate at which the battery loses charge while not in use. Self-discharge rate can become an issue for end users who use a particular portable system infrequently, yet want to be able to rely on it when necessary. Self-discharge for NiCd batteries is moderate compared to other types.

Nickel cadmium is the most familiar secondary-battery technology and is therefore widely used. Charging circuits are relatively simple and charging is relatively rapid, but take care to avoid extended periods at high temperatures during charging.

Although NiCd cells are popular and well-understood, they have some drawbacks. First, most batteries of this type available today exhibit "memory effect," which is the loss of capacity that results when the battery is recharged before it is fully discharged. Second, these batteries contain cadmium, a hazardous substance, and must be recycled in most areas.

Another common rechargeable alternative is nickel metal hydride (NiMH) batteries. NiMH cells offer increased volumetric efficiency (190 WH/I ) over even the most advanced NiCd types. Energy density is also better than that of NiCd, at 55 Wh/kg. Open-circuit voltage for NiMH cells is 1.25 V, which is identical to that of NiCd cells. As a result, some designers have used NiMH batteries as drop-in replacements for NiCd packs. However, NiMH cells cost significantly more than NiCd cells (up to twice as much, depending on form factor) and require special charging circuits that are substantially different than the relatively simple ones used for NiCd. For NiMH, charging time, rate, and temperature must be accurately controlled. NiMH batteries also have the highest self-discharge rate of any of the types discussed here, making their use in some types of portable systems of questionable value.

Because NiMH cells contain no hazardous substances, disposal is not an issue. They also do not exhibit the memory effect associated with NiCd types, but their longevity is directly related to depth of discharge.

A third rechargeable option is lithium-based battery systems, which have always been considered attractive because of their high level of electrochemical performance. On the other hand, safety and environmental considerations originally required complex and costly construction techniques and safety systems. These factors raised costs and restricted early lithium batteries to critical military applications. Newer lithium-based systems have overcome the safety and environmental obstacles and are, in general, the most efficient types available.

The two lithium-based systems available today are lithium ion and lithium metal. While both types exhibit the overall advantages of lithium-based systems, they differ in some important respects specifically related to portable applications.

Lithium-ion rechargeable batteries were first used in small video camcorders and are now seeing some use in other portable applications. The relatively high voltage (3.6 V) of the lithium-ion cell offers the advantage of fewer cells being required to achieve a given voltage.

One major drawback to lithium-ion technology is its relatively high cost/performance ratio. The cost per watt-hour of a lithium-ion cell is significantly higher than that of other types, but some performance figures are not in proportion (Table 1, again). For example, the volumetric efficiency of a lithium-ion cell is significantly less than that of a lithium-metal cell and only slightly better than that of a nickel-cadmium cell. Energy density is about 64% better than NiMH, but at a potentially greater cost penalty. Energy density is also 36% below that of lithium metal.

Another shortcoming of the lithium-ion system is its primarily nonlinear discharge characteristic. Typically, an AA-sized lithium-ion battery, discharged at a rate of 250 mA, will drop in voltage from 4.3 V (fully charged) to about 3 V in about 90 minutes. The voltage will remain at 3 V for the next 90 minutes and then drop off rapidly to 2 V, at which point the battery is considered discharged. Depending on the design of the system this discharge characteristic can be troublesome in some portable applications that require a minimum voltage for operation.

The more recently developed rechargeable lithium-metal (Li/LixMnO2) batteries offer energy density and volumetric efficiency unmatched by any other battery type. Lithium metal cells, exemplified by Tadiran's In-Charge AA-size cells, sport energy density of 140 Wh/kg and volumetric efficiency of 300 Wh/I. The cells are totally safe and are immune to practically all types of physical or electrical abuse conditions. The increased safety factor is due primarily to a Tadiran-patented fail-safe, self-quenching electrochemical system and a built-in safety vent.

Lithium-metal batteries have no memory effect and have the lowest self-discharge rate of all rechargeables. A typical lithium-metal battery stored at room temperature (70'F, 20'C) retains 85% of its capacity after one year. A portable system powered by a charged lithium-metal battery will always be available for use regardless of how long it sits idle. Operating temperature range (-30'C to +55'C) is also greater than other battery types.

The discharge curve of a lithium-metal cell is practically flat. At a 250-mA discharge rate, after a brief (about 10-minute) drop from the fully charged voltage of 3.4 V, the voltage remains at 2.8 V for the remainder of more than three hours, dropping off to 2 V (the "discharged" point) after that. This also means that when two cells are used in series (to create a battery with a nominal voltage of 6 V) the voltage remains above 4 V for the entire discharge cycle. That 4-V level is the minimum operating voltage for many portable systems, including many cellular phones.

Lithium-metal batteries are capable of delivering up to 2 A of current under continuous or pulse demand. The latter is especially important in cellular phones, where in a typical 600-mW unit, current demands can jump from a standby current of 40 mA to a 0.6-ms talk pulse of 1.4 A with a 200-mA floor between pulses. Under these conditions, the mean talking current is 333 mA. A 4-cell (AA cells) lithium-metal battery pack with a capacity of 1600 mAh, can provide nearly four hours of talk time combined with over 13 hours of standby time between recharges. This is accomplished in a four-cell battery pack that weighs only 68 g (2.4 oz.), which is 70% of the weight of an equivalent 6-V NiMH pack and about 60% of a lithium-ion pack. It can be useful to make a comparison of rechargeable-battery technologies (with the exception of sealed lead acid) and how they stack up ion 6-volt battery packs for cell phones (Table 2).

The cost of lithium-metal batteries per watt-hour is nearly equal to that of NiMH batteries, making them the closest thing available today to "the lowest possible weight and highest possible capacity crammed into the smallest possible volume at the lowest possible cost."

Dr. Pnina Dan is director off research and development for rechargeable lithium batteries at Tadiran Battery Division, Rehovoth, Israel. She received her PhD in chemistry from the Weitzman Institute For Science, Rehovoth.