Wireless Systems Design

Batteries Bear The Digital Load's Burden

With Popular Battery Chemistries Failing To Eradicate Some Major Problems, Battery Maintenance May Be Today’s Only Answer.

As the world moves from analog to digital communications devices, new demands are being placed on the battery. While analog transceivers draw a steady current, the digital radio loads the battery with short, heavy current spikes. For example, the Terrestrial Trunked Radio (Tetra) system that's being implemented in Europe draws current pulses of up to 3 A when transmitting. Other systems, such as North America's Project 25, have similar requirements.

For two-way radios, low internal resistance forms one urgent battery requirement. Measured in milliohms (mΩ), the internal resistance is the battery's gatekeeper. To a large extent, it determines the talk time. If the resistance is lower, the battery encounters less restriction when delivering the needed power spikes. In contrast, a high-milliohm reading often triggers an early "low-battery" indication on a seemingly good battery. The available energy cannot be delivered fully, so it remains in the battery.

Cold and hot temperatures also impact battery performance. Like humans, the battery performs best at room temperature. Although it varies according to the battery chemistry, the performance at freezing temperatures is generally reduced by 20% to 50%.

Table 1 examines analog and digital radio transceivers. It compares peak power and current requirements, which the battery must be able to supply during transmission. Obviously, moving from analog to digital communications devices reduces the overall energy need. During load pulses, however, it will increase the peak current. The wattage varies in terms of signal strength.

THE LITHIUM-BASED ALTERNATIVE
Today's battery research is heavily focused on lithium systems. In fact, the extent of this focus is so great that one could assume that all future applications will be lithium-based. But how well do these new battery systems perform in the rather harsh environment of digital transceivers? Table 2 examines the relationship between energy density (capacity) and internal resistance. It compares nickel-cadmium (NiCd), nickel-metal-hydride (NiMH), and lithium-ion (Li-ion) batteries. To address longevity, "best cycle life" also is included. Note that periodic discharge cycles are required to achieve the indicated cycle life of nickel-based batteries.

Both nickel-metal-hydride and lithium-ion batteries perform well for cell phones and laptop computers. For nickel-metal-hydride, the two-way-radio track record isn't as encouraging. A shorter-than-expected service life prompts some nickel-metal-hydride users to switch back to nickel-cadmium. Or they experiment with the more expensive lithium-ion batteries.

Nickel-cadmium—and to some extent nickel-metal-hydride—are high-maintenance batteries. They must be fully discharged once a month to prevent "memory." Here, the word "memory" is actually a misnomer. The modern nickel-cadmium battery is mostly void of this phenomenon. Instead, "memory" should be explained as the crystalline formation that occurs on the cell plates. If no maintenance is applied for a period of four months or more, this formation causes the battery's capacity to drop by as much as one third. At this stage, discharging the battery to 1 V per cell may restore lost performance. The full restoration becomes more difficult, however, the longer that the service is withheld.

It is not recommended to discharge a battery before each charge. Such activity wears down the battery and shortens life. Nor is it advisable to leave a nickel-based battery in the charger for more than two days. When it's not in use, the battery should be put on a shelf. It can then be charged before use. Lithium-ion boasts a characteristic that cannot be claimed by most other chemistries: low maintenance. To prolong the lithium-ion battery's life, no scheduled cycling is required. In addition, the pack is lighter and holds more energy than a nickel-based pack of the same size.

THE LITHIUM-ION DOWNSIDE
Despite its advertised advantages, however, lithium-ion does have drawbacks. An example is its fragility. To maintain safe operation, lithium-ion requires a protection circuit. In addition, its maximum charge and discharge current are lower than the currents of nickel-based batteries. Price also is an issue that must be taken into consideration.

Aging is another concern for lithium-ion. Whether it has been used or not, a battery frequently fails after two or three years. Although manufacturers are constantly improving lithium-ion, this age limitation has not been solved. Keeping the battery at cool temperatures extends service life.

A digital transceiver requires less overall energy than its analog equivalent. Yet the batteries for digital transceivers must still be capable of delivering high-current pulses. Often, these pulses are several times higher than their own rating. Take a look at battery rating when it's expressed in C-rates:

  • For a battery that's rated 1000 milli-ampere-hours (mAh), a 1C discharge equals 1000 mA. In comparison, a 2C discharge of the same battery is 2000 mA. A Tetra transceiver, which is powered by a 1000-mA battery drawing 3-A pulses, loads the battery with a whopping 3C discharge pulse.
  • For a battery with very low internal resistance, a 3C-rate discharge is acceptable. Aging batteries pose a challenge, however. The milliohm readings increase with usage and time.

    Improved performance can be achieved by using a larger battery, which also is known as an extended pack. The bulkier and heavier extended pack offers a typical rating of about 2000 mAh or roughly double the rating of the standard pack. In terms of C-rate, the 3C discharge is reduced to 1.5C when using a 2000-mAh battery instead of a 1000-mAh battery.

    WHEN GOOD BATTERIES FAIL
    While researching to find the best battery system for wireless-communications devices, Cadex Electronics examined nickel-cadmium, nickel-metal-hydride, and lithium-ion at various discharge rates. These batteries had been in use for a while. They generated good capacity readings when they were tested with a battery analyzer, which draws a mild load. They were then discharged at a higher rate, which is the case in a digital transceiver. At this point, the batteries' performance dropped sharply. On some packs, this drop occurs because of high internal battery resistance. Nickel-cadmium showed a low 155 mΩ. In contrast, nickel-metal-hydride had a high 778 mΩ. Lithium-ion showed a moderate 320-mΩ resistance reading. At capacities of 113%, 107%, and 94%, respectively, the batteries appeared normal.

    Clearly then, talk time is directly related to a battery's internal resistance. Here, nickel-cadmium performs best. It provides a talk time of 120 minutes at 3C discharge (FIG. 1). Nickel-metal-hydride only performs at 1C. It fails at 3C (FIG. 2). In contrast, lithium-ion allows 50 minutes of talk time at 3C (FIG. 3). For these examples, the batteries tested were used for Global System for Mobile Communications (GSM) phones. A performance parallel does happen to exist, however, between GSM phones and the Tetra system.

    Although battery performance can be reliably measured by applying a full discharge, the service is slow. In addition, a discharge with a steady current fails to assure battery performance under a digital load. During the last few years, instruments have been introduced that measure internal battery resistance. Even though these measurements are fast, the resistance diagnosis often provides conflicting capacity readings. As a result, the predicted battery performance is highly unreliable.

    A BETTER BATTERY MEASUREMENT
    To measure a battery's state of health (SoH), Cadex Electronics developed a method called QuickTest. Using an inference algorithm, this approach fuses data from a number of different variables. The Cadex Electronics QuickTest method evaluates capacity, internal resistance, self-discharge, charge acceptance, discharge capabilities, and electrolyte mobility. All of these variables are combined into one number, and that is what is known as the battery's state of health.

    This battery SoH measurement program has been built into the Cadex Electronics' C7200 and C7400 battery analyzers. Figure 4 shows the four-station Cadex C7400 battery analyzer with QuickTest. The QuickTest method makes use of the battery-specific matrices that are obtained in the analyzer's trend-learning process. This ability to learn makes it possible to adapt to new batteries in the field.

    THE BOTTOM LINE
    In summary, it is important to keep in mind that portable-communications devices are only as reliable as their batteries. To this day, the battery remains the renegade—especially after the pack has been in service for a while. No alternative technology has appeared on the horizon to replace today's temperamental electrochemical battery.

    Although batteries have been improving, emphasis should immediately be placed on battery maintenance. Such maintenance can take many forms. It might, for example, entail exercising batteries to prolong their lives or reconditioning those batteries that have become weak. It might also involve simply retiring of the packs that have become unserviceable. A battery fleet can only remain reliable through a properly managed maintenance program. Reduced operating costs are sure to follow.

    TAGS: Components
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