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Choosing a Battery that Will Last

What causes a battery to wear down - is it mechanical or chemical? The answer is both. A battery is a perishable product that starts deteriorating right from the time it leaves the factory. Similar to a spring under tension, a battery seeks to revert back to its lowest denominator. The speed of aging is subject to the depth of discharge, environmental conditions, charge methods and maintenance procedures, or the lack thereof. Aging and user-conditions affect each battery chemistry differently.

As part of an ongoing research program to find the optimum battery system for wireless applications, Cadex has performed life-cycle tests on nickel cadmium (NiCd), nickel metal-hydride (NiMH) and lithium-ion batteries. The batteries received an initial full-charge, and then underwent a regime of continued discharge/charge cycles. The internal resistance was measured and the self-discharge was obtained by reading the capacity loss incurred during a 48-hour rest period. The test program involved 53 batteries of different models and chemistries.

When conducting battery tests in a laboratory, it should be noted that the performance in a protected environment is commonly superior to those in field use. Elements of stress and inconsistency that are present in everyday use cannot always be simulated accurately in the lab.

Figure 1

Figure 1. Characteristics of the NiCd, NiMH and lithium-ion batteries in terms of energy density, internal resistance, self-discharge and cycle life.

1 Internal wiring, contacts and protection circuits are taken into account. Readings vary with cell rating, charge state and number of cells connected in series.
2 The discharge is highest in the first 24 hours, then tapers off. Self-discharge increases with higher temperature.
3 Cycle life is based on use and maintenance procedure. Failing to apply periodic full discharge cycles may reduce the cycle life by a factor of three.
4 Cycle life is based on depth of discharge. Shallow discharges provide more cycles than deep discharges.
Battery Performance as Life Cycling


In terms of life cycling, the standard NiCd is the most enduring battery. In Figure 2, we examine the capacity, internal resistance and self-discharge of a 7.2V, 900mA NiCd battery with standard cells. Due to time constraints, the test was terminated after 2,200 cycles. During this period, the capacity remained steady, the internal resistance stayed flat at 75mW and the self-discharge was stable. This battery received an A grade for almost perfect performance.
Figure 2

Figure 2. Capacity, internal resistance and self-discharge of a 7.2V, 900mA NiCd battery with standard cells.

The readings on an ultra-high capacity NiCd were less favorable, but still better than other chemistries in terms of endurance. Although up to 60 percent higher in energy density than the standard NiCd version, Figure 3 shows a steady drop of capacity during the 2,000 cycles delivered. At the same time, the internal resistance rose slightly. A more serious degradation is the increase of self-discharge after 1,000 cycles. This deficiency manifests itself in shorter run times because the battery consumes some energy, even if not in use.
Figure 3

Figure 3. Capacity, internal resistance and self-discharge of a 6V, 700mA NiCd battery with high-capacity cells.

Figure 4 examines the NiMH, a battery that offers high energy density at a low cost. Initial performance was good, but past the 300-cycle mark, performance started to drift downwards with internal resistance and self-discharge increasing after cycle count 700.
Figure 4

Figure 4. Capacity, internal resistance and self-discharge of a 6V, 950mA NiMH battery.

The lithium-ion battery offers advantages that neither the NiCd nor NiMH can meet. In Figure 5, examine the capacity and internal resistance of a typical lithium-ion. A gentle and predictable capacity drop is observed over 1,000 cycles, and the internal resistance increases only slightly. Because of low readings, self-discharge has been omitted on this test.
Figure 5

Figure 5. Capacity and internal resistance of a 3.6V, 500mA lithium-ion battery.

Is Lithium-Ion Truly Superior?



Today's battery research is heavily focused on lithium systems, and one could assume that all future applications will be lithium-based. This lithium hype is especially apparent when attending battery seminars where battery manufacturers worldwide meet. The emphasis is on lithium-polymer; older chemistries such as NiCd and NiMH are hardly mentioned.

In many aspects, the lithium-ion battery is superior to nickel or lead-based batteries. There is one weak point with the lithium-ion that, for unknown reasons, is seldom mentioned by the battery manufacturer. It is aging. Capacity deterioration is noticeable after one year, whether the battery is in storage or use. Past two years, the battery frequently fails.

Although less of a concern in the fast-moving cellular phone and notebook market, storage at operational readiness is an important factor in defense applications. In Figure 6, we examine the capacity loss as a function of charge level and storage temperature.
Table 2

Figure 6. Recoverable capacity after storage at different charge levels and temperatures.

It is not recommended to keep lithium-ion batteries in storage for a long time, and packs should be rotated like perishable food. The buyer should be aware of the manufacturing date when purchasing a replacement lithium-ion battery. Unfortunately, this information is often encoded in an encrypted serial number and is only available to the manufacturer. The recommended charge level is 40 percent and the storage temperature should be 15 degrees or less.

How Much Battery Maintenance?

The NiMH and NiCd are considered high-maintenance batteries that require regular discharge cycles to prevent memory. Although the NiMH was originally advertised as memory-free, both NiCd and NiMH are affected similarly by memory. The nickel plate, a metal that is shared by both battery systems, is the main contributor to crystalline formation. Memory may not be as visible on the NiMH because of its shorter cycle life as compared to the NiCd.

The lithium-ion is a low-maintenance battery that does not require periodic discharges. No trickle charge is applied once the battery reaches full charge and the lithium-ion may remain in most chargers until used. With newer battery systems such as lithium-ion, battery maintenance shifts from applying periodic discharges to quick testing. Quick testing is possible by reading the battery's internal resistance, also known as impedance. Measured in milliohms (mW), the impedance is the gatekeeper of the battery that, to a large extent, determines the runtime. The lower the resistance, the less restriction the battery encounters in delivering the needed power.

There are a number of techniques available with various results. The most common is the dc load test, which applies a discharge current to the battery while measuring the voltage drop. Voltage over current provides the internal resistance. The ac method, also known as the conductivity test, measures the electrochemical characteristics of a battery by applying an alternating current. Battery corrosions and other defects contributing to capacity loss can thus be identified.

Parameters that affect the mW readings are battery chemistry, cell size (mAh rating), the type of cell, number of cells connected in series, wiring and the contact type. For best results, measure a good battery with known performance and use the readings as a reference. Solid terminal connection is essential because a poor contact will provide a high reading. Alligator connections and long battery leads are not suitable. A battery must have at least a 50 percent charge to obtain a meaningful mW reading.

Summary


Research has brought about a variety of battery chemistries, each offering distinct advantages, but none providing a fully satisfactory solution. However, with today's selection of battery systems, better choices can be made to tailor to specific user applications.

Relentless downsizing has pressured manufacturers to invent smaller battery sizes. By packing more energy into a pack, other qualities may be neglected, one of which is longevity. Long service life and predictable low mW reading are found in the NiCd family. However, this chemistry is being replaced, where applicable, with systems containing higher energy density. In addition, negative publicity about the memory phenomenon and concerns of toxicity in disposal are causing equipment manufacturers to seek alternative choices.

Once hailed as the superior battery system, the NiMH has also failed to provide the universal battery solution for the 21st century. Shorter-than-expected service life remains a major complaint by users.

For a fast-moving consumer market that replaces the equipment every two years, the lithium-ion battery is the best choice. However, for applications that need fully charged batteries that often must be kept in warm storage conditions, the lithium-ion does pose a problem in terms of longevity.

So far, the emerging lithium-polymer system has been unable to overcome the shortcomings of the lithium-ion. Other than lighter packaging and more flexible design, the lithium-polymer, as we know it today, may not yet offer the ultimate solution to portable power.

With the rapid developments in technology occurring today, battery systems may soon become viable that use neither nickel, lead nor lithium. Fuel cells, which do not rely on electro-chemical process but allow refueling similar to a vehicle, may solve the portable energy needs for the future. Instead of a charger, the user carries a bottle of liquid energy. Such a battery would truly change the way we live and work.

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