New battery technologies hold promise, peril for portable-system designers: for the near future, most portable systems will have to rely on some form of the now-venerable lithium-ion battery. Fortunately, lithium-ion cells are improving in cost, robustness, and even energy capacity. But beware the perils that "clone" battery packs can pose for your system.(Cover story)

For portable systems relying on a battery or a battery pack, the ever-increasing complexity and speed of the system's ICs and the resulting increase in power requirements are problematic: Although ICs can surf the processing and speed wave of Moore's Law, battery technology has improved at a much slower pace. The increase in energy capacity for a lithium-ion cell has little more than doubled in 10 years, from 280Whr/1 (watthours per liter) in 1995 to 580Whr/1 in 2005. Compare this figure with the IC's ability to double its complexity every 18 months, and you can see the mismatch between power need and power capacity for portable devices.

Although batteries don't enjoy the same aura of the IC, lithium-ion technology has nevertheless made significant advances. By changing the chemical formulation of the cathode and anode, lithium-ion-battery manufacturers are tweaking cells' energy capacity, as well as cell cost and robustness. However, even as vendors make advances, variations will emerge. System designers will need to pay attention to battery formulations and be aware that battery charging and output voltages are changing, affecting system requirements.

Lithium ion is not the only type of rechargeable cell that battery packs use. Older but still popular types are NiMH (nickel metal hydride) and NiCd (nickel cadmium). Lithium ion, which manufacturers introduced in 1991, is the newest technology, but it has taken over the field with its combination of relatively low cost and high energy density (Table 1). (For a definition of battery terms, such as "cell" and "pack," see box "A battery of definitions.")

What's been driving the change in lithium-ion chemistry? In addition to the need for higher power density, battery manufacturers want a cheaper formulation as well as a safer, more robust mix. Tweaking the lithium and other chemicals in a battery cathode affects the battery cost, capacity, ruggedness, and voltage.

In today's lithium-ion cells, which typically come in a 18650size cyclindrical package, the anode is a graphite mixture, and the cathode is a combination of lithium, nickel, and cobalt. When lithium batteries first became available in the 1990s, cobalt was relatively inexpensive, and its price was stable. But cobalt prices began to rise at the end of the decade, and battery manufacturers worked on new cathode materials that moved away from cobalt.

According to Robin Tichy, product-marketing engineer with Micro Power, a battery-pack developer that incorporates cells from several manufacturers in its packs, "The road map for lithium-ion batteries based on current LiCo[O. …

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