Power-management issues are fundamental concerns in the design of portable electronic equipment. They affect all elements that make a design portable, influencing device performance, run time, size, and weight. While power management encompasses many aspects of circuit design relating to power conversion and consumption, the topic always leads back to the power source—typically a battery. The portable device's performance depends heavily on the capabilities of available battery types, which very often are secondary (or rechargeable) cells.
For applications that are portable in the sense that they can be carried around, designers generally select one of three basic battery chemistries—NiCd, NiMH, and Li-ion. These batteries have found their way into cell phones, pagers, camcorders, PDAs, notebook computers, power tools, and a variety of other applications. Choosing from among the three chemistries is very often a tradeoff between size, performance, and cost (Table 1). When minimizing the size and weight of the battery is paramount, though, high energy density makes Li-ion the chemistry of choice.
Li-ion doesn't just offer the highest energy density. It also provides moderate to high power density, the highest charge retention, no memory effect, and a wide operating-temperature range. And, it isn't considered hazardous for use on airplanes or in disposal. Some additional safety concerns are associated with Li-ion cells, and charge termination requires a high degree of accuracy. But overall, charging Li-ion cells can be simpler than charging nickel-based cells.
Li-ion cells were originally developed by Sony for use in cellular phones. These cells were cylindrically shaped. The 14500 model, an AA-size cell, had dimensions of 14 (diameter) by 50 (length) mm. The 20500 model was 20 by 50 mm. Sony then developed the higher-capacity 18650 cell—now a popular size—as well as other cylindrical sizes tailored to various applications.
Prismatic Li-ion cells, which have a flat, rectangular shape, were developed in response to a need for thinner batteries that better fit cell phones, notebook computers, and other consumer portables. Cells that are 8 to 10 mm thick have become widely available, and it has been possible to obtain over 1000 mAh of capacity in a prismatic measuring 34 by 48 by 8 mm. More recently, 6-mm cells have arrived—albeit with battery capacity closer to 700 mAh—to satisfy the requirements of thinner products.
Industry efforts now focus on shaving cell thickness to 4 mm and below, making Li-ion cells as low-profile as many chip-sized components. To achieve this goal, battery developers are optimizing existing Li-ion cell designs, which rely on a liquid electrolyte. They're also developing new Li-ion cells, commonly known as Li-polymer, which exploit a polymer-style electrolyte.
In the established Li-ion technology, cells consist of negative (anode) and positive (cathode) electrodes based on intercalation compounds—materials with lithium ions mixed in. The anode starts as pure carbon but then has lithium intercalated into it on the first charge. The cathode is a lithium metal oxide where the metal is either manganese, cobalt, or nickel-cobalt. Energy storage and release depends on the interelectrode flow of lithium ions through a liquid electrolyte . The electrodes, which are wound in a "jelly-roll" fashion, are packed in a steel or aluminum can that provides internal stack pressure.
Li-polymer batteries replace the liquid electrolyte with a polymer in gel or solid form. The polymer electrolyte provides the required electrode stack pressure, so the metal can is no longer required and it becomes possible to enclose the cell in a foil pouch. A laminate of aluminum foil and plastic, the pouch occupies less space and weighs less than the metal can. The foil pouch permits greater room for active material, and it can be readily formed in a shape closely tailored to the space available in the application. Moreover, the foil can reduce the cell weight by 50% or more.