The fast increasing density of integrated circuits has provided the basis for implementing a growing number of applications as portable systems, like notebook computers, cellular phones, video systems, satellite navigation, etc., some of them spawning new markets of their own. This has created an equally large demand for light-weight, inexpensive portable power sources which are in most cases batteries. However, despite the fact that batteries have been in use for centuries the progress of battery technology is slow compared to the advances of microelectronics. This could be a major limitation to the growth of the portable-electronics market unless the power efficiency of the electronic circuits is improved.
While ULP technology helps to increase battery life substantially, the reduced supply voltage also helps to enhance and simplify the powering of many portable applications such as pagers which normally would require multiple-cell batteries or DC-DC converters [53]. This enables single-cell operation with NiCd or NiMH batteries where otherwise lithium-ion cells would be required.
A battery cell is simply a device for electro-chemical energy storage. It consists of two separated electrodes of different conducting materials, an electrolyte, and appropriate packaging. The properties of the battery depend on the materials used and on the manufacturing. The main stream battery technologies are carbon-zinc (CZn) and alkaline-manganese for primary cells, and nickel-cadmium (NiCd), nickel-metal hydride (NiMH), and to some extent sealed lead-acid for secondary (rechargeable) cells. In the recent years, lithium based primary and secondary cells have been developed, which offer a much higher energy density than other cells due to their higher open-circuit voltage at he expense of higher cost and technological problems caused by the reactiveness of lithium. Zinc-air cells offer high energy density because the cathode reactant, oxygen, is not contained in the cell but is absorbed from the surrounding air. However, they are less robust than other cells and are not suitable for continuous high-current discharge [56]. Table 2.6 gives an overview of the key features of the most important battery technologies [59].
| Type | ![$\textstyle \parbox{4em}{\centering{\bf nominal voltage [$\rm V$]}}$](img324.gif) |
| alkaline | 1.5 |
| NiCd | 1.2 |
| NiMH | 1.2 |
| lead-acid | 2.0 |
| zinc-air | 1.2 |
| lithium-ion | 3.6 |
All of these batteries, except the lithium-based, have in common that the end-of-discharge cell voltage is about 0.9V and even less at higher discharging currents [53,59]. Worse yet, NiCd cells (to some extent also NiMH cells) exhibit a so-called memory effect, i.e., the capacity reduces when they are repeatedly discharged only partially. This shows in an additional voltage drop of about 150mV when the cell is discharged below its reduced capacity. Another cause of this so-called voltage depression is the growth of abnormally large crystals on the cadmium electrode during long-term trickle (i.e. low-current) charging. These effects altogether preclude single-cell powering of many applications in traditional technologies. Ultra-Low-Power CMOS, on the other hand, works perfectly with these low voltages and enables even mixed-analog-digital applications.