Two ways to power remote, portable devices are tiny, long-life batteries and energy harvesting techniques. The choice depends on the demands of the particular application. Power conservation is a critical component of the wireless sensor networks used in hundreds of commercial, industrial, medical, consumer, and military applica-tions characterized by low power requirements and low data rates. Long battery life (typically 5-10 years) is essential in wireless sensor networks (WSNs) where line power is not available or where the sensor nodes are mobile. Because the philosophy of sensor networks is "wireless anywhere," the size of a sensor node is also critical. Many applications require nodes not much larger than a dime (see Figure 1). In these cases, even AA batteries are too bulky and coin cell batteries are the only option. WSNs are capable of operating on sub-milliampere power, allowing 3 VDC coin batteries to power a sensor node for up to 10 years and beyond, depending on the sampling rate.
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