March 29, 2012 -- The advent of inexpensive, energy-efficient embedded processors and smart-object technologies has fueled the growth of intelligent distributed networks for applications for smart buildings, the Smart Grid, and environmental monitoring. The same technologies also are being used to add intelligence to existing products such as utility meters and health-monitoring and portable medical equipment.

While the operational and design requirements vary widely, all of these applications must operate reliably for years using the limited energy resources available from their batteries or energy harvesting systems.

Many of these applications such as smart wireless water and gas meters, which may be buried or otherwise difficult to access, present significant design challenges. To achieve service lives of 10 to 20 years, they use batteries with specialized low-leakage chemistries that can cost $0.75/Wh or more.

As a result, the meter's energy budget must be aggressively trimmed to maintain pricing that's competitive in the global marketplace. Even when a system is accessible, like the condition-monitoring system for a shipping container, it isn't realistic to expect it to be maintained more than a few times during a 20-year service life, if at all.

A typical design might have an energy budget that allocates 20% of the available capacity to transmit activities, 30% for receiving commands, and 20% for data collection and system maintenance. This leaves a mere 30% of the battery's capacity to support the system's idle mode. At first glance, this may seem a high proportion of the total battery life dedicated to doing no useful work. The problem is that the device will spend more than 99.9% of its life in this mode.

By Anders Guldahl. (Guldahl is an application engineer at Energy Micro AS.)