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Spotlight on long-life battery dilemma

16 Jul 2015  | Bill Schweber

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Selecting the ideal long-life battery, even at very low discharge levels, encompasses more than the basic current-and-time versus battery-capacity analysis. The rising interest in long-life embedded applications including remote data logging or utility meters brings up the eternal engineering question: how are we going to power these devices? These devices, where some are operating under difficult environmental conditions, should work on a small battery for 10, 20, or more years without any attention.

Long-life battery

Behind this deceptively simple symbol is a complex world of electrochemical and even radioactive activity.

For shorter-life installations on the order of a few years, the battery decision analysis begins with some basic analysis of the current drain on the battery under various duty cycles and operational modes compared to the energy capacity of the battery (mA-hour). This can get fairly involved for applications with complicated operational cycles, but it's not too hard to at least put an upper-bound, worst-case number on requirements. However, when you need to run for a decade or more, the basic electrical analysis of load current and power versus capacity is only a small factor. Issues such as self-discharge, chemical deterioration and enclosure corrosion become major concerns.

Which is why I was intrigued when I read "Designing and Fabricating a Multiple-Decade Battery in Aerospace & Defence Technology." The article detailed a thermoelectric generator (TEG) based on radioactive decay that could run for 150 years, in theory. The architecture used a two-step process that I had not read about before, where the decay generates light, and then that light generates power via photovoltaic cells. The authors allude to the low efficiency, but unfortunately give no numbers, although I suspect it is in the <5 per cent range.

TEGs powered by radioactive decay have been used successfully for decades, especially for deep-space vehicles where solar radiation is minimal. These TEGs use a single-stage conversion process based on heat of radioactive decay rather than the photons of the two-step process, with Seebeck-junction thermocouples to generate the electrical power from the decay's heat.

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