'Energy Harvesting AND a Battery?'

Energy Harvesting AND a Battery?

By Eva Celia GTech at 01/28/09 14:23

Energy harvesting - whether powering a device from solar, RF, vibrational or other energy sources, has been the talk of the town. And only recently, there has been talk surrounding using energy harvesting to recharge a battery, particularly microbatteries (very small batteries). US companies like Cymbet and Infinite Power Solutions claim their rechargeable batteries can last years by hooking up an e-harvesting method - and they are absolutely correct. But both companies offer batteries that cannot provide above 1 MILLIamp hours of capacity - that's a problem.

Most of us sensor developers need capacities of 10 to 100s mah with expectations for recharging under 30 mins. Which is why I was interested in a recent rumor, along with an demo I heard of this past fall, that a "microbattery" company is launching a 10mah thin battery, complete with a wireless energy harvester to charge the battery, early this year. The company, Planar Energy Devices, already demonstrated the device to rave reviews during the Embedded Systems Conference. I'll be watching their site, www.planarenergy.com, for further updates and post when I get a chance to try out the battery.

E/H & a battery is indeed anathema to many E/H manufacturers; however, the truth of the matter is that for mesh networked sensors, the sensor node must be able to wake up periodically and listen for communications from neighboring nodes. Being dependent on the availability of harvestable energy cannot guarantee that; furthermore, I have to agree that 50 - 100microA-hr cells only provide back-up for a short period of time should the ambient energy go away (e.g., after dark). There are a few companies, however, that have been developing E/H systems that recharge NiMH or NiCd cells (think of those garden lights you can buy at home improvement stores). The challenge is temperature, limited cycle life under partial discharge conditions (memory effect), toxicity (NiCd). Li ion overcomes these problems, but requires careful control to avoid overcharging and over-discharging and cannot be trickle charged.

There is another new product on the market, known as EnerPak, that offers lithium ion back up capability. Features of the designinclude uninterrupted delivery of power to the sensor under dynamically varying harvested energy conditions, dynamic peak power tracking, a highly efficient two-stage energy storage system, low output impedance, and the ability to maintain the user’s desired data rate with a power system that lasts as long as its sensor host, eliminating many of the design trade-offs that exist between data rate and power source life for mesh networked sensors.

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