Battery-Charging Controllers for Energy Harvesters

Whether your energy harvesting application uses large solar panels with high voltages and currents or, more often the case, must make do with minute amounts of power derived from various other ambient energy sources, one thing is almost certain: some type of energy storage is on board, whether in the form of a small rechargeable lithium ion battery, a super capacitor, or solid-state energy storage technology.
For the engineer this means that not only do we need to design circuits to harvest and convert ambient energy, but we also have to include an energy-harvesting interface (and protection circuitry) as well as a charge controller.
This article looks at single chip energy harvesting devices that also provide some form of charge control. It discusses the different conditions under which energy can be extracted as well as what to expect when trying to squeeze power out of the ambient environment. Finally, the article will present some typical integrated solutions for small-sized low-power energy-harvesting designs.
Sleep and startup
Low power memory and micros can sit idle and asleep for long periods of time, drawing only nan amperes. During sleep cycles, the energy harvesting techniques employed should provide more power than the sleeping circuits require. When enough excess energy has been accumulated to do something useful, such as charging a capacitor or battery, the micro can wake up.
In some cases, the amount of energy needed for the task at hand is easily determined. For example, Figure 1 shows a sampling application in which a wireless low power sensor wakes up every so often, takes a reading, transmits it, and then goes back to sleep. It can be programmed to slowly draw microamperes from the environment to maintain a charge on a capacitor or battery.
Event driven designs are trickier. If an event needs to be responded to quickly, then the micro needs to be awakened more frequently to assure the necessary response time. The more it wakes up, the more energy it burns, and the more of a demand it creates on your harvesting system and storage cell.

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