Thin film battery recharging from micropower energy harvesting sources

Abstract

Energy harvesting systems can eliminate the need for battery maintenance. However, in many cases the amount of instantaneously available harvested energy may not be sufficient to power an electronic circuit. Therefore, it is desirable to efficiently accumulate and store energy until enough energy is available to do something practical, such as power a wireless sensor. We conducted a review of energy storage options to allow wireless circuitry to operate in the presence of intermittent ambient energy and to provide an energy reserve for operation when the source of ambient energy has subsided, i.e. the machine is ??????off??????. We found that conventional battery chemistries are not compatible with the low power outputs provided by energy harvesters. However, thin film lithium batteries have nearly ideal characteristics for use as an energy storage element in energy harvesting circuits. Thin film lithium batteries have high energy density per unit volume (450 WHh/L), low loss due to self discharge (<1%/year), and are extremely thin (<20 microns thick) and flexible. Furthermore, they can be trickle charged at extremely low currents, they can tolerate more than 60,000 charge/discharge cycles, and they exhibit high charging efficiency. However, thin film batteries have several downsides, including low capacity and strict recharging requirements once the battery has been fully depleted. Successful use of these batteries requires careful design of the recharging and application load disconnect circuitry. This paper will review charging circuitry requirements and present several practical energy harvesting wireless sensor applications utilizing thin film batteries.