There is extensive literature available regarding the use of batteries and other energy storage devices, most focused on large energy storage for EV’s and backup power applications. Relatively little is written about selection of energy storage for IoT applications, or technologies and methods to maximize the life of energy storage to power wireless sensors.
This article will focus on the use of rechargeable energy storage technologies to improve the operating lifetime of wireless sensors. In particular, we will discuss the characteristics of high Cycle Life technologies such as Supercapacitors, Hybrid Supercapacitors and a lithium based battery technology, Lithium Titanate Oxide (or LTO), with use cases where and how these technologies can be used to boost sensor operational life.
Selecting and designing energy storage into an IoT device adds several layers of complexity to required lifetime or duration estimations and hence, technology selection. For example, wireless sensor operation with periodic measurement and transmission cycles that require relatively short bursts of energy can adversely impact battery storage capacity.
In addition, how frequently the energy is required, and over what time duration, will favor one type of energy storage technology over another, each with for different energy delivery profiles.
The energy storage characteristics of interest to wireless sensor designs include:
- High efficiency charge / discharge with less internally generated heat (safety), with simpler, lower cost Battery Management Systems (BMS).
- Ability to deliver high amounts of energy to the load over a short time duration (power) repeatedly, e.g. current impulse during radio transmission, without negatively impacting long term capacity.
- Low self-discharge (leakage), which robs the energy storage of its ability to deliver the rated capacity.
- High cycle life and charge current levels supportive of using Energy Harvesting (EH) as primary or supplemental source of energy.
As in any electrical application where the voltage may vary for any reason due to changes in the load, capacitors can be used to smooth out the available voltage by safely delivering energy (current) quickly to the load in high amounts when needed. ‘Supercapacitor’ technologies such as Electric Double Layer Capacitors (EDLC) have been developed for this purpose, as a complementary technology to batteries. EDLC technology can also deliver high power over many charge/discharge cycles with minimal degradation or aging of the device capacity when compared to battery technologies. Supercapacitors are often used in tandem with battery technologies for these reasons, delivering high power to the load rapidly when needed then to be recharged by the battery.
Supercapacitors store large amounts of charge electro-statically, such that when discharged the voltage drops steadily relative to the load. Rather than supply steady energy to a circuit with a relatively constant voltage as a battery does, the Supercapacitor can deliver power within a very short time duration, depending on the load. Note also that Supercapacitor datasheets describe the energy capacity in terms related to the physics of capacitance, such as Equivalent Series Resistance (ESR) and drop in capacitance, rather than the mAh capacity specified for batteries.
As an alternative technology option, so called ‘Hybrid Supercapacitors’ (also known as Li-Ion Capacitors) are …