There is extensive literature available regarding the use of batteries and other energy storage devices focused on large energy storage for EV’s and backup power applications. Relatively little is written about energy storage for IoT applications, or how the various technologies might be applied to increase the operating life of energy storage in wireless sensor applications.
In this series of articles we will discuss small capacity energy storage technologies that may be applied in wireless sensors, the various technologies available, their characteristics and how they can be leveraged to extend the operating life of a wireless sensor.
Figure 1. A selection of battery technologies, left to right a) Coin cell, lithium manganese dioxide chemistries (Li-MnO2), b) Alkaline, replaceable batteries commonly used for consumer applications, c) Lithium Thionyl Chloride (Li-SOCl2), a high density technology with a flat discharge voltage often found in wireless sensors and d) Lithium Ion (Li-Ion), e) Lithium Polymer (Li-Po) both are high capacity rechargeable technologies that can be manufactured in various sizes and shapes, hence likely used in your mobile device.
Battery capacity is defined by the amount of steady state current that the battery can deliver at 25°C before the voltage drops below a pre-defined useable level (or on a single charge, if rechargeable), typically defined as the Cut-off voltage. Battery vendors typically specify charge capacity (in A-Hr) associated with a nominal average discharge current, providing voltage as a function of discharge time.
Various battery technologies have been developed to deliver a variety of charge voltage levels, charge capacity (size), energy density (chemistry and manufacturing), recharge capability and form factors, each suited for different applications.
Supercapacitor based devices broaden the energy storage device category by storing charge between electrode plates electrostatically. These devices are optimized to deliver power to a load in a short time e.g. behaving like capacitors.
Hybrid Supercapacitors (also known as Lithium Ion Capacitors) mix battery technology with supercapacitor construction, trying to optimize the power delivery with charge storage capacity, such that these devices will also be included in this discussion.
To complete the survey, we should point a relatively recent addition to Lithium based variations, Lithium Titanate Oxide, or LTO. This technology delivers much higher Cycle Life than other lithium technologies with lower self leakage. Products available today cite 25,000 charge/discharge cycles and self-discharge less than 5%, performance associated with Supercapacitors. This ‘hybrid’ battery technology provides modest capacity like a battery, though has lower energy density than other lithium technologies.
A partial list of energy storage technologies[1] available today are …
