Batteries in the Internet of Things

The potential applications for the Internet of Things (IoT) are myriad in market sectors such as industrial, environmental monitoring, energy management, home and building automation, medical and healthcare devices and systems, and many other diverse markets including a growing multitude of mobile consumer and wearable gadgets.

Central to the IoT paradigm is the deployment of autonomous and wirelessly connected sensor based devices and systems that both collect and communicate data, often via a gateway and up into the cloud. Many IoT devices will be describing environmental conditions in some very remote locations in many applications. And how these potentially distant sensor-based devices are powered is a critical issue: system requirements will demand energy storage sources that deliver either a long operating life to keep maintenance schedules manageable, or in many cases will need to be recharged in conjunction with complementary energy harvesting mechanisms and storage systems – although rechargeable batteries have limitations in terms of the number of charge-discharge cycles and will also need eventual replacement. One potential alternative approach is the use of rechargeable solid-state bare-silicon-die batteries developed by Cymbet. The Enerchip range offers 3.3 and 3.8V outputs, an operating temperature range of –20 to +70°C, and can be highly cost effective for use in highly integrated and lightweight designs, as well as having the advantage of a minimal negative environmental impact.

In many applications, these data-capturing sensor-based devices are quite likely to run in sleep or standby mode for much of their existence, only consuming significant amounts of energy intermittently and then only for brief periods of time, thereby significantly extending battery life. While energy-harvesting techniques are seeing continual improvement, IoT based sensors that are more active will certainly demand the more reliable resources of a primary source such as a battery.

There are many trade-offs in the choice of energy source including electrical characteristics such as voltage output, lifetime power requirement, energy density and costs such as those for assembly and maintenance. Other important factors are that sensor devices will most likely need to be compact and lightweight, hence the attractiveness of the coin cell or button battery format. Fortunately, many products that employ technologies required for this universe of IoT sensor devices have already been developed for many different markets by leading battery makers for portable products such as watches, cameras and calculators and are being increasingly used in the embryonic and fast growing wearables market. The lithium based CR2032 coin cell battery is a classic example.

A major supplier of button/coin cell non-rechargeable batteries is Panasonic. The company’s 3V CR type batteries employ the highly common lithium manganese dioxide based chemical makeup (lithium anode and manganese dioxide cathode), feature capacities from 35 up to 1000mAh and are suitable for intermittent high load current applications. The Panasonic BR type batteries are based are lithium polycarbon monofluoride and offer a more moderate energy density than the CR type with capacities from 48 to 225mAh, but also deliver high reliability and higher operating temperatures up to +80°C.

An alternative choice is the RS branded range of 3V lithium manganese dioxide coin batteries with capacities from 36 to 610mAh or the RS branded range of 1.55V silver oxide coin batteries with capacities from 14 to 165mAh and offering a longer lifetime and very good low temperature performance characteristics.

 

 

Figure 1: RS branded 3V CR2032 lithium manganese dioxide coin button battery

 

Finally, a further option is the 3.6V PCB-mounting lithium thionyl chloride non-rechargeable coin cells from Tadiran Batteries. This type of chemistry is more commonly used in batteries for industrial and medical applications rather than in consumer markets, and features the highest energy density and voltage of all commercial lithium types. The Tadiran SL-840 (0.42Ah) and SL-889 (1Ah) offer very high energy density, high terminal voltage, level voltage discharge and a very long shelf life. Able to last from 15 to 25 years, they are particularly suitable for remote locations and challenging environmental conditions and are ideal for applications that require low continuous currents and/or moderate pulse-currents (up to 10mA and 20mA, respectively, for the SL889 battery for example).