As the IoT Expands into Warmer Climes, we Need to Rethink Battery Selection

Mobile IoT, or licensed LPWA – the family of technologies incorporating LTE-M and NB-IoT – provide without question the network technology of choice for cellular IoT devices with lower energy requirements. This is for several important reasons, but in particular the reliability of Mobile IoT makes it the go-to option in use cases which cannot easily be serviced, but must nonetheless be depended upon to provide continuous functionality.  This is the case for instance in applications like smart metering, where many devices in often hard-to-reach locations must be connected reliably for very long periods.

The need for consistency in connectivity is mirrored in how such IoT devices must be powered.  Any manufacturer selecting batteries for smart objects faces a challenge here – being able to provide units that are both appropriately configured and optimally sized, while also meeting the customer’s expectations in terms of lifespan.

So how can OEMs and developers select the most appropriate model, and choose batteries which maximise the working lives of IoT devices?

There are a few things for them to bear in mind.  Among the most important of these, they need to consider environmental factors like operating temperature, which has a profound impact on the performance of batteries: low temperatures will slow down electrochemical reactions, and higher ones encourage passivation, particularly for lithium thionyl chloride cells (Li-SOCl2)– a process whereby a layer of lithium chloride builds up on the battery’s anode while it’s dormant, or only solicited at very low discharge currents – which can lead to malfunctioning. The thicker this layer, the higher the impedance within the cell becomes, the lower the voltage response, and the slower the voltage recovery of the battery – which can lead to a shorter life for the entire device itself.

Another phenomenon to consider is self-discharge, whereby a battery leaks power by essentially consuming itself, a condition which affects all battery models to some degree. Self-discharge too is accelerated with rising temperatures; as the geographical reach of IoT applications expands to warmer parts of the globe, as the IoT expands in particular through Asia, but also in Africa and South America, OEMs are increasingly tasked with choosing the right battery for optimal performance in extreme temperatures.

In view of this, what are the appropriate technologies available to those going through this selection process? It’s a given that only lithium batteries will suffice here – but which specific chemistries?

Li-SOCl2 cells are highly adept at serving devices like smart meters, reliably and over long periods, and millions of such cells have already been deployed successfully worldwide. With a proven combination of high energy density, wide temperature ranges in which they can operate, low self-discharge and stable nominal voltage. These models have a 20-year lifespan and are optimised for legacy electronics, or those with high energy but low power requirements.

New specifications and protocols for communication devices however bring new challenges and demands on autonomous power supplies – and in practice that means they require higher power pulse capabilities, and the ability to operate in a wider range of temperatures. A new generation of cells based on Li-MnO2 has been developed. They have the same 20-year lifespan as the Li-SOCl2 variants, but are optimised for the latest generation of electronics, making them a particularly good candidate for IoT devices where more power is needed.

We interviewed Cecile Joannin, Market Manager for IoT at SAFT, on her views on battery selection. “We’ve built on our extensive experience to develop a mathematical model to forecast a battery’s run time,” Ms Joannin explained, “which now plays a critical role in helping determine the optimum battery chemistry and size for a specific application, and in helping industries assessing the life they can expect from batteries. It’s clear that for now most applications will do well with the 2 existing Li-SOCl2 ranges: high energy and high power cells. These feature exceptional operating temperature range, and are environmentally friendly power solutions thanks to their life duration, which helps avoid costly field replacements.”

As the IoT expands globally however, and IoT devices grow in sophistication, the role of Li-MnO2 will become more central. For use cases at the higher power end of the LPWA spectrum, the safest and most reliable batteries of choice will increasingly rely on lithium manganese dioxide – and SAFT’s recognised best-in-class manufacturing of these places us in pole position to meet this growing need around the world in the years ahead.