How to precisely monitor battery discharge

The number of interconnected devices that monitor everything from heart rates to room temperatures or building occupants is ever expanding. It is now commonly referred to as the Internet of Things (IoT). New applications are created every day to measure and report all types of data via wireless local networks which in turn may connect via gateways directly to the Internet. If the pundits are correct, we will soon have the ability to monitor the health and operating status of every appliance in our homes, turn off all the lights, and learn the exact location of our pets, all with a few finger swipes on our smart phones. Ubiquitous wireless monitoring will enable observation and control of our surroundings anytime, anywhere.

On a more utilitarian note, the Internet of Things has also manifested itself in industrial settings in the form of wireless sensors arrayed in vast mesh networks. Such wireless sensor networks are used in factories, industrial sites and on vehicles and machinery around the world to monitor critical parameters and improve safety, reliability and timely maintenance. Regardless of their intended use, such wireless devices all share a common problem: how do they get their power?

Clearly, there are many alternatives to consider. Wireless monitors should be small and unobtrusive, and they should require minimal maintenance. In the IoT world of tomorrow, experts suggest that many of these devices will be self-powered via optimised energy harvesters capable of providing an endless source of power. While such a prospect sounds ideal, and considerable progress has been made to improve the practicality of energy harvesting, solutions today often fall short in terms of size and performance, and there will always be cases where power is needed and no harvestable energy is available. Fortunately, battery technologies exist which are optimised for long lifetime, low average power applications such as those on the IoT spectrum.

Lithium Thionyl Chloride: The ideal wireless sensor energy source
IoT applications tend to have similar power and energy requirements. The average power for remote monitors is typically very low, with an occasional need to measure and broadcast data in a bursty fashion. The ideal battery for such applications would therefore favour energy density over power density. In addition, battery self-discharge should be minimised to enable the longest possible operating time and to reduce the need for costly downtime and maintenance to replace batteries. An excellent battery technology for such applications is lithium thionyl chloride (Li-SOCL₂).

This battery chemistry provides extremely low self-discharge (shelf life of 20 years plus claimed by several suppliers), very high energy density and a relatively high 3.6V typical operating voltage. Li-SOCL₂ batteries are widely available from numerous suppliers in many different shapes, sizes and capacities. However, as with most highly specialised technologies, usage comes with a set of trade-offs.

Challenges using long lifetime batteries
Realising the lifetime (capacity) benefits of Li-SOCL₂ batteries requires particular care when designing the application circuitry. As can be seen in figure 1, lithium thionyl chloride batteries have a very high output impedance. The chemical reaction that enables extremely low self-discharge and long shelf life (passivation formation) has the unwanted effect of limiting the available output current. Even when the passivation layer has dissipated due to periodic loading of the battery, the peak currents that may be supplied are low compared to other battery chemistries for a given amp-hour capacity rating.

With lithium thionyl chloride, high current draw results in not only a reduced operating voltage but also a reduced battery capacity. Operating the battery of figure 1 with a 100mA DC load results in 9 amp-hours of capacity, considerably below the peak value of 19 amp-hours, which occurs with a 4mA load. Hence, applications requiring momentary high peak currents must employ capacitor storage in parallel with the battery to handle the periodic short-term power bursts, as well as some form of battery current limiting during peak loads in order to maximise available capacity.