Maximize the Energy from Long-Life Batteries
Battery lifetime is a key consideration for the development of the wireless sensor nodes that will populate the Industrial Internet of Things (IIoT). In many applications, the sensor nodes will need to be installed in locations that are difficult to reach let alone service. The sensor nodes need to be autonomous in terms of energy because it is too costly and difficult to run power lines to them or to have maintenance workers replace batteries regularly.
As well as low power consumption in the processing electronics, the battery itself needs to be able to support very long service times: potentially as long as 20 years. Many battery chemistries cannot support such a long service life even when supporting specialized low-energy electronics because of their self-discharge rate.
Lithium thionyl chloride chemistry, however, has a very low self-discharge rate. As a result, the chemistry provides the longest life and highest energy autonomy seen so far for primary battery technology that is suitable for IoT sensor nodes and other equipment where small size is important. The service life of the lithium thionyl chloride chemistry has been demonstrated over a period of close to 40 years. AA-size cells have exhibited a service life of more than 20 years in utility meters.
However, as the applications for long-lifetime battery-powered systems widen, it is important to take into account the characteristics of the lithium thionyl chloride chemistry. The addition of wireless communications to sensor nodes as well as actuation functions – such as the ability to open and close gas or liquid valves – increases the peak current required from the battery.
The microcontroller in a typical sensor-node or metering application will be asleep for much of the time, waking up to take readings at regular intervals to store them in local memory. At less frequent intervals, the microcontroller will activate the wireless communications module and send a packet of stored data to a gateway or server. When the wireless interface is transmitting, the current needed may reach 500 mA; but it needs to be only a period of a few hundred milliseconds.
Although the nominal ratings of the battery may appear to support such short-term peak currents, the inevitable effects of ageing can reduce the in-field lifetime. The available capacity from the battery is affected not only by the self-discharge rate, but also by a gradual rise in impedance caused by the generation of substantial current pulses.
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