The IIoT (Industrial Internet of Things) concept is rapidly expanding into remote sites and extreme environments that require intelligent battery-powered solutions. These applications include asset tracking, supervisory control and data acquisition (SCADA), environmental monitoring, machine-to-machine communications, artificial intelligence/machine learning and more.
Most remote wireless devices are powered by primary (non-rechargeable) lithium batteries, operating primarily in a stand-by state, drawing average current measurable in micro-amps, alternating with multi-amp pulses to power wireless communications.
As the lightest non-gaseous metal, lithium delivers the highest intrinsic negative potential, highest specific energy (energy per unit weight) and highest energy density (energy per unit volume) of any battery chemistry. Lithium batteries feature an operating current voltage ranging from 2.7 to 3.6V. They are also non-aqueous, thus less susceptible to freezing.
Among primary lithium chemistries, bobbin-type lithium thionyl chloride (LiSOCl2) is overwhelmingly preferred for remote deployments due to its extended temperature range, high capacity, high energy density and extremely low annual self-discharge rate.
Minimizing self-discharge
Numerous factors need to be considered when specifying an ultra-long-life battery. These parameters include the amount of current consumed during active mode, including the size, duration and frequency of pulses; the amount of current consumed during ‘stand-by’ mode (i.e., the base current); storage time—where self-discharge diminishes cell capacity; thermal environments (during storage and operation); and cut-off voltage, which drops as cell capacity is exhausted or from prolonged exposure to extreme temperatures.
Perhaps of greatest importance is the cell’s annual self-discharge rate, which often exceeds the amount of energy consumed while operating the device. Self-discharge results from the chemical reactions that occur even when a cell is not in use or is disconnected. Self-discharge can be significantly reduced by harnessing the passivation effect.
Unique to LiSOCl2 chemistry, passivation occurs when a thin film of lithium chloride (LiCl) forms on the surface of the anode to separate it from the electrode, thereby reducing the chemical reactions that cause self-discharge. Whenever a continuous load is applied, the passivation layer causes initial high resistance and a temporary drop in voltage until the layer of LiCl begins to dissipate, which is a continually repeating process. How a cell is manufactured, and the quality of the raw materials, can significantly impact the passivation effect and thus self-discharge. Superior quality bobbin-type LiSOCl2 cells can feature a self-discharge rate as low as 0.7% per year, meaning they are able to retain 70% of their original capacity after 40 years. By contrast, inferior quality LiSOCl2 cells can lose up to 3% of their capacity each year due to self-discharge, exhausting 30% of their total energy every 10 years, making a 40-year battery life unachievable.
