Harsh environments complicate the choice of power management system, especially if a wireless device is to be deployed in a remote, inaccessible location. When reliable, long-term power is required, and battery replacement or recharging are not viable options, lithium chemistry is preferred due to its intrinsic negative potential, which exceeds that of all other metals. The lightest non-gaseous metal, lithium offers the highest specific energy (energy per unit weight) and energy density (energy per unit volume) of all chemistries, with normal OCVs ranging from 2.7 and 3.6V. Lithium batteries are also non-aqueous, able to withstand far colder temperatures than water-based chemistries.
While numerous types of primary lithium batteries are available, bobbin-type lithium thionyl chloride (LiSOCL2) batteries are overwhelmingly preferred for remote wireless applications due to unique performance characteristics, including extremely high energy density (1420 Wh/l), high capacity, and the ability to withstand extreme temperatures (-55oC to 150oC), with certain models modified to operate in the cold-chain, where temperatures can reach -80°C.
Certain bobbin-type LiSOCL2 cells feature exceptionally low annual self-discharge rate (less than 1% per year), allowing a 40-year operating life. Low annual self-discharge is critical, as many remote wireless devices spend the vast majority of time in a “standby” mode, where daily energy consumption is nil or negligible, causing the total lifetime self-discharge of the battery to be greater than the total amount of energy consumed by the device.
Battery self-discharge can be negatively impacted several ways: through the chemical composition of the electrolyte; the manufacturing processes used to make the battery; storage and operational environments; high levels of impurities in the electrolyte; or impedance between internal components, which can be controlled by blending special additives into the electrolyte.
High pulse applications require customized solutions
A growing number of wireless devices require bursts of energy for data sampling and transmission, requiring standard LiSOCL2 batteries to be modified to handle higher pulses. If the application involves dormant periods at elevated temperatures, alternating with periodic high pulses, lower transient voltage readings can result during initial battery discharge. This phenomenon, known as transient minimum voltage (TMV), affects bobbin-type LiSOCl2 batteries due to their low-rate design.
One alternative is to create a hybrid lithium thionyl chloride battery that combines a standard long-life bobbin-type LiSOCL2 cell with a patented hybrid layer capacitor (HLC). The battery and HLC work in parallel, with the battery supplying long-term low-current power while the HLC supplies pulses up to 15 A, thus eliminating the voltage drop that normally occurs when a pulsed load is initially drawn. The single-unit HLC works in the 3.6–3.9 V nominal range to eliminate TMV and deliver high pulses while avoiding the balancing problems, current leakage, and bulkiness associated with supercapacitors.
If the application requires low to moderate pulses, a cost-effective solution may be to use an LiSOCL2 cell that has been specially modified without the use of an HLC to virtually eliminate the TMV and power delay issues common to standard LiSOCL2 cells These modified LISOCL2batteries operate very efficiently and can extend battery operating life up to 15% for certain applications involving extreme temperatures.
Bobbin-type LiSOCl2 batteries are not created equal
When designing a wireless device for use in a harsh environment, avoid batteries made with inferior raw materials or sub-standard manufacturing processes, as they can be prone to electrolyte leakage, short circuits, and reduced service life. For instance, the annual self-discharge rate of a leading brand of LiSOCL2 battery can be less than 1% per year, enabling 40-year operating life, while a lesser quality LiSOCL2 battery may deliver an annual self-discharge of 2.5% to 3% per year, which, over time, translates into much shorter battery life expectancy.
Careful due diligence is required throughout the vendor selection process, as claims of exceptionally long battery life need to be validated based upon the size of the battery, its method of construction, as well as environmental considerations. Start by demanding full product traceability all the way back to the raw materials, as well as fully documented and verifiable test results for battery pulses, low-temperature pulses, discharge, and repeatability.
To ensure that your remote wireless device is robust and reliable, select a primary lithium battery that has been field proven to deliver long-term performance in harsh environments.