Wireless Power for Harsh Environments

Remote Wireless Devices bound for extreme environments and hard-to-access locations demand more ruggedized power management solutions. Two technologies have emerged: lithium thionyl chloride (LiSOCL2) batteries; and energy harvesting devices coupled with rechargeable lithium-ion batteries modified for extreme temperatures.

When recharging or replacing a battery is not an option, the preferred choice is bobbin-type lithium thionyl chloride (LiSOCL2) chemistry due to its intrinsic negative potential, which exceeds that of all other metals. Lithium is the lightest non-gaseous metal, offering the highest specific energy (energy per unit weight) and energy density (energy per unit volume) of all available battery chemistries.

Lithium cells, all of which use a non-aqueous electrolyte, have normal open-circuit voltages (OCVs) of between 2.7V and 3.6V. The absence of water also allows certain lithium batteries to operate in extreme temperatures (-55C to 125C), with certain models adaptable to cold-chain temperatures down to -80C. Tadiran placed LiSOCl2 cells in a cryogenic chamber and subjected them to progressively lower temperatures down to -100C without failure.

Certain brands of bobbin-type LiSOCl2 batteries have been proven to last up to 40 years. However, not all batteries are created equal, as the use of inferior raw materials and/or non-standardized battery manufacturing techniques can severely limit battery service life. Claims regarding extremely low annual self-discharge at ambient temperatures may not be valid depending upon the size of the battery, its method of construction, or the application-specific temperature requirements, as a difference of just a few microamps in annual self-discharge rate can shave years off battery life expectancy.

Every application has unique power requirements based on numerous parameters, such as energy consumed in dormant mode (base current); energy consumed in active mode (size, duration, and frequency of pulses); storage time (which diminishes capacity); thermal environments; cut-off voltage (where a device stops operating as battery capacity diminishes); and annual self-discharge rate (which can be higher than the current draw).

If the application involves dormant periods at elevated temperatures, alternating with periodic high current pulses, then lower transient voltage readings can result during initial battery discharge. This phenomenon, known as transient minimum voltage (TMV), is common to bobbin-type LiSOCl2 batteries due to their low-rate design.

The use of supercapacitors in conjunction with lithium batteries to solve TMV has drawbacks due to high self-discharge rates, the need for balancing circuits, and limited temperature range. Bobbin-type LiSOCL2 cells have also been successfully modified to address TMV issues.

For example, PulsesPlus batteries for high pulse applications combine a standard bobbin-type LiSOCl2 battery 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 15A, thus eliminating the voltage drop that normally occurs when a pulsed load is initially drawn. The single-unit HLC works in the 3.6V to 3.9V nominal range to deliver high pulses and a high safety margin, thus avoiding the balancing and current leakage problems associated with supercapacitors.