Specifying the ideal power management solution for remote wireless devices found in extreme environments and hard-to-access locations requires more ruggedized solutions. Fortunately, two viable options are now available: lithium thionyl chloride (LiSOCL2) chemistry that can operate for 40+ years, and energy harvesting devices coupled with special rechargeable lithium-ion batteries designed for extreme environments that can deliver up to 20+ years of battery life. Lithium thionyl chloride chemistry is proven for use in extreme environments.
When recharging or replacing a battery is not an option, the preferred choice is bobbin-type 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 2.7 and 3.6V. The absence of water also allows certain lithium batteries to operate in extreme temperatures (–55 °C to 125 °C), with certain models adaptable to cold-chain temperatures down to –80 °C. LiSOCl2 cells were placed in a cryogenic chamber and subjected to progressively lower temperatures down to –100 °C, and the batteries remained operational.
Bobbin-type LiSOCl2 batteries have been field-proven to last more than 28 years, with plenty of unused capacity to last up to 40 years. However, bobbin-type LiSOCl2 batteries are not created equal, as superior-grade LiSOCl2 batteries are constructed using high-quality materials and advanced manufacturing techniques that reduce the potential for electrolyte leakage or short circuits. Use of inferior raw materials or non-standardized battery manufacturing techniques can lead to batch-to-batch inconsistency, which severely limits battery service life.
As a result, design engineers need to be leery of battery manufacturer’s claims regarding low annual self-discharge at ambient temperatures, as these claims may not be valid depending upon the size of the battery, its method of construction, or the application-specific temperature requirements; a difference of just a few microamps in annual self-discharge rate can translate into years of reduced battery life expectancy.
Matching Performance to Applications
Increasingly, remote wireless devices feature two-way RF communications and/or remote shut-off capabilities that can reduce battery life expectancy. To maximize battery life, these devices typically operate in a “dormant” mode with nominal average daily power consumption, periodically requiring high pulses for data acquisition and transmission that range from hundreds of milliamps for short-range RF communications, up to a few amps for certain GPRS protocols.
Every application has unique power requirements based on application-specific parameters, such as:
- Energy consumed in dormant mode (the base current)
- Energy consumption during active mode (including the size, duration, and frequency of high-current pulses)
- Storage time (as normal self-discharge during storage diminishes capacity)
- Thermal environments (including storage and infield operation)
- Equipment cut-off voltage (as battery capacity is exhausted, or in extreme temperatures, voltage can drop to a point too low for the sensor to operate)
- Battery self-discharge rate (which can be higher than the current draw from average sensor use)
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. One alternative solution is to use supercapacitors in conjunction with lithium batteries. However, supercapacitors have performance limitations related to high self-discharge rates, their need for balancing circuits, and limited temperature range, which is not ideal for extreme environments.
Bobbin-type LiSoCl2 chemistry has been successfully modified to address TMV issues. Tadiran’s PulsePlus batteries 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 15 A, thus eliminating the voltage drop that normally occurs when a pulsed load is initially drawn. The single-unit HLC works in the 3.6–3.9V nominal range to deliver high pulses and a high safety margin, thus avoiding the balancing and current leakage problems associated with supercapacitors. The batteries can also be programmed to deliver low battery status alerts.