Energizing Remote Wireless Devices

Wireless devices deployed in remote locations and extreme environments require industrial-grade lithium batteries.

Resensys structural stress monitoring of a bridge.
Resensys structural stress monitoring of a bridge.
Tadiran

Remote wireless devices require specialized battery-powered solutions, especially for low-power applications that operate mainly in a โ€œstand-byโ€ state while periodically drawing pulses in the multi-amp range for an average current measurable in micro-amps.

Common applications include asset tracking, system control and data automation (SCADA), seismic, infrastructure and environmental monitoring, M2M, AI, and machine learning, to name a few.

When designing a low-power solution, you need to factor in the amount of current consumed during active mode (including the size, duration, and frequency of pulses); energy consumed during โ€œstand-byโ€ mode (the base current); storage time (as normal self-discharge during storage diminishes capacity); thermal environments (including storage and in-field operation); equipment cut-off voltage (which drops as cell capacity is exhausted or during prolonged exposure to extreme temperatures); along with the annual self-discharge rate (which typically exceeds the amount energy consumed while operating the device).

As the lightest non-gaseous metal, lithium features the highest intrinsic negative potential, specific energy (energy per unit weight) and energy density (energy per unit volume) of all, operating within a normal operating current voltage (OCV) range of 2.7 to 3.6V. Lithium batteries are also non-aqueous, thus able to survive Arctic temperatures.

Available primary (non-rechargeable) battery chemistries include iron disulfate (LiFeS 2 ), lithium manganese dioxide (LiMNO 2 ), lithium thionyl chloride (LiSOCl 2 ), and lithium metal oxide, along with consumer alkaline. Table 1 shows that bobbin-type LiSOCl 2 batteries are unrivaled for their wider temperature range, higher capacity and energy density, and lower annual self discharge.

Table 1Table 1

Controlling battery self-discharge

Self-discharge is common to all batteries, as chemical reactions occur even when the battery is not in use: a phenomenon best controlled by harnessing the passivation effect.

Passivation involves a thin film of lithium chloride (LiCl) that forms on the surface of the lithium anode to separate the anode from the electrode, thus limiting the chemical reactions that cause self-discharge. Whenever a load is placed on the cell the passivation layer causes initial high resistance and a temporary drop in voltage until the discharge reaction begins to dissipate the passivation layer: a process that keeps repeating.

The level of passivation can be influenced by numerous factors, including the cellโ€™s current discharge capacity, the length of storage, storage temperature, discharge temperature, and prior discharge conditions, as partially discharging a cell then removing the load increases the level of passivation over time. Passivation is ideal for reducing self-discharge, but too much of it can overly restrict energy flow.

Passivation and self-discharge can vary based on the method of manufacturing and the quality of the raw materials. For example, a superior quality bobbin-type LiSOCl 2 cell can deliver a self-discharge rate of just 0.7% per year, retaining 70% of its original capacity after 40 years. By contrast, a lower quality bobbin-type LiSOCl 2 cell can have a self-discharge rate of up to 3% per year, losing 30% of its capacity every 10 years, limiting potential operating life to 10-15 years.

Two-way wireless communications draws additional energy

Remote wireless devices increasingly require periodic high pulses to power two-way wireless communications. However, standard bobbin-type LiSOCl 2 cells cannot deliver high pulses due to their low-rate design. This can be solved with a hybrid solution that adds a patented hybrid layer capacitor (HLC). The bobbin-type LiSOCl 2New PulseplussiloTadiran delivers low-level background current during โ€˜standbyโ€™ mode while the HLC delivers up to 15A pulses during โ€˜activeโ€™ mode when data is being queried or transmitted. As an added benefit the HLC features a unique end-of-life voltage plateau that permits โ€˜low batteryโ€™ status alerts.

This hybrid solution is preferred over supercapacitors for industrial applications. While found in many consumer electronic devices, Supercapacitors have significant drawbacks for industrial applications, including: short-duration power; linear discharge qualities that do not permit full discharge of available energy; low capacity; low energy density; and a very high self-discharge rate of up to 60% per year. Supercapacitors linked in series also require bulky and expensive cell-balancing circuits that draw additional energy to further shorten their operating life.

Does your application require a battery that can last as long as your device? Then do your due diligence and demand long-term test results, in-field performance data under similar environmental conditions, and multiple customer references. This knowledge could help to reduce your cost of ownership since the impact of higher annual self- discharge may not become apparent for years and is commonly underestimated by theoretical models.

Sol Jacobs is the vice president and general manager of Tadiran Batteries.

More in Products