Wireless sensors are becoming increasingly essential to everyday life, with new applications emerging that offer enhanced system functionality such as periodic two-way communications and remote shut-off capabilities.
Satisfying the power-hungry demands of increasingly sophisticated wireless devices requires the selection of the ideal power management system; a decision-making process that takes on added importance if the device is intended for long-term use in extreme environments and/or hard-to-access locations.
Why LiSOCL2 chemistry is preferred
Among the different types of battery chemistries available for extended use in remote locations, bobbin-type lithium thionyl chloride (LiSOCL2) is overwhelmingly preferred because of its high energy density, wide temperature range and low annual self-discharge rate.
Bobbin-type LiSOCL2 batteries have a proven track record of success in powering remote wireless sensors, including AMR/AMI meter reading devices. For example, hundreds of thousands of meter transmitter units (MTU) that were installed in the mid-1980s were still operational after 28+ years on their original Tadiran batteries.
Achieving such incredibly long life demands an extremely low annual self-discharge rate as, in many instances, the total lifetime self-discharge rate of the battery can be greater than the total amount of energy consumed by the device itself.
However, not all battery brands are alike in terms of their annual self-discharge. Tadiran bobbin-type LiSOCL2 batteries can deliver an annual self-discharge rate of approximately 0,7%, enabling the Tadiran battery to retain about 70% of its initial capacity even after 40 years of self-discharge.
Other brands of LiSOCL2 batteries have a significantly higher self-discharge rate of between 2,5% to 3% per year. As a result, these competing batteries retain only 70% of their initial capacity after 10 years.
Powering advanced two-way communications
Choosing the right battery is especially important if the remote wireless device is designed to offer advanced two-way communications, which demand more energy from the battery.
In order to deliver enhanced functionality without sacrificing operating life, design engineers need to find ways to conserve energy so as to maximise battery life. For this reason, many remote wireless sensors are designed to operate mainly in ‘dormant’ mode, during which time daily power consumption ranges from nil to a few microamps, followed by brief periods where the device is in an ‘active’ mode that requires high current pulses of up to several amps to energise the device as it reads and communicates data before returning to its ‘dormant’ mode.
Wireless sensors that remain dormant at elevated temperatures and periodically require high current pulses may experience lower transient voltage readings during the initial phases of battery discharge. This phenomenon, known as transient minimum voltage (TMV), is strongly correlated to the chemical make-up of the electrolyte and/or the design of the cathode.
Tadiran offers two effective solutions to combat TMV: PulsesPlus batteries for high current pulse applications; and TRR Series batteries for applications requiring moderate current pulses.
PulsesPlus lithium thionyl chloride batteries combine a long-life bobbin-type LiSOCL2 cell with a patented hybrid layer capacitor (HLC) that stores and delivers high current pulses, when required.
If the application requires moderate current pulses, then Tadiran Rapid Response (TRR) Series batteries could offer a cost-effective solution, as they do not require the use of an HLC but still deliver high capacity and high energy density without experiencing voltage drop or power delay.
TRR Series batteries virtually eliminate this transient minimum voltage level, resulting in zero delay during the voltage response. They also utilise available capacity more efficiently, and can extend the operating life of the battery by up to 15% under certain conditions, especially in extremely hot or cold temperatures.
Rechargeable lithium batteries support energy harvesting
While bobbin-type LiSOCL2 batteries remain the preferred choice for long-term use in extreme environmental conditions, new applications are emerging that are well suited to energy harvesting technology in conjunction with rechargeable lithium batteries.
Energy is harvested from light (photovoltaics), heat (Peltier devices), flow (dynamos), as well as vibration (Piezo devices). Photovoltaic and Peltier devices produce continuous trickle charges, but not enough current to power a pulse. Dynamos and Piezo devices produce bursts of energy, but not always continuously. In both the ‘trickle’ and ‘burst’ devices it is necessary to store energy in order to deliver the high current pulses needed by remote wireless sensors.
Energy harvesting devices are typically combined with rechargeable lithium batteries that store harvested energy. Unfortunately, standard consumer rechargeable lithium-ion cells have inherent drawbacks that limit overall performance, including short operating life (maximum five years), low maximum cycle life (1000 cycles), a high annual self-discharge rate (up to 60% per year), and a limited temperature range (0°C to 60°C) with no possibility of charging at low and high temperatures.
To address these problems, Tadiran recently introduced TLI Series lithium ion batteries, which utilise a modified version of the patented HLC to deliver improved performance, including 10-year service life, 5000 recharge cycles, extremely low annual self-discharge (less than 5% per year), the ability to deliver high current pulses (up to 5 A), plus a much wider temperature range (-40°C to 85°C, with storage up to 90°C) with the ability to be recharged at extreme temperatures (10 hour rate). They are available in AA-size (1550), 1530 and 1520, as well as custom battery packs.
For example, TLI batteries were selected by a leading manufacturer of photovoltaic-powered parking meters to deliver extended life, increased recharge cycles, and deliver the high current pulses required for advanced cellular communications.
Choosing between rechargeable and non-rechargeable lithium batteries
Powercast Corporation specialises in low-power RF energy harvested from broadcast radio or television signals, and/or RF transmitters located within a 15 metre range. This solution can be an ideal alternative for networked wireless sensors that require microamps of power to operate and which are located in environments where there is access to suitable amounts of ambient RF energy.
For a different application – the WSN-1101 – Powercast chose to use Tadiran PulsesPlus batteries. The WSN-1101 is a wall-mounted sensor that measures indoor temperature, humidity and other variables in HVAC, lighting control, energy management, industrial monitoring and medical applications.
Designed for use in temperatures ranging from -20°C to +50°C, the WSN-1101 can transmit data once per minute for more than 25 years to the Powercast WSG-101 wireless gateway, which interfaces with wired building automation system (BAS) networks via industry-standard protocols.
Use of an LiSOCL2 battery enables Powercast to offer a highly cost effective and reliable 25-year solution that converts buildings into smart buildings, providing an ideal upgrade for older structures with concrete or cinder block walls that cannot be easily retrofitted for hard-wired solutions.
Extreme environments call for more robust batteries
If a remote wireless sensor is being designed to withstand extreme temperatures, then LiSOCL2 chemistry is generally preferred over rechargeable lithium batteries due to its wider temperature range and lower self-discharge rate.
Achieving reliable long-term battery performance under such extreme environmental conditions requires the use of an LiSOCL2 battery constructed with superior grade materials as well as proprietary manufacturing techniques. Use of inferior raw materials or non-standardised battery manufacturing techniques can affect batch-to-batch consistency and increase the risk of battery corrosion, which can severely jeopardise system operation and performance.
To ensure consistent performance and mitigate any risk of anomalies in the field, Tadiran uses state-of-the-art total quality management tools during all phases of battery manufacturing, including six sigma methodologies and statistical process controls (SPC).
As a result, Tadiran batteries are UL approved, able to withstand challenging tests for temperature, humidity, shock, vibration, puncture and other parameters
Having designed and manufactured lithium primary batteries for over 30 years, the company continuously collects test data on numerous parameters, including cell size, temperature, load size, etc.
Battery testing at Tadiran is an intensive process that is conducted in numerous ways to cross-check results. All batteries undergo long-term tests under different loads and test profiles, with the oldest batteries still operating after 24 years of continuous operation in the lab. The company also conducts accelerated testing, calorimetric testing, lithium titration and monitored field results. It has covered over 100 000 application points and tested the equivalent of over 10 000 operating years in the lab.
With battery replacement costs estimated at ten times the initial cost of the original battery, it is incumbent upon the end user to first verify the accuracy of all claims involving battery life expectancy based on typical annual self-discharge. In addition, it is important to know the heritage of the battery and the consistency of its manufacturing must be known in order to differentiate superior quality batteries from inferior knock-offs.
For more information contact Dylan Smith, Arrow Altech Distribution, +27 (0)11 923 9600, [email protected], www.arrow.altech.co.za
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