There are many application areas where the design engineer would like to have, or truly must have, a system that needs no servicing at all for many, many months, or many, many years. Remote sensors of all types—such as forest fire sensors, earthquake recorders and sensors that monitor the changing thickness and location of icebergs—are some examples. Home smoke and carbon monoxide sensors are two more.
Choosing the right power supply can be a highly application-specific decision. If your design is sufficiently low power, a good choice might be a primary lithium battery. You may be surprised how long they can keep working.
Back in 1984, an automatic meter reading (AMR) device manufacturer named Aclara (formerly Hexagram) started using bobbin-type lithium thionyl chloride (LiSOCL2) batteries to power early versions of their meter transmitting units (MTUs). The powerful little batteries operated continuously for over 28 years. When it came time to replace these older devices with newer generation technology, tests showed that these decades-old batteries still had plenty of available capacity.
This example shows that certain primary lithium cells can work for up to 40 years without any maintenance at all. However, a designer's due diligence is required to verify that the particular manufacturer's battery can perform as promised. In order to maximize operating life, these cells need to be produced to the highest standards possible using superior quality raw materials and precisely controlled manufacturing processes. Engineers should be certain that they can receive verified test results.
Recently, I looked over the landscape of primary (non-rechargeable) batteries, which are becoming more important as so many applications involving low-power wireless devices become connected to the fast-growing industrial internet of things (IIoT). There are a number of primary lithium chemistries available that offer big advantages over consumer type alkaline batteries (MnO2). One example is the lithium iron disulfide (Li-FeS2) type that is commonly available at any drug store. While lithium batteries usually deliver 3 volts or higher, Li-FeS2 provide 1.5 V, which is compatible with consumer type AA and AAA formats. This technology offers an energy density of 650 Wh/l.
Another popular primary lithium battery is the CR123A, which uses lithium manganese dioxide (LiMnO2) chemistry to deliver 3.0 V to 3.3 V, also with an energy density of 650 Wh/l.
The Most Popular Choice for Remote Wireless Applications
The cell type most commonly preferred for remote wireless applications is lithium thionyl chloride (LiSOCl2). This battery chemistry delivers the highest energy density available—up to 1420 Wh/l. This cell chemistry also features a wide operating temperature range and, critically, an extremely low self-discharge rate. Common applications for LISOCl2 batteries include AMR/AMI, mobile asset tracking, security devices, emergency beacons, GPS devices, and other remote sensing applications.
Two Types of LiSOCl2 Batteries
Two types of lithium thionyl chloride cells are available: bobbin and spirally wound construction. Bobbin-type cells have higher impedance than spirally wound batteries, but offer a higher energy density and a much lower self-discharge rate. You can achieve lower impedance and higher pulse current with spirally wound cells due to the increased surface area of the wound cathode. However, the trade-offs are significant, as spirally wound cells have lower energy density (due to more inactive material within the cell) as well as a much shorter operating life than bobbin-type construction.
Bobbin type construction is particularly well suited for applications that draw low average daily current. These cells feature a self-discharge rate as low as 0.7%/yr, enabling certain cells to operate for up to 40 years. Bobbin-type lithium thionyl chloride cells also offer an extended temperature range of up to -55° C to 125° C, with specially modified cells designed for use in the cold chain at -80° C. The ability to withstand broad fluctuations in pressure, temperature and shock make bobbin-type cells ideal for deployment in remote locations and extreme environments.
One type of bobbin type LiSOCl2 battery is the TL-4955 2/3 AA size from Tadiran, which features a nominal capacity of 1.65 Ah at a 0.5 mA discharge rate. Nominal voltage is 3.6 V and the maximum recommended continuous current is 35 mA. Pulse capability is 75 mA for one second. The TL-4955 battery weighs in at only 12.5 g (0.441 oz) and has a volume of 5.2 cc. Operating temperature range is -55º C to 85º C, and a high temperature version is available. This cell is U.L. recognized (MH 12193), and various termination types are available.
A Talk With An Expert
To further check out my findings on these types of cells, I contacted Sol Jacobs, VP and General Manager of Tadiran Batteries, a leading manufacturer of bobbin-type LiSOCI2 cells. I asked him how they confirm the extremely low self-discharge rates.
“First off, for long lifetime, you must ensure the cell is made from high-quality materials in a very carefully controlled and highly automated production line,” Sol said. “Secondly, Tadiran does continuous testing to ensure our cells will operate continuously even in the most challenging performance environments. For example, we use accelerated testing run at 72° C, which is the equivalent of about 32x the lifetime operation at 22° C. And, we test at both 72° C and room temperature. We test with a one-month discharge rate, and at two, three, four, five, and six-month rates, and some at a one-year rate. We then cross check our accelerating testing data against other well-recognized test procedures along with data from the field to ensure that our predictive models are highly accurate.”
For decades, test engineers at Tadiran have accumulated more than 100,000 application test points over the equivalent of over 10,000 operating years in the lab, then double checking these test results against actual field data to provide redundant proof that certain bobbin-type LiSOCI2 batteries can achieve 40-year battery operating life by featuring a self-discharge rate of 0.7% per year. This testing conclusively demonstrates that Tadiran batteries can last up to 4x longer than competing cells that have a much higher self-discharge rate of up to 3%/Yr. As a result, an inferior made bobbin-type LiSOCl2 battery can lose up to 30% of its available capacity every 10 years, making 40-year battery life virtually impossible.
Designer engineers need to be mindful that if a standard bobbin-type LiSOCl2 battery is left completely off for an extended time, a passivation layer can form between the anode and the cathode that increases cell impedance. However, this passivation effect dissipates soon after a load is applied. Also, when a fairly heavy pulsed load is applied, such as the initiation of two-way wireless communications, the cell’s voltage can drop temporarily, an effect known as transient minimum voltage (TMV). If the engineer employs a dc-dc converter with a low drop out voltage, this may not be a problem. Otherwise, a de-passivation operation may be required after six months of storage, which may involve discharging the battery for 10 seconds prior to applying any type of pulsed load. Storage for more than 12 months may require a 30-second discharge time.
A Modified Version Solves Two Problems
A hybrid version of the LiSOCl2 battery (the PulsesPlus Series) was developed specifically for applications that require high pulses to power two-way wireless communications. The PulsePlus Series battery adds a hybrid layer capacitor (HLC) that works in conjunction with a standard bobbin-type LiSOCl2 cell. The standard LiSOCL2 cell conserves energy by delivering the low background current when the device is operating in its “stand-by” mode. The HLC works like a rechargeable battery to store and deliver high pulses, thus eliminating any passivation or TMV effect. For example, a D-size PulsesPlus hybrid LiSOCl2 battery can operate for decades while delivering pulses of up to 19 Ah.
There is a growing need for remote wireless sensors that are connected to the IIoT. Primary lithium batteries typically offer the best long-term power source for these applications. Within the lithium family, Bobbin-type LiSOCl2 cells stand apart by offering the highest energy density and extremely low self-discharge, which translates into operational life of up to 40 years. However, these batteries are not all created equal so design engineers must perform the proper due diligence to ensure that the battery will deliver as promised.
For more information, visit http://www.tadiranbat.com/.