Wireless technology is growing rapidly, now encompassing consumer-grade devices as well as industrial-grade products used in utility meter reading (AMR/AMI), wireless mesh networks, system control and data acquisition (SCADA), data loggers, measurement while drilling, oceanographic measurements, emergency/safety equipment, and M2M communications. The rise in wireless technology is closely tied to the development of low-power communications protocols such as ZigBee, Bluetooth, DASH7, INSTEON, and Z-Wave.
Each application is unique, so design engineers need to specify the right power supply based on application-specific requirements, including:
- Energy consumed in dormant mode (the base current)
- Energy consumption during active mode (including the size, duration, and frequency of pulses)
- Storage time (as normal self-discharge during storage diminishes capacity)
- Thermal environments (including storage and in-field 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)
- Initial cost and anticipated long-term maintenance costs, including scheduled battery replacement, where applicable
The performance parameters dictate whether the application requires a primary or rechargeable battery.
Consumer Batteries Carry Hidden Costs
If long battery operating life is not a major priority, either because the battery is easily accessible for replacement or the device’s life expectancy is relatively short, then a primary (non-rechargeable) alkaline battery may suffice. Alkaline cells are extremely inexpensive and readily available, but have certain drawbacks, including low voltage (1.5V), a limited temperature range (-0 °C to 60 °C), a high annual self-discharge rate, and crimped seals that can leak.
The low initial cost of a consumer alkaline battery can be highly misleading, as this investment is relatively short-lived, and carries downstream risks associated with loss of productivity and/or data due to premature battery failure. Since alkaline batteries need to be replaced every few months, there are hidden labor costs associated with future battery replacements, which increases the total lifetime cost of ownership. If the device is placed in a remote and inaccessible location, these labor costs could be substantial.
Using Lithium Primary Batteries
Lithium remains the preferred choice for powering remote wireless devices due its intrinsic negative potential, which exceeds that of all other metals. Lithium is the lightest non-gaseous metal, and offers 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 OCVs of between 2.7 and 3.6V. The absence of water also allows certain lithium batteries to operate in extreme temperatures (-80 °C to +125 °C).
There are numerous lithium primary battery chemistries to choose from, including lithium iron disulfate (LiFeS2), lithium manganese dioxide (LiMNO2), lithium thionyl chloride (LiSOCL2), and lithium metal oxide. A comparison of primary lithium cells is presented in the table on page 18.
Consumer-grade LiFeS2 cells (1.5V) are relatively inexpensive and deliver the high pulses required to power a camera flash. However, these batteries have limitations, including a narrow temperature range (-20 °C to 60 °C), high annual self-discharge, and a crimped seal.
Lithium manganese dioxide, cells, such as CR123A cells, offer space-saving solutions for cameras and other consumer devices, as one 3V cell can replace two alkaline batteries while delivering moderate pulses. However, LiMNO2 cells suffer from low initial voltage, high annual self-discharge, a limited temperature range, and crimped seals.
Lithium thionyl chloride cells are the preferred choice for wireless applications that require long-term power, especially in extreme environments. These batteries can be constructed two ways: spiral wound or bobbin-type construction. Bobbin-type LiSOCL2 chemistry offers the highest capacity and highest energy density of any lithium cell, along with an extremely low annual self-discharge rate (less than 1% per year). Bobbin-type LiSOCL2 batteries also deliver the widest possible temperature range (-80 °C to 125 °C), and feature a glass-to-metal hermetic seal to prevent battery leakage.
An inferior quality LiSOCL2 battery may deliver as little as 10-year operating life with an annual self-discharge rate of 2-3% per year, while a superior-grade LiSOCL2 battery can feature an annual self-discharge rate of just 0.7% per year, enabling this better quality cell to operate maintenance-free for up to 40 years. Due diligence is required during vendor evaluation, as not all LiSOCL2 batteries are manufactured to such high standards.
Selecting a top-grade lithium battery could substantially reduce total cost of ownership by extending battery operating life, as the labor costs associated with battery replacement typically far exceed the cost of the batteries themselves.
Lithium metal oxide batteries are designed for applications that require long battery shelf lives of up to 20 years, and deliver continuous high-rate current and/or high pulses to applications such as surgical power tools, automatic external defibrillators (AEDs), emergency beacons, smart munitions, and other high-power applications. Lithium metal oxide batteries feature a very low annual self-discharge rate as well as an extended temperature range of -45 °C to 85 °C. The AA sized cells can deliver up to 15A pulses.
Industrial Applications Require More Robust Batteries
A growing number of industrial applications demand long-life batteries that can operate at extreme temperatures, including automotive toll tags, GPS tracking devices, scientific and oceanographic instruments, remote sensors, automatic utility meters, process controls, and other M2M devices. Many of these remote wireless devices also require high pulses to support advanced two-way communications and/or remote shutoff capabilities. Standard LiSOCl2 batteries, due to their low-rate design, may experience a temporary drop in voltage when first subjected to this type of load — a phenomenon known as transient minimum voltage (TMV).
One solution is to combine a standard bobbin-type LiSOCl2 cell with a patented Hybrid Layer Capacitor (HLC). The battery and HLC work in parallel, with the battery supplying long-term, low-current power in the 3.6–3.9V nominal range while the single-unit HLC delivers high-current pulses with a very high safety margin, thus avoiding the balancing and current leakage problems associated with supercapacitors. This hybrid battery features a distinctive end-of-life performance curve that allows the device to be programmed to provide low battery status alerts.
Bobbin-type LiSOCL2 batteries can also be specially modified to deliver moderate current pulses without the use of an HLC, thus delivering high capacity and high energy density without experiencing voltage drop or power delay. These specially modified batteries also utilize available capacity very efficiently, thus extending battery life up to 15% in extremely hot or cold temperatures.
Choosing Between Consumer and Industrial Rechargeable Batteries
Certain applications may be well suited for energy harvesting devices working in conjunction with rechargeable lithium batteries or supercapacitors to store the harvested energy and to deliver high pulses.
Consumer-grade lithium ion (Li-ion) batteries are very popular due to their high efficiency and high power output. The popular 18650 cell was developed by laptop computer manufacturers as an inexpensive solution for five years and 500 full recharge cycles. However, consumer- grade Li-ion cells do not perform well in extreme environments due to their limited operating life, high annual self-discharge rate, narrow temperature range, and crimped seals. These batteries also experience a gradual degradation of the cathode that makes them less receptive to future recharging, thus reducing their service life.
Slim-profile smartphones and tablet computers typically use thin lithium polymer batteries that are reasonably inexpensive, but also deliver limited service life, poor performance at extreme temperatures, and have a tendency to swell in size over time.
The use of supercapacitors to store harvested energy has severe limitations, as they are bulky, not well suited to extreme temperatures, and have a very high rate of self-discharge. Solutions involving multiple supercapacitors also require balancing circuits. To address these limitations, an industrial-grade rechargeable battery was recently developed that can operate maintenance-free for up to 20 years and 5,000 full recharge cycles. These highly rugged batteries feature very low annual self-discharge rate, the ability to be recharged in extreme temperatures (-40 °C to 85 °C), the capability to deliver up to 15A pulses with an AA-sized cell, and a glass-to-metal seal to withstand harsh environments.
The choice between an industrial-grade and a consumer-grade battery involves numerous factors related to power and performance, reliability, and total lifetime cost of ownership.
This article was written by Sol Jacobs, Vice President and General Manager at Tadiran Batteries, Lake Success, NY. For more information, Click Here .