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 selfdischarge, 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.