By Dr. Thomas Dittrich
Batteries are the lifeblood of a wireless device, providing the essential power required for reliable data collection, storage and communication. Specifying the right battery is essential, especially for wireless communication modules designed for long-term where battery replacement is costly and life cycle costs of the equipment are crucial.
This decision making process is complicated by the breadth of choices between competing chemistries such as alkaline, carbon zinc, zinc-air and lithium, and by the fact that it is often difficult to differentiate a superior quality battery from an inferior knock-off. So getting it right demands thorough product knowledge and due diligence to ensure that the optimal power management solution has been specified.
START BY REVIEWING YOUR OPTIONS
Application-specific requirements often dictate the solution. For example, consumer alkaline cells can suffice for applications requiring only a few months of service life in a moderate temperature range and where battery replacement is relatively simple due to easy accessibility. While inexpensive and readily available, alkaline batteries are ill suited for applications that require extended service life under extreme temperatures.
Choosing the optimum power management solution begins with a careful evaluation of power and performance requirements, from which design engineers can compare competing chemistries based on a prioritised checklist of desired attributes such as voltage, capacity, size, weight and/or special packaging requirements, expected service life, temperature and/or environmental issues and cost. Special requirements such as the need for high current pulses or high discharge rate should also be considered.
LONG TERM APPLICATIONS CALL FOR LITHIUM BATTERIES While primary lithium batteries lack the media “buzz” associated with rechargeable batteries, they offer dynamic opportunities within growth markets such as smart grids, utility automatic meter reading (AMR), wireless mesh networks, system control and data acquisition, data loggers, oceanographic, emergency and safety equipment, as well as other remote sensing devices where recharging is not practical or even impossible.
Lithium is the preferred choice for utility meters due to its intrinsic negative potential, which exceeds that of all other metals. Lithium is the lightest solid 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 1.5 V and 3.6 V. The absence of water also allows certain lithium batteries to operate in extreme temperatures (-55 OC to +150 OC).
Despite the popular misconception that all lithium batteries are essentially equal, the lithium family of primary batteries is actually quite diverse. The choices include poly carbon monofluoride (Li/CFX), manganese dioxide (Li/MnO2) and lithium thionyl chloride (Li/SOCl2). Each chemistry offers unique advantages and disadvantages, so trade-offs are inevitable.
Poly carbon monofluoride (Li/CFX) and manganese dioxide (Li/MnO2) are best suited for applications that do not require long operating life or possible exposure to extreme temperatures. However, when extremely long battery life, extended temperature range and reduced battery size and weight are important considerations, the lithium battery of choice is lithium thionyl chloride (LTC), which is available in two styles: bobbin or spirally wound construction.
Bobbin-type versus spirally wound construction
Both spirally wound and bobbin-type lithium thionyl chloride styles use a non-aqueous electrolyte, resulting in relatively high impedance. Spirally wound cells reduce this impedance by increasing the surface area of the anode. This implies more inactive materials such as separators, substrate and current collectors, whereby overall performance is reduced and energy density is lowered. Also, operating life is shorter as extra surface area leads to more self-discharge. As a result, spirally wound LTC batteries deliver only 800 Wh/l energy density with a temperature range of -55 OC to +85 OC and a maximum service life of about 8 years.
In contrast, bobbin-type LTC cells are capable of delivering far higher energy density (1,420 Wh/l), have more capacity and are able to withstand extreme temperatures (-55 OC to +150 OC). Due to low annual self discharge (less than 1% per year), they also offer extremely long service life – more than 25 years according to customer testimonials.
While the theoretical service life of a bobbin-type lithium thionyl chloride battery is over 20 years, actual service life varies based on the self-discharge rate, which is governed by the chemical composition of the electrolyte, the manufacturing processes used, as well as mechanical and environmental considerations. Battery performance and self-discharge can also be negatively affected by high levels of impurities in the electrolyte, as well as by impedance resulting from the internal resistance created by the electrolyte, the anode and the cathode. Experienced battery manufacturers can control impedance by blending special additives into the electrolyte.
Lithium thionyl chloride batteries have a proven track record for utility metering applications. Since the 1990s, these batteries have been used in the E6 electronic gas meter. Also, in 1984, Hexagram (now Aclara), introduced its first automatic meter reading (AMR) devices for the gas and electric utility market, which were powered by AA-size lithium thionyl chloride batteries. Over 3 million of these devices have been deployed worldwide, and virtually all continue to operate on their original batteries after 25 years. Long term reliability is especially critical to the utility industry, as extended battery life translates into higher productivity and profitability by eliminating the need for system-wide battery change outs.
High current pulse “hybrid” solutions
In addition, there are a growing number of applications that require high current pulses, presenting technical challenges to both spirally wound and bobbin-type lithium batteries. Typically, these applications involve low continuous current (or no continuous current) coupled with high pulse currents of up to several Amperes.
Devices requiring high current pulses must be carefully designed to extend battery life. For example, these devices are often programmed to operate in multiple modes, including a sleep or standby mode, where power consumption is nil or a low background current; a measurement or interrogation mode, where the unit requires a few hundred milliamps of energy; and a transmission mode that requires high current pulses before returning to sleep or standby status.
Spirally wound lithium thionyl chloride batteries deliver the energy density required by high current pulse applications, but lack the required capacity and have a comparatively high rate of self discharge, which limits their long term operation. Another fact is that the internal resistance increases with depth of discharge, thus also limiting battery life of this style.
Bobbin-type cells have the ideal capacity and energy density, but only allow low current pulses due to their low rate design. To overcome this, engineers at Tadiran developed PulsesPlus™ technology by combining a bobbin-type primary cell with a patented high rate, low impedance HLC (hybrid layer capacitor) to deliver extremely high currents with an excellent safety margin. The rate at which energy can be stored by the HLC varies from 280 As with smaller HLCs to 1,120 As with larger size HLCs. Variations of this hybrid technology have also been utilised to provide short duration high rate power for military and medical applications.
EVALUATING BATTERY SUPPLIERS
Once the right battery chemistry is chosen, design engineers need to perform due diligence to verify that the lithium battery they have specified can deliver as promised. With the rapid growth in knock-off products now flooding the marketplace, vigilance is required to ensure product quality and authenticity, including 100% product traceability back to the raw materials. As part of the vendor selection process, it is therefore recommended that potential battery suppliers be required to provide a list of customer references along with fully documented and verifiable test results for parameters such as battery pulse, low temperature pulses, discharge and repeatability.
Faithfully applying these and other common sense practices during the vendor selection process will help ensure years of trouble-free battery performance.