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Today's United States military relies heavily on portable electronic systems in the battlefield. The fighting soldier might, for example, pack communications equipment, a thermal sight, night-vision goggles, a global positioning system, and even a laptop computer (Figure 1). On the ground, they supply power to remote sensors used to secure a perimeter. In the air, batteries power portable radios, emergency beacons, and unmanned vehicles. And the list keeps growing.

For the most part, the batteries used during combat operations are primary (non-rechargeable) batteries. Although non-rechargeable batteries offer the highest capacity, the problem is that once a battery is used on a mission, the returning soldier, uncertain of its remaining run time, will replace it with a fresh one rather than take a used battery back out in the field, an understandable action considering the risks associated with running out of battery power. The result is that many of the discarded batteries have sufficient capacity remaining for a second mission, and the loss of this capacity represents a significant cost to the military.

“No soldier on the battlefield is going to trust a battery the second time he takes it out,” said Michael T. Brundage, Power Applications branch chief at the U.S. Army's Power Division (Fort Monmouth, NJ). “Today,” he explained, “a used battery doesn't give the soldier the warm fuzzy feeling that it will have enough energy to last,” and indeed chucking it for a fresh unit makes sense when faced with a life-and-death combat situation.

As it turns out, however, many of those used batteries are being discarded with a significant amount of remaining energy, according to the U.S. Army and the Defense Logistics Agency, which together oversee the development, manufacture, and acquisition of communication-electronics batteries for all the military services. Studies vary, said Brundage, but the Army estimates that the batteries are being discarded with 30% to 50% of their energy remaining. “That's millions, if not tens of millions of dollars, being wasted each year.”

As a result, the Army is eager to find ways to stem the waste without endangering its fighting soldiers. One recent breakthrough is the development of a low-cost, built-in state-of-charge indicator for the military's most popular primary battery, the BA-5590. By showing a battery's remaining energy level, the indicator gives the soldier the information he needs to decide whether it's safe to take a battery he's already used back into combat. This technology will allow the military to get more use out of each battery, reducing the number of batteries needed and the overall cost of providing power to the soldier. Another effort is under way by the Army to find ways to increase the use of rechargeable batteries in combat situations, where recharging them is problematic.

What is the Army's idea of the perfect battery? When answering that question, Brundage likes to joke that, “it would be safe, long lasting, lightweight, small, low-cost, available anywhere, and when it's used up, edible.” In practice, at least most of these goals are within reach.

The Army stockpiles about 13 different types of lithium-based primary batteries, unique to the military, to meet the different sizes, shapes and voltage requirements, ranging from 3 V to 24 V, required by the military's various portable electronic systems. The most common chemistry on which the cells are based is lithium sulfur dioxide (LiSO₂), a formulation that meets the military's need for safe operation, high energy density, long shelf life, wide operating temperature (typically from -40°C to +55°C), and light weight.

Also in use, on a smaller scale, is the lithium manganese dioxide (LiMnO₂) chemistry. LiMnO₂, while heavier, delivers about 50% more energy for a given volume. Thus, where a BA-5590 LiSO₂ might deliver 7.5 A hours and weigh 2.2 lbs., an LiMnO₂ version of the same size battery (designated BA-5390) would deliver 10.5 A hours and weigh about 2.9 lbs. The drawback to LiMnO₂ is its significantly higher cost.

Long shelf life, typically five years, is especially important, as batteries are stockpiled during peacetime. Similarly, wide operating and storage temperatures are key to batteries that could wind up being used in extremely hot or cold climates. Another factor the military needs to consider is a battery's signature; that is, whether it gives off heat or noise (some batteries have moving parts) that can be detected by the enemy.

Safety is the first criteria when the military picks a battery chemistry and technology, and for the high energy they pack, both the lithium sulfur dioxide and lithium manganese cell chemistries meet that requirement. In addition, a number of safeguards are added to the design, including over current and over temperature protection and, as a last line of defense, a safety vent in each cell to release any gas that builds up inside. These measures are needed mostly to ensure safety in the event of a short circuit or other malfunction that would push the battery's temperature too high. Primary cells are also fitted with diodes to prevent damage or destruction if they are inadvertently put into a charger.

Needless to say, military batteries are made to be rugged. Designed to withstand humidity, shock, vibration, impact and extreme temperatures, the batteries must work everywhere, every time. The batteries comprise specific cell types (typically welded steel cans) and are housed normally within plastic cases with the necessary electronics. Features include the means to fully discharge the battery after use, making it safer for disposal. Initial qualification of the battery to MIL-PRF-49471-B (CR), the standard for non-rechargeable high-performance batteries, is costly and time-consuming but necessary to provide the performance and safety the military needs.

In addition to the built-in state-of-charge indicator, another avenue for cutting battery usage costs that has been actively pursued by the military for several years is the use of lithium-based rechargeable cells. Although a rechargeable battery typically carries a price tag that is five times that of its disposable counterpart, military rechargeables are good for at least 224 recharging cycles, yielding significant savings over their lifetime.

Another advantage of rechargeable batteries is that improvements in the technology are being driven by commercial applications, and the Army can leverage these advances at little internal cost.

The problem, of course, is the lack of electrical outlets or other charging sources on the battlefield. In addition, for safety's sake, charge rates must be carefully controlled, putting an added burden on the associated electronics. For these reasons, the use of rechargeable batteries in the military is largely reserved for training, where at the end of the day soldiers can conveniently recharge their batteries overnight.

Nevertheless, several studies have shown that with the proper logistical support in the battlefield, rechargeable batteries can indeed serve in certain tactical situations. What's more, several Army units in southwest Asia have been trained from the beginning to work with nothing but rechargeables. Said Brundage, “They were provided with whatever chargers, cables and adapters they needed. They set up standard operating procedures on how they would cycle the batteries through the unit, getting them to the chargers and back out to the soldiers. They are totally energy independent.” The overall key to this success was that the units had all the required procedures and equipment in place and operational prior to deploying.

In addition, the Army is looking at fuel cells and zinc-air power sources for recharging batteries in the field. While these technologies are available as prototypes, each presents significant technical challenges to wide-scale adoption. Fuel cells, for example, which use cartridges of methanol, diesel fuel or hydrogen, require a lot of hardware, including valves and pumps, which are difficult to shrink into a manageable size. That said, however, with an energy content several times that of a traditional battery, they are, according to Brundage, “too attractive to ignore.”

New battery technologies are important to support the Army's Land Warrior system, part of the military's push away from a reliance on conventional heavily armored forces to those that are lighter, more agile, and more deployable. Seen as a way to transform the soldier into a complete and highly lethal weapons system, Land Warrior aims to equip each soldier with a number of electronic subsystems. These include a radio, computer, laser rangefinder and digital compass, daylight video camera, laser aiming light, and thermal sight. The Land Warrior helmet alone will contain a night sensor with flat panel display and a laser detection module.

As with existing systems, the Land Warrior will need to accept rechargeable batteries for training purposes and disposable primary batteries for use in combat. Moreover, the system will make use of the system management bus, or SMBus, a power and battery management communication protocol developed jointly by Intel Corp. and Duracell originally for portable consumer equipment.

By communicating with “smart” batteries, the SMBus reports a wealth of information such as a battery's state of charge, its remaining run time, temperature, age, and number of recharging cycles, as well as monitoring its charging and discharge rates, adapting the charging cycle for different battery types, and ensuring overall safe operation.

Originally wary that the SMBus lacked the features needed to serve its needs, the Army has since adopted it for use in Land Warrior, and is currently studying ways to improve it for military service. Indeed, SMBus has already helped the Land Warrior effort by allowing the system to smoothly accommodate, on one hand, successive improvements in battery technology, and on the other, batteries and chargers from different vendors.

As a result of its commercial applications, lithium rechargeable technology has been advancing so quickly, battery capacity has tripled since the time the contract started. Consequently, the Army has several different generations of the same battery type, each with a different capacity, as well as battery chargers from four different vendors. This has made the use of SMBus more critical because the batteries and chargers all comply with, and therefore communicate over, the SMBus. Noted Brundage, “We can take any one of those batteries, regardless of its generation, and use it with any of the chargers. You get the most efficient charge, and it's done safely.”

But most important, said Brundage, the SMBus tells the soldier accurately how much time is left for powering his Land Warrior system. “The last thing we want to do is tell a soldier in a life or death situation, he has a half hour of capacity left when in reality he has only 10 minutes.”

In the final stages of qualification testing is a unique primary (i.e., single-use non-rechargeable) military battery manufactured by EaglePicher Technologies, LLC for powering portable equipment. What makes the battery unique is its low-cost built-in state-of-charge indicator (Figure 2). With it, soldiers can for the first time check whether a battery's remaining energy level is enough to take it back into the field. Without that knowledge, soldiers are likely to discard a battery, even, as it turns out, one with plenty of power left, costing the military millions of dollars that could otherwise be saved.

The familiar finger-activated state-of-charge indicator on commercial Duracell brand alkaline batteries, however, belies the difficulty of fitting a similar feature on military batteries. Thus, while the remaining energy in an alkaline battery maps nicely to the terminal voltage, the LiSO₂ chemistry used in today's military primary batteries, makes no such accommodation.

“Lithium sulfur dioxide is a very difficult chemistry when it comes to measuring state of charge,” said Gary Davison, CEO and cofounder of POWERPRECISE Solutions Inc., (Herndon, VA), the company that invented the low-cost state-of-charge circuit. “The chemistry has a very flat discharge curve so you can't easily tell where you are on the discharge cycle until the battery is drained. It's almost ruler flat, with a cliff where usability ends.” The same is true of emerging LiMnO² chemistry which, with its higher volumetric efficiency (i.e., more energy per unit volume) compared to LiSO₂, makes the state-of-charge indicator even more valuable as a cost saver.

Without the advantage of an easy to measure voltage-to-energy relationship, POWERPRECISE designers had to find another way to track the battery's remaining power. They discovered the answer in what they call compensated coulomb counting; in effect, tracking the remaining capacity by measuring how much energy the battery has used. To do that accurately and reliably, the built-in electronics take into account the total current over time, the rate of discharge, and the battery temperature (Figure 3). The electronics also monitors parameters to ensure a safe discharge rate. “The trick,” said Davison, “is to do a maximum amount of math without using a lot of power and at a low enough cost to put the circuit in a disposable battery.”

Designers met those goals using what they call Virtual Analog^™ technology. “Effectively, Virtual Analog^™ technology is the secret sauce that allows us to do very high accuracy analog-to-digital conversion and signal processing using fewer gates,” stated Davison. “Fewer gates mean less power and smaller chip size, which translates into low cost.”

Gregg Bruce is director of product development, and Brad Audette is director of sales and business development at EaglePicher Energy Products Corp.