Mobile Power Sources Impact Military Operations

Mar 1, 2010 12:00 PM
Sam Davis, Editor in Chief

Power sources for mobile systems are now a major concern of the military as it strives to cut the size and weight of its tactical systems, while improving performance.

As modern military systems become more mobile and electronically sophisticated, there is a greater reliance on associated power sources. The growing need for mobile power impacts the total size and weight of its associated system, whether it is an unmanned air vehicle (UAV) or part of a war fighter's load. This includes the primary and backup power sources for communications, navigation, imaging, displays, computing, sensors, etc.

Today, mobile military equipment must operate reliably in extreme conditions: dry/hot deserts, humid tropical jungles, and frigid arctic locations. To date, their relatively light weight, small size, high power per unit volume, and availability makes batteries the logical choice for many of today's military systems.

Applying a battery to an electronic system requires power management that depends on the specific battery employed in the system. Too often, powering the system is an afterthought, instead of being considered at the design's outset. Early battery considerations are important because battery-system interactions can affect system performance.

PRIMARY VERSUS SECONDARY BATTERIES

System designers must decide whether to use non-rechargeable or rechargeable batteries. Non-rechargeable batteries are called primary cells, with carbon-zinc, alkaline, and lithium types among the most widely used. When these batteries fail they become throw-away items. In contrast, rechargeable batteries (called secondary cells) may often be recharged without removing them from the system.

Because they usually have a lower self-discharge rate than rechargeable batteries, primary batteries are useful when it is necessary to store them for long periods. Self-discharge is the amount of charge they lose when not being used. Applications that require a small current for a long time use primary batteries, because the self-discharge current of a rechargeable battery would exceed the load current and limit service time to a few days or weeks. Although, if they could be easily recharged this would not be a problem.

Commonly used in portable devices that have low current drain, primary batteries are only used intermittently, or are used well away from an alternative power source — such as in alarm and communication circuits — where other electric power is only intermittently available. Disposable primary cells cannot be reliably recharged, since the chemical reactions are not easily reversible and active materials may not return to their original forms.

Generally, primary batteries have higher energy densities than rechargeable batteries, but they do not fare well under high-drain applications with loads under 75 Ω. Compared with rechargeable lithium cells, primary lithium batteries have higher capacity, lower self-discharge rates, and often different voltages. Primary lithium batteries can operate from as low as -55° to as high as 125°C. Due to its organic liquid electrolyte, primary lithium batteries typically have higher internal impedance and more limited maximum current than their rechargeable counterparts.

With primary batteries, the important characteristics are energy density (Wh/kg), output voltage, and safety considerations. For rechargeable batteries, it is important to understand the battery characteristics in terms of the specified charge-discharge cycle times, energy density, self-discharge, output voltage, and safety considerations.

The majority of rechargeable battery-based military electronic systems employ one of the following types:

• Sealed Lead-Acid (SLA) batteries were among the first to be used in military systems because of their availability and high power-handling capability, but they suffer from heavy weight.
• Nickel Cadmium (NiCd) is used where long life, high discharge rate, and economical price are important. Its disadvantage is a memory effect that requires periodic discharge to prevent charging problems.
• Nickel-Metal Hydride (NiMH) has been used extensively, at the expense of reduced cycle life compared with NiCd. It uses environmentally friendly metals and offers about 30% to 40% higher energy density than NiCd. Also, NiMH batteries are less prone to memory effects.
• Lithium Ion (Li-ion) is used where high energy density and light weight are of prime importance. The lightest of all metals, lithium has the greatest electrochemical potential and provides the largest energy density per weight. Rechargeable batteries using lithium metal anodes (negative electrodes) can provide both high voltage and excellent capacity. However, there are potential safety problems with this type of battery. Li-ion batteries have good cold and hot temperature charging performance. Some cells allow charging at 1C from 0° to 45°C. Most Li-ion cells prefer a lower charge current when the temperature gets down to 5°C or colder.
• Li-ion Polymer has a chemistry similar to Li-ion in terms of energy density. but differs from Li-ion in terms of fabrication, ruggedness, safety and thin-profile geometry. Unlike Li-ion, there is no danger of flammability because it does not use a liquid or gelled electrolyte.

In many systems, Li-ion batteries are configured in packs consisting of multiple batteries. But, some Li-ion battery packs consist of only one cell because of its relatively high cell voltage (4.2 V). Its life expectancy is 300 charge-discharge cycles, with 50% capacity at 500 cycles.

An added requirement for Li-ion battery packs is a protection circuit that limits each cell's peak voltage during charge and prevents the voltage from dropping too low on discharge. The protection circuit limits the maximum charge and discharge current and monitors cell temperature. This protects against overvoltage, undervoltage, overcharge current, and overdischarge current in battery packs.

The charge and discharge capacity of a rechargeable battery is in terms of “C,” given as ampere-hours (Ah). Most portable batteries are rated at 1C, which is a discharge current equal to the rated capacity. For example, a battery rated at 1,000 mAh provides 1,000 mA for one hour if used at 1C rate. The same battery used at a 0.5C rate provides 500 mA for two hours.

BATTERY LIFE

Users of battery-based systems are sensitive to the amount of usable life left in the battery. In addition, their operating environment can vary over a wide range of temperatures, which affects a battery's efficiency, rate of charge and discharge, and therefore battery life.

One approach for battery-based systems is a so-called “gas gauge” that indicates battery conditions with a display on the equipment itself. Another approach is a battery-monitoring IC that accurately measures charge and discharge currents in rechargeable battery packs.

These devices contain all the necessary functions to form the basis of a comprehensive battery-capacity management system. Battery monitors work with the host controller in the mobile system to implement the battery life management system.

Today's Li-ion batteries are based on an intrinsically unstable materials platform. Chemical degradation can lead to premature failure in some applications, and relatively poor lifetimes prevent Li-ion cells from addressing all tactical applications. The problem is due to the flammable liquid electrolyte employed in early Li-ion batteries.

There are current efforts to reduce battery size and catastrophic failures of Li-ion batteries. Seeo (Berkeley, CA) is working on an entirely solid-state electrolyte for Li-ion batteries. At the core of this technology is a solid polymer electrolyte material that can transport lithium ions while providing inherently safe and stable support for very high-energy electrode chemistries. This could offer dramatic improvements in energy density while also improving product lifetime and safety.

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