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.
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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Khalil Amine, manager of the advanced battery technology group at Argonne National Laboratory, says “the lithium anode could be a show-stopper.” Lithium has a tendency to get roughened at the surface and grow crystal dendrites that can reach the cathode and short the battery. The company will need to do long-term tests to show that its polymer is hard enough to block the dendrites.
Another factor to consider is that the present solid-state Li-ion batteries don't charge as fast as liquid-based types. However, their high energy density could make them perfect candidates for some applications.
BATTERY-POWERED POWER UAVs
Only batteries power one of the smallest UAVs now flying; the AeroVironment (Monrovia, CA) WASP III that has a 2.4-ft wingspan (Fig. 1). Also called Micro Air Vehicles, or MAVs, their power source is rechargeable Li-ion batteries that run its electric motor and electronic systems.
The nearly 1-lb Wasp III carries all the sensors found on most high-end UAVs. In addition, it has an onboard payload of either a pod with forward- and side-looking high-resolution electro-optical color cameras, or a pod with forward- and side-looking high-resolution IR cameras. Digital stabilization allows the plane to keep a target in sight.
The WASP III fits in a backpack and is small enough so that soldiers can launch it by hand. Once airborne, a ruggedized handheld ground station can control its flight profile. The ground station remains within line-of-sight with the WASP III as it beams down video signals. This MAV can fly for up to 45 min and up to 5 km while being controlled manually or by auto-pilot.
Another AeroVironment MAV is the Raven (Fig. 2) with a 4.5-ft wingspan, and weighing more than four times the Wasp. This MAV uses a pusher prop mounted in back of the main fuselage so it can employ forward- and side-looking color cameras or a forward- and side-looking IR imager mounted in the nose. This allows it to look down the aircraft's flight path. Each payload weighs about 6.5 oz.
Although the Raven will not fit in a backpack, it can be controlled by the same ground station used by the WASP III. The Raven has about 45 to 60 min of flight time on its batteries.
When the ground station operator gets a low-battery indication from a WASP III or Raven, he can land it, remove the old batteries, install new ones, and return the MAV to its flight pattern. Then, he can recharge the old batteries.
The Raven and the WASP III offer a real-time, up-to-date, over-the-horizon view of trouble spots. This live, detailed, day or night coverage allows units to conduct intelligence, surveillance, and reconnaissance of danger zones without committing personnel to perform this task.
While the military continuously looks to technology to improve the war fighter's ability to dominate the battlefield, conventional batteries are unable to keep up with all power demands. The Office of Naval Research is sponsoring projects to boost power density in batteries while U.S. Army planners are also funding research into mobile power generation. One possible answer is the use of fuel cells to provide mobile power.
A fuel cell employs a fuel tank and an electrochemical process to produce a dc output voltage. The dc voltage is generated by the reaction, triggered in the presence of an electrolyte, between the fuel (on the anode side) and an oxidant (on the cathode side). The reactants flow into the cell, and the reaction products flow out of it, while the electrolyte remains within it. Fuel cells can operate almost continuously as long as fuel remains and the necessary flows are maintained.
Fuel cells are different from conventional electrochemical cell batteries because they consume reactant from an external source, which must be replenished. Fuel cells are a thermodynamically open system, whereas batteries store electrical energy chemically, representing a thermodynamically closed system.
Many combinations of fuels and oxidants are possible. A hydrogen fuel cell uses hydrogen as its fuel and oxygen (usually from air) as its oxidant. Other fuels include hydrocarbons and alcohols. Other oxidants include chlorine and chlorine dioxide.
Some fuel cells have become reliable power sources for mobile military applications. Direct methanol fuel cells (DMFCs) provide logistical, safety, and functionality advantages, including virtually undetectable operation. They also are immune to extreme weather, generate power only when needed, and operate almost silently without producing exhaust. Unlike batteries, which store energy, DMFCs generate power by chemically converting methanol into electrical energy.
As shown in Fig. 3, the DMFC's fuel cell power-producing heart is a mixture of methanol and water introduced to the anode side, which is connected to the cathode by an electrical circuit. A patented water-management system enables the use of 100% pure methanol with a very high energy density in the fuel cartridges. Ambient air is pumped into the stack on the cathode side. Upon contact with a platinum catalyst, methanol releases its electrons, which flow in the direction of the cathode, thus producing power. At the same time, protons are released and penetrate the membrane to the cathode. There, the oxygen reacts with the protons and electrons to form pure water.
To address soldiers' growing power demands, the Smart Fuel Cell (SFC) AG (Brunnthal-Nord, Germany) system is a fuel-cell/battery-hybrid system solution that offers a lightweight alternative for non-stop equipment operation. The energy network (Fig. 4) consists of a fuel cell, fuel-cell cartridge, and intelligent power manager, along with a rechargeable Li-ion battery, is also universally usable and compatible with current and future power-consuming requirements.
The energy source of the network, the portable Jenny fuel cell already in use by various military organizations, weighs only 3.7 lbs and measures 10 × 7 × 3 in. It provides 25 W continuous nominal power directly to electrical devices, or for charging secondary batteries. Its nominal voltage is 16.8 V and it can be adapted to other voltages (output voltage 10 to 30 Vdc). Fuel consumption at 25 W is less than one millilitre per watt hour. The network produces power automatically, as needed, continuously as long as there is fuel. The fuel cell itself remains maintenance free throughout its entire life. The only maintenance required is occasional fuel cartridge replacement.
The SFC Power Manager is an intelligent portable power-management device that ensures continuous operation of any electrical equipment carried by special operations soldiers, as well as for charging batteries. The Power Manager enables smart energy harvesting, including the option of assigning various priorities during charging/discharging of multiple batteries and powering several devices simultaneously. It automatically recognizes the voltage demands of the individual devices and adapts the output power accordingly. The SFC Power Manager also provides a constant indication of each battery's state of charge and other parameters. The system readily hybridizes with conventional power sources, including vehicle power, solar, and fuel cells.
The fuel-cell/battery hybrid system is a flexible and intelligent solution that provides continuous operation of any electrical equipment. The combination of the fuel cell and the power management system enables versatility. For example, it can hybridize a solar-energy system as well as provide lightweight and reliable energy for soldiers in the field.
For example, a fuel-cell/solar combination with a Jenny fuel cell weighing 3.7 lbs, plus two 0.8-lb fuel cartridges, would reduce the weight of the soldier's power supply in the 96-hour mission example with three consumers (radio, notebook, and thermal imaging device) by almost 70% compared to non-rechargeable batteries. The hybrid fuel cell system is 31 lbs lighter than batteries with equivalent power. As described above, fuel cells have applications on land. In addition, fuel cells can be used in the air for UAVs.
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Most current UAVs employ reciprocating engines that are smaller versions of those used in small aircraft. Militarily, a better approach would be to use electric motors that run quieter and are more difficult to detect. However, that requires a lightweight power source for the motor and the complement of electronic equipment in the UAV.
UAVs designed for surveillance and other special missions require extended flight times that cannot be achieved with batteries. Therefore, advanced fuel cells offering quiet, reliable, and energy-dense power would enable extended flight times and the ability to carry a greater payload.
To date, the Naval Research Laboratory has completed two test flights using a prototype power system comprised of Protonex' fuel cell technology and a compressed hydrogen fuel source. While fuel cells coupled with compressed hydrogen improve flight durations compared to those of battery-powered UAVs, compressed hydrogen is not the most effective means of storing hydrogen on a per-weight basis.
The ProCore UAV system combines Protonex (Southborough, MA) high-performance fuel-cell technology with an advanced chemical hydride fueling solution for hydrogen generation based on Millennium Cell's Hydrogen on Demand technology. The result is a propulsion system that provides small UAVs with up to four times the energy density of advanced batteries, significantly extending the capabilities of these systems.
EnergyOr Technologies Inc. (Montreal, Canada), a developer of proton exchange membrane (PEM) fuel-cell systems, has demonstrated its advanced fuel-cell system technology for long endurance UAV applications. PEM fuel cells (Fig. 5) have been developed for transport applications as well as for stationary and portable fuel cell applications.
The EPOD EO-210-LE and EO-210-XLE are lightweight, rugged UAV propulsion systems designed specifically for extended flight endurance under the most demanding weather conditions. Their performance has been optimized over the last four years based on extensive testing in several different UAV platforms, including the EO-360-UAV The systems are fully integrated and include all of the necessary subsystems to provide reliable and efficient turnkey UAV propulsion power.
These hybrid UAV power systems were designed to take full advantage of fuel cells for their high energy density and LiFePo4 batteries to provide short bursts of power during take-off, climb, and severe weather conditions. The outcome is that UAVs powered by EnergyOr's fuel-cell systems have a flight endurance that is two to three times longer than those powered by the best rechargeable batteries (LiFePo4).
The EO-210-LE and EO-210-XLE offer a proprietary power management system which includes in-flight battery charging to ensure high power levels are always available, a modular design for optimal UAV integration, low heat and noise signature, exceptional system efficiency, and a system level energy density of over 450 Wh/kg.
Lack of mobile power-source standardization and unification has made the logistics burden of supplying mobile power sources to the battlefield more difficult. For example, there are many different battery types from multiple manufacturers. A standardized minimum number of batteries would alleviate these logistical concerns.
In addition, there is no standard fuel cell size and voltage. And, the method of recharging fuel cells varies from one type to the other. Recharging a fuel cell requires replacement of the electrolyte, for example, a methanol cartridge in the DMFC type. That is, recharging fuel cells does not involve application of a charging current, which is done with rechargeable batteries.