Virtually every modern naval weapons system, in fulfilling its mission, is highly dependent upon the performance characteristics of its electrochemical power supply. For example, batteries are used to power detection, guidance, control, ignition, propulsion, arming, fuzing, transmitting and jamming devices. The need for weapons with increased range, speed, endurance, sensitivity and accuracy created battery
requirements which could not be fulfilled with conventional technology. In many instances, batteries have become the limiting factor in
improving the performance of weapon systems.
Batteries have been used on underwater applications for more than a century, for instance, lead-acid batteries have been used in conventionally powered submarines since the end of the 19th century. Since World War II, submarines have used lead acid batteries. Lead acid batteries are heavy, but they’re also a proven technology. They are utilized to power the vehicle’s main propulsion, or as a stand-by battery. Lead-acid battery (LAB) technology, even with its drawbacks in power and energy density, has survived a century as a source of stored energy for main propulsion or as stand-by batteries for submarines.
However, increased endurance and speed demands have stimulated the development of a new generation of energy storage technology, based on mature Lithium-ion battery (LIB) technology. In addition, the lead-acid battery loses power over time and must be charged more frequently as the service life increases. They’re also obsolete in the consumer world, replaced in the 1990s by nickel metal hydride batteries. Today’s devices are powered by an even better technology, lithium ion batteries.
The present grid-scale energy-storage sector is dominated by lithium-ion batteries, because of their higher energy density & specific power and long cycle life. However, there are some serious concerns regarding Li-ion batteries, such as safety risk, limited resource supply, high cost and lack of recycling infrastructure. This necessitates the development of an alternate battery system with lower environmental concerns, economical and higher energy density.
In this concern, lead acid batteries are still one of the most reliable, economical, and environmentally friendly options. However, electrodes in the lead acid batteries suffer from the problem of heavy weight, corrosion, poor thermal stability and diffusion of electrolyte in one dimension which ultimately affect the output power.
Lead-acid batteries are temperature sensitive, providing optimal performance between 10-deg C (50-deg F) and 50-deg C (110-deg F). At minus 25-deg C (-15-deg F), the battery will typically deliver 20% of the power that it would deliver at 25-deg C (75-deg F). Repeated deep-discharge from 100% to below 10% of storage capacity greatly reduces service life expectancy. Sulfur build-up on lead plates reduces storage capacity while fully drained batteries often cannot be recharged.
While power designers have used several clever techniques to compensate for these limitations (perhaps most notably the use of sophisticated charge monitoring, or the use of ultracapacitors in hybrid power systems), major advances in lead-acid battery technology itself hold the greatest potential for enabling new capabilities cost-effectively.
The lead plated battery that contains 5% sulfuric acid and 95% silicon dioxide is also dubbed the lead-crystal battery and easily sustains the operation of battery-powered, low-speed, short-distance boats during cold weather. Lead-crystal batteries cost 1/3rd as much as lithium batteries and can deliver over 2,500-cycles of service when repeatedly operated at 50% depth of discharge. At 50% the cost of lithium batteries, the carbon foam battery is currently only available with sulfuric acid electrolyte and can deliver over 3,500-cycles at 50% depth of discharge and over 1,000-cycles at 80% depth of discharge.
While most operation of small boats occurs in warm weather, a small number of boats continue to operate during colder weather. When air temperature drops to the freezing point of water, salty seawater remains liquid and especially when waves, tidal currents and ocean currents are present. At such temperatures, flowing river water also remains liquid and allows for boat navigation. Lead-crystal batteries can deliver over 65% of its rated power at sub-freezing temperatures as low as 0-degrees F. Other than propulsion, these batteries can fulfill a wide range of other applications on small boats powered by internal combustion engines.
Among all carbon materials, carbon foam is one of the lightweight (<0.5 g/cc), highly porous (> 85%), which have highly resistive to corrode, good electrical and thermal conductivity (>100 W/(m.K)) with high surface area and have recently attracted a lot of attention owing to their potential applications in various field.
Microcell technology used in Firefly’s 3D and 3D2 carbon-graphite foam batteries represents a significant advance in lead-acid battery performance.
According to Mil Ovan, Sr. Vice President of Firefly, the fundamental advance embodied in Microcell technology is the use of carbon-graphite foam, which replaces the conventional lead grid as an electrode. During manufacturing, the foam is exposed to a lead slurry, creating a lead-coated surface having the dual advantages of greatly reduced weight and greatly increased surface area. The 3D battery uses a carbon-graphite foam for the negative electrode, and a conventional plate for the positive electrode. In the 3D2 battery, both positive and negative electrodes are made of carbon-graphite foam. These two configurations allow designers to optimize for either cost or performance.
Ovan states the lead-acid electrochemistry of conventional batteries is unchanged in Microcell technology. Rather, new physical geometries enabled by the use of carbon foam allow a greater utilization of the full energy potential of lead-acid battery chemistry. The theoretical limit for the chemistry itself is 170 Whr/kg. Conventional batteries harness only 30 Whr/kg to 50 Whr/kg.
This full utilization is possible because the new geometry brings all materials undergoing chemical reactions into closer proximity, providing the electrodes greater access to the full quantity of the electrolyte. It also expands the surface area of the electrodes, the sites where reactions occur. The increase in electrode surface area combined with the small distances between the positive and negative electrodes (through which ions must flow within the electrolyte, reduced from several millimeters to less than 400 μm) increases the reaction rate. The reaction rate in a lead-acid battery is electrically modeled as the internal series resistance, which in turn is a reflection of battery power capacity.
According to Kurt Kelley, CTO for Firefly, while exact comparisons are difficult, tests have been conducted that compare a 1-C rated Firefly battery discharged at a 5-C rate against a 1-C low-resistance, spiral-wound, lead-acid battery. At 93% state-of-charge (SOC), the discharge internal resistance of the spiral-wound was 11 mΩ, and for the Firefly battery this was 3 mΩ. At 10% SOC, this increased to 13 mΩ for the spiral-wound, and 4 mΩ for the Firefly battery.
In addition to these electrical performance advantages, there are mechanical and thermal advantages. Most notably, Microcell technology enables superior cold-temperature performance. Ovan states, for example, that a battery at -20°C using Microcell technology may have 60% of its room-temperature capacity, while a conventional lead-acid battery may have only 20%.
Also, because lead-coated carbon foam is much lighter than all-lead electrodes, total volume or weight reductions on the order of 50% over conventional batteries are possible, with faster discharge rates (under 1 hour) and cold temperature operations showing the greatest gains in weight/volume reduction. Another advantage derived from using foam is the ability to accommodate swelling caused by the production of sulfate crystals formed as lead-acid batteries are discharged, without the mechanical failure that can occur in conventional batteries. The smaller electrode geometries of carbon-graphite foam also keep these crystals small enough to be easily decomposed upon battery recharging, thereby preventing sulfation.
Both the lead-crystal and carbon foam batteries have potential application in a variety of port service vehicles. Fork-lift trucks and container picker trucks require considerable counter-weight, enhancing the suitability of banks of heavy lead-based batteries for such service at smaller ports. Some smaller ports use industrial-type battery-electric locomotives for short-haul shunting service. Both lead-crystal and carbon foam battery technologies offer superior cold weather performance than traditional lead-acid batteries, allowing for shunting operations with air temperature at 0-deg F (-18-deg C), when either battery would offer over 65% of energy availability compared to 70-deg F.
Both lead-crystal and carbon foam batteries can sustain several hours of operation of engine heaters that require battery power to operate a water pump and combustion of a small amount of hydrocarbon fuel. Prior to engine starting, these batteries can also sustain operation of electric oil pumps that pre-lubricate the IC engine. It will then recharge an ultra-capacitor that will dump high starting current into the engine electric starter motor, before blending in to maintain engine cranking prior to ignition. Battery power can also recharge a new spring-loaded engine cranking technology to initially turn over the engine.
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