Carbon Breathes New Life Into Lead-Acid Batteries

May 2, 2007 5:11 PM
By Mark Valentine, Technical Editor, Power Electronics Technology

The Microcell technology used in Firefly’s 3D and 3D² carbon-graphite foam (see Figure 1) batteries represents a significant advance in lead-acid battery performance. The company has now teamed with NorthStar to bring the technology into production with advanced lead-acid battery manufacturing techniques. A large portion of the funding for this phase of development is provided by the U.S. Army, which has awarded Firefly a contract under its “Silent Watch” program. This is a project that seeks to develop batteries with sufficient capacity to power electrical equipment in military land vehicles during stealth operations.

Figure 1. The carbon-graphite foam electrode used in Microcell technology was originally developed for heat dissipation, providing enhanced thermal conductivity as well as good electrical conductivity and immunity from corrosion when used as a battery electrode.

Conventional lead-acid battery technology is the most mature and effective battery technology for vehicle electrical systems. Reflecting this fact, 95% of all military vehicles use the T6 lead-acid battery, according to a market study by Frost & Sullivan. However, the limitations of the technology, such as weight, slow recharge-ability, limited lifespan, sulfation and internal corrosion of the lead electrodes (see Figure 2), are equally well established. 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.

Figure 2. In conventional lead-acid batteries, Lead grids can corrode in the acidic electrolyte in the presence of lead-dioxide, the positive plate active material.

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.

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