NiMH batteries are prime candidates for HEV applications. They normally have a specific energy of 80 W-h/kg, a specific power of 200 W/kg, and an energy efficiency of 65%. With a great degree of abuse tolerance, these batteries offer a longer cycle life than lead-acid batteries. The main challenges to NiMH battery development are high cost, low cell efficiency, high self-discharge, the need to control hydrogen loss, and the need for a recycling infrastructure. A price comparison of vehicles supplied by lead-acid batteries and those supplied by NiMH batteries reveals the high cost of NiMH technology. For instance, the cost of one electric vehicle, the 1996 Solectra Force sedan, more than doubled when its lead-acid batteries were upgraded to NiMH.
Nevertheless, a sealed NiMH battery is employed in a current HEV, the Toyota Prius, which optimizes this technology for the hybrid environment. Essentially, the Prius battery is a peak-power device that enables regenerative energy storage. It's comprised of 228 1.2-V cells connected in series and organized into 38 sealed NiMH modules for a nominal system voltage of 273.6 V. Nominal energy storage is designed to be 1 kW-h.
One of the leaders in NiMH technology research is Ovonic Battery Co., Troy, Mich., a subsidiary of Energy Conversion Devices Inc. With high power delivery, the Ovonic NiMH batteries have fast recharge capability, charging up to 60% of capacity in 15 minutes. These batteries have drastically increased typical energy density with ratings of up to 80 W-h/kg. Furthermore, the company has a patented NiMH technology capable of achieving a specific energy of 95 W-h/kg.
Ovonic also is investigating the use of a bipolar approach to constructing NiMH batteries. While it may lead to free electrolyte leakage and corrosion of core components, improvements to this approach may render it useful in NiMH technology. The Ovonic monopolar NiMH cells demonstrate power densities of 600 W/kg and over 1700 W/L. With the reduction of the resistance factor through a bipolar approach, researchers at the company believe that they can achieve an NiMH battery with rates of 1000 W/kg and 2500 W/L.
A second technology slated for use in HEVs by PNGV, Li-ion batteries offer high specific power and high energy efficiency. With low self-discharge, these batteries provide efficient high-temperature performance. Li-ion batteries supply good energy density. Also, to gain high power in the batteries, energy density may be compromised. Presently, these batteries are in the experimental phase as a long-term option for HEV use. Major improvements must be exacted to increase the possibility of using Li-ion batteries in HEVs. The calendar and cycle life have to be drastically improved. Cell and battery safety measures need to be integrated into the technology, and abuse tolerance measures must be added. Overall, the manufacturing costs of this technology must be reduced before Li-ion batteries are commercially viable.
Currently, Saft America Inc. is developing high-power Li-ion cells that achieve between 1350 and 1500 W/kg. They also demonstrate a relatively good specific energy of 64 to 70 W-h/kg. Nominal voltage is 3.6 V, and energy density is 135 W-h/dm3. The power density of the Saft Li-ion cells is 3100 W/dm3.
Additionally, the efficiency of Li-ion batteries is under investigation by T/J Technologies Inc., Ann Arbor, Mich. This company has developed a new tin-based anode material for Li-ion batteries that enables lowering first-cycle losses in Li-ion batteries.
As far back as 1996, the USABC slated lithium-polymer batteries as the most promising long-term technology for HEV applications. This organization projects that lithium-polymer batteries will offer four to five times the energy density of lead-acid batteries and be produced in the U.S. for close to $100/kW-h. Now, the USABC is in the second phase of an ongoing development contract with 3M Corp., St. Paul, Minn., and Hydro-Quebec, Montreal, to develop efficient lithium-polymer batteries. Lithium-polymer batteries offer high specific energy and high specific power as well as good cycle life and long calendar life. Plus, these batteries offer a sufficient degree of safety measures. To heighten the usefulness of this technology, the specific power of these batteries must be raised while manufacturing costs are lowered.
Today, there's a multitude of research into the battery technology that may effectively replace lead-acid batteries in HEV applications. Saft and Argonne National Laboratory, Argonne, Ill., are both developing lithium-iron-disulfide batteries. On the zinc-bromine technology forefront, Powercell Corp., Cambridge, Mass., and Daewoo Motor Corp., Korea, are in the middle of a joint development project.
Acme Electric Corp., East Aurora, N.Y., is evaluating nickel-cadmium battery technology. Meanwhile, Electrosource Inc., Austin, Tex., is focusing on the use of composite materials to reduce the weight and increase the energy of traditional lead-acid packs. While experts vary in their projections, the consensus is that NiMH and Li-ion batteries will suit near- and mid-term HEV applications. The opinion largely held is that lithium-polymer configurations will dominate the long-term HEV battery market.