26 May 2004—For decades, the lead acid battery has dominated the US $16 billion industrial and automotive markets. But lead has not always led in hybrid electric vehicles, where far pricier nickel metal hydrides (NiMHs) are found under the hood of popular models like the Honda Insight and Toyota Prius. Now a team of researchers from the Indian Institute of Science (IIS), Bangalore, thinks it has found the way for lead acid to match NiMH’s performance.
The IIS group has developed a technique for making lead acid batteries substantially lighter, and therefore more energy dense—a key determiner of a battery’s usefulness in electric vehicles (EVs) and HEVs. Currently, lead acid batteries have a density of 30 watt-hours per kilogram, but the new process delivers batteries with more than 50 Wh/kg, which is the minimum required energy density for EVs and HEVs, according to A.K. Shukla, a professor in the solid state and structural chemistry unit at IIS.
Switching to lead batteries should drive down the cost of EVs and HEVs, because the battery constitutes about 15 to 20 percent of the total cost for lead battery-driven cars, compared with up to 50 percent for NiMH and lithium-ion battery-driven vehicles.
Most efforts to enhance the energy density of lead acid batteries rely on using lightweight materials in the grids that act as the battery electrodes. In conventional batteries, one grid is made of lead, the other of lead with a lead dioxide coating. They are submerged in sulfuric acid to form the battery. When discharging, current flows from one grid to the other via a chemical reaction that turns the surfaces of both electrodes into lead sulfate. Researchers have tried to make the grids lighter by using lower density metals like aluminum, or plastics such as polyvinylchloride, and then coating those with lead. These attempts failed because the plastics melted at the typical temperatures involved in making batteries, and the sulfuric acid tended to eat into and disintegrate the grid.
Still, the IIS team thought it could succeed with a plastic substrate. ”It was important for us to keep the cost low. Therefore, plastic-based substrate material became imperative,” says Shukla. The key was figuring out a way to make the battery without melting the grid and, at the same time, to protect the grid from the acid.
To accomplish that, IIS turned to a technique ordinarily used in the semiconductor industry, but adapted it to work on plastic instead of silicon. First, they used an electrochemical reaction to deposit 10- and 100-micrometer coatings of copper and lead, respectively. The lead coating is particularly thin, helping reduce the grid weight, and the electrochemistry won’t melt the plastic.
Another innovation is the use of halogen lamps to drive a rapid chemical reaction that applies a 10-µm-thick layer of tin oxide onto the grid to protect it from the acid. Because the coating takes just five seconds, the grid doesn’t melt. The result was a 75 percent lighter electrode, says S.A. Shivashankar, an associate professor in the materials research center at IIS. As an added benefit, the process should be less environmentally onerous because it uses much less lead.
While experts consider this new process a scientific advancement, they say it’s too early to celebrate. According to Patrick T. Moseley, program manager, Advanced Lead-Acid Battery Consortium, Research Triangle Park, N.C., ”One of the misgivings about the idea that it will revolutionize the prospects for HEVs is the economic viability of the process for industrial manufacture.”
”This concern is justified,” admits IIS’s Shivashankar, ”and we have calculated the likely constraints in the industrial environment. Since we have worked with the industry right from the beginning, we are confident that this process can be incorporated into assembly line production. This is not a [deal breaker].”
A Hyderabad, India�based company, NED Energy Ltd., has been testing the lab prototypes. Says S.A. Gaffoor, vice president of NED, ”Our data shows improved performance in energy density, fast charging, and corrosion resistance. We are scaling up our efforts for field action.” Gaffoor sees a lot of potential for this technology not only in EV applications, but also in areas where high power is required for short durations—such as tripping circuits in power plants, complex control circuits in petrochemical plants, and firing circuits in weapons systems.
Experts like Moseley and Isamu Kurisawa, of the Japan Storage Battery Co., Kyoto, argue that one of the more serious challenges for lead acid batteries in the HEV environment is dealing with the rapid accumulation of sulfate on the negative plate. Says Moseley: ”The problem for lead acid is that in the HEV, the battery is required to operate almost all the time at a partial state of charge and to supply and receive charge at high rates. In conventional designs of a lead acid battery, this leads to sulfation on the negative plate and early failure. It is this problem that needs to be solved for HEV operation, rather than the energy density problem, which was the preoccupation with EVs.”
The IIS researchers claim their battery will see use in both EVs and HEVs. They admit that they have not addressed the issue of sulfation in HEVs yet. But the reduced weight and increased energy density achieved through this research might resonate for designers hoping to solve the sulfation syndrome in other projects. One example is the European Advanced Lead Acid Battery Consortium’s Rholab project, which aims to adapt traditional lead acid technology for use in HEVs.
In an announcement in February this year, Rholab collaborators said they had designed a new lead acid battery that they planned to road-test shortly. For the 80 000-kilometer trial, the Rholab team will fit the new battery into a Honda Insight HEV. According to Rholab researchers, the negative-plate sulfation in the lead acid battery was addressed by a so-called intelligent battery management system. The system optimizes battery performance by monitoring and controlling the individual cells within the battery—for instance, heating or cooling them and estimating their state of charge.
The manufacturers of the Indian electric car, Reva, have devised and patented something similar, says Chetan Maini, managing director of Reva Electric Car Co., in Bangalore.
Maini says the lead acid weight issue is important as well, and he has been watching the NED-IIS project closely. ”I would indeed try out this new lead acid technology in Reva if it actually reduces weight by 40 percent and is made viable for commercial use,” he says.
Commercialization is probably three years and about $300 000 away, with about a third of the funding coming from the Indian federal Department of Industrial and Scientific Research.
The NED-IIS battery will likely see its first use in a developing country, in a Reva or another car. ”We feel that an interim solution in battery technology for EVs and HEVs is required for a country like India, where it is difficult to think of directly going for sophisticated and costly lithium-ion batteries or fuel cells,” says NED’s Gaffoor. EV requirements in the West are different from those in India, in terms of vehicle size and speed, level of comfort, and average distance traveled between chargings. For instance, a suitable EV for India would be small and lightweight, without air conditioning or heating. The distance traveled between chargings would be about 60 to 70 km per day, with a maximum cruising speed of 40 to 50 km per hour, much lower than that required in western countries, explains Gaffoor.