Rethinking lead-acid for mild- and micro-hybrids

  • 20-Sep-2012 10:56 EDT
SubhashDhar1-EPS.JPG

Energy Power Systems founder and CEO Subash Dhar spent years in lithium-ion development—and grew disenchanted with the industry's ongoing claims about materials cost reduction. (Image by Lindsay Brooke) 

Thirteen years after Toyota and Honda launched the modern electrified-vehicle era with their Prius and Insight hybrids, the global market for HEVs and EVs remains miniscule compared with conventional ICE-powered vehicles. Even with an increasing number of nameplates from virtually every OEM (more than 40 as of 2012), Prius still accounts for the biggest slice of the segment, by far. No other hybrid or EV comes close.

What’s the problem? Battery cost is still the primary hurdle to the widespread adoption of electrified vehicles, industry experts note. But the promise of better, less expensive battery technology has not come close to bearing fruit. In 1992 the U.S. Advanced Battery Consortium set a 10-year cost target of $150/kW·h for the “advanced” lithium-based solutions then emerging. At the time, nickel metal hydride packs of the size used in the original Prius were estimated to cost the OEM about $3500. Automakers targeted a cost reduction to $1400 per pack as needed to drive high-volume vehicle sales, but in 2012 the same pack is estimated to cost nearly $2000.

“Despite huge technological advances made in nickel metal hydride and lithium-ion batteries and their processing in the past 21 years, the problem still is cost,” said advanced battery industry veteran Subash Dhar. “We can have a debate on what the real cost is for current hybrid and EV batteries, but in my view it is north of $700/kW·h. We need to be at $250/kW·h or less. That’s a factor of three.”

Dhar, a chemical engineer, is founder and CEO of Energy Power Systems (EPS). The Troy, MI-based start-up is moving steadily down a technology path most others in the hybrid and EV battery industry had long abandoned: lead-acid. He is convinced there is a major opportunity for the world’s oldest, most ubiquitous battery chemistry that has provided safe and reliable energy storage since 1859.

“What made me think about starting this company and focusing on lead-acid was my disenchantment with what I was hearing from the battery industry regarding lithium,” he told AEI. “People claiming 300-400 W·h/kg and that $150 kW·h was right around the corner. It doesn’t matter who says what, but I’d realized that NiMH and lithium battery costs are not coming down anytime soon. I thought something different is needed, that we can’t depend on these claims that keep getting repeated every 10 years.”

Dhar said that though he applauds and supports the many development programs, he believes the auto industry must become “more pragmatic about what we can do during the next 10 years to reduce the battery burden on the vehicle. We can either use a smaller battery or reduce the cost of the battery by a factor of three or four.”

To do that requires materials that provide four times the energy with about one-quarter the amount of material. That was Dhar’s focus while he was president of NiMH pioneer ECD-Ovonic Battery (from 1982-2003), then later vice chairman of EnerDel (and president of its parent, Ener1), and Chairman and CEO of Envia. “Like everyone, I was chasing the dream of electric cars that go 250-300 miles between charges at costs comparable to an ICE vehicle,” he said.

“But I shifted my thought process from range and distance between charges to the question, ‘How do we go about storing electrons?’ The focus has to shift, I think, from range to fuel economy.”

While Dhar noted that energy density is one of the drivers behind Li-ion development, cost continues to be an issue. “Can such expensive materials, $10 to $30 per kg, provide economically acceptable solutions? Can $10/kg material give you 3X energy density? That’s a tough proposition, and people are working on it,” Dhar said. “I’m focused more on how to control battery cost per vehicle by reducing the size of the battery and starting with materials that are inherently much less expensive.”

Lead continues to be the least expensive material used to store electrons. “At $700/kW·h, lithium can provide those electrons, yes. But rather than start with the highest-energy-density battery and work on it over time and volume, and try to reduce the cost, we came at this from the other end: Start with low-cost materials and improve their performance.”

Assembling the EPS team

Dhar recalls he came across some technical papers on lead-acid batteries and wondered: Can power be quadrupled? Can the failure mechanisms of lead-acid chemistry be fully broken down and understood? Why do the plates have to be X thick and the particle size 10 microns? Why is a cast grid needed? And what if the plates are thinner and particle size is 0.5 micron?

Investors secured, Dhar and his initial nine-member team put EPS’s plan in motion in 2011. During the past year the core team has grown to 31—primarily materials scientists, design and manufacturing engineers, and technicians—with another nine positions planned by the end of 2012.

“I could have put together a team with lots of lead-acid experience. But I didn’t want to contaminate our thought processes with people who already know what doesn’t work,” Dhar explained. “Instead I chose people who had helped me develop NiMH and lithium batteries. We used our knowledge gained in developing nickel and lithium—our microscopic understanding of the materials, the morphology, the electrochemistry, the porosity and structure. We applied it at the same level of attention to lead-acid, which was, I believe, outside the scope of USABC, the U.S. Department of Energy, and Argonne National Laboratory.”

That’s not to say the battery industry has neglected lead-acid technology. Exide, Varta, and others continue to refine their products (http://www.sae.org/mags/sve/11378) and develop new ones aimed at electrified vehicles.

After 10 months, Dhar brought in two lead-acid chemistry “gurus” who helped the EPS team shorten its product-development time. Their strategy: Concentrate development only on batteries used in micro-hybrid (stop-start) and mild-hybrid applications (EPS’s target markets) and on improving power and cycle life.

Their first stage of development (known internally as Gen-1) is described by Dhar as a “quasi-bipolar” design with multiple stacked lead plates. It’s designed to be air-cooled and features a unique thermal management system EPS acquired from a Chicago-based firm.

“We can put our battery under the hood if that’s what is required,” Dhar said.

“The fundamental problems of the lead-acid battery are life and energy density,” he noted. “We don’t care about energy density because we’re focused only on making small-sized batteries—no more than 2 kW·h. For stop-start and mild-hybrid applications with some assist and regen, you don’t need more than that—maybe no more than 1.5 kW·h. We need to increase power because when you have a 1-kW·h battery and you need 18 kW from it, the power-to-energy ratio needs to be much higher than it has been with lead-acid.”

Dhar concedes that lead-acid, at least in its current form, will never achieve specific energy levels of 150 W·h/kg. “It’s just a fundamental thermodynamic limitation, and it’s why everybody working on hybrids and EVs gave up on it. At best we said maybe it can do 70 W·h/kg,” he said, “but then when you go from 25 W·h/kg to 70 W·h/kg, what other trade-offs might you encounter? What will happen to cycle life, among other things? But if we can improve the power of this chemistry and improve its cycle life, we can address all the needs of micro- and mild-hybrid applications.”

Aiming to beat eAssist

Industry analysts have described General Motor’s eAssist system as “providing 80% of the efficiency of a full-hybrid system for about 20% of the cost.” For EPS, the eAssist is both a technical bogey and an opportunity for the company’s lead-acid battery solution. The relatively compact mild-hybrid system used in various Buick and Chevrolet models uses a 115-V, 480-W·h Hitachi Li-ion battery that Dhar estimates is roughly 40% of the $1200-$1400 system cost.

“This is a very powerful battery—very high power density and reasonable energy density,” he said, adding that it is designed to provide a small amount of boost and moderate regen capability. “Our target is to provide a lead-acid battery that meets the eAssist performance and dimensional requirements for less cost.”

He shows a PowerPoint slide with a photo of the Gen-1 EPS battery pack fitting within the footprint of the eAssist battery. The package volume of the eAssist battery is about 14.6 L; the EPS battery consumes 12 L. The lead-acid EPS pack (air-cooled, like the eAssist) weighs 4-5 lb (1.8-2.3 kg) more than the Hitachi lithium battery, Dhar admits, due to lead-acid’s lower energy density. However, he explains that with a small battery around 0.5 kW·h, weight is not as critical as the overall package size.

Energy density of the complete EPS battery pack is 45 W·h/kg, Dhar claims, vs. 36 W·h/kg for the eAssist pack. That’s due to less parasitic weight—primarily less conductive copper that is required for the Hitachi unit’s 32 interconnected, cylindrical cells. “All of our half-kilowatt-hour of energy comes from one container,” he said. “Our energy density is flat at 40-45 W·h/kg regardless of whether it’s a 0.5 kW·h, 1 kW·h, 1.5 kW·h, or 2 kW·h battery.”

In terms of maximum power density, the EPS Gen-1 had achieved approximately 2000 W/kg in early-summer testing. By comparison, the best lead-acid power density is about 450-535 W/kg. “And I’m not talking about a single cell in a little beaker on a lab bench—these are real 20 a·h, 12-V batteries,” Dhar noted. He said development is now focused on increasing cycle life, which currently is roughly double the 1000 cycles at 80% depth of discharge cycle life of the leading lead-acid batteries in production.

Without revealing greater details of the EPS battery’s architecture and manufacturing plan, Dhar quietly said there is no radical enabler within the Gen-1’s case. “We haven’t added a little silver, or a little cobalt, for example, or some unique manufacturing process. Our cost structures are the same as JCI uses to make DieHards,” he said.

'Hybrid chemistry'

Besides the micro- and mild-hybrid markets, the EPS engineers also are investigating what they call a “hybrid battery chemistry.” This is combining the smaller lead-acid pack with a larger lithium pack. The resulting “hybrid” energy storage system benefits from the high-energy-density attributes of Li-ion and the high-power-density attribute of the EPS battery.

For example, in a Chevrolet Volt the large, heavy, and costly 16-kW·h Li-ion pack could be paired with a 3.5-kW·h EPS pack (the high-power pack), which would enable the Volt battery to be downsized to 12 kW·h or 9 kW·h to provide high energy.

“So when you need 136 kW from this battery, shift the power load to the EPS lead-acid battery, because it has enormous power, 120 kW. And shift the energy to the lithium-ion pack,” Dhar opined. The cost per kilowatt-hour would be reduced from about $800 (estimated in the current Volt) to about $500, he said.

Last summer the USABC came out with a request for proposal (RFP) for development of a new 12-V battery aimed at micro- and mild-hybrid requirements. Reading into the text, with its limitations on battery weight, the focus is on lithium-ion. The EPS team read the RFP and realized their lead-acid battery can meet all the proposed performance requirements.

“We might be a couple or three pounds heavier, but we’re the only ones who can meet the cost—$260 per battery if it’s located under the hood and $210 if located elsewhere on the vehicle,” Dhar said. “We’re there. And I don’t believe lithium can get there on cost.”

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