DETROIT – As auto makers struggle to design more efficient batteries – key components in hybrid-electric vehicles – hydraulic hybrid technology is getting a second look.

Hydraulic systems are “showing efficiency numbers better than you get with batteries – a lot better,” says David Hermance, executive engineer-environmental engineering at Toyota Technical Center, U.S.A., Inc.

“If, in volume production, those prove out to be reasonable numbers, then maybe you start looking seriously at how would I package it?”

Packaging has been the bane of hydraulic hybrid technology because, to date, such systems require large cylinders to store the required energy. For this reason, heavy-duty vehicles have been the testing ground for hydraulic systems.

Just last month, China Daily reported that a fleet of 50 hydraulic hybrid buses were ready to hit the streets of Beijing. Last year, the Environmental Protection Agency agreed to work on a hydraulic hybrid UPS truck with Eaton Corp., which previously had outfitted a garbage truck with such a system.

“They work really best as garbage trucks because that duty cycle really, really, really has lots of high-power pulses and lots of high-power discharges during acceleration,” Hermance says. “And the hydraulic systems are really good at that. There are challenges with packaging hydraulic systems in a small vehicle. Will we see them in smaller vehicles? Maybe.”

As Toyota marches toward its 2010 goal of putting one million hybrids on the road annually, it is doing “a huge amount of work” on “a wide variety” of hybrids, Hermance says during an interview at the 2006 Society of Automotive Engineers World Congress.

Among them is a system that uses lithium-ion batteries instead of the currently popular nickel-metal hydride.

Lithium-ion batteries charge and discharge more efficiently. Therefore, a hybrid system could realize weight savings because its battery could be made smaller.

“In the battery world, the operation most damaging to the battery is the top 15% of charge and the bottom 15% of discharge,” Hermance says. “At those ends the chemical processes start to get a little wacky, and you start doing things that deteriorate the battery.”

For this reason, Toyota’s hybrid batteries stay within 20% of each extreme.

“It never sees anything more than 80% state of charge; never sees anything less than 40% state of charge,” Hermance says, adding Toyota is not worried about its nickel-metal hydride battieries.

“We’ve got lab data for 180,000-mile (290,000-km) equivalent (use) with no deterioration. Not with a little deterioration – no deterioration.”

Hermance notes that, to date, Toyota has not had a single warranty claim related to a hybrid battery. And the auto maker does not expect to ever replace a spent battery, he adds.

The next plateau for powering hybrids could involve adding ultra-capacitors.

“While they’re great power devices – you can smack ’em really hard, you can capture all the re-gen pulse – you can’t store much energy in them,” Hermance says, noting the battery would solve the storage problem.