An advanced start-stop system using two different battery chemistries is now in the prototype phase, Johnson Controls officials announced during media days at the 2015 North American International Auto Show.
Most conventional start-stop systems use a single lead-acid battery, typically a 12-volt absorbent glass mat (AGM) battery or an enhanced flooded battery (EFB). But a dual-battery design can provide a bigger performance payback for micro-hybrid electric vehicles.
The breakthrough with Johnson Controls’ two battery system is the combination of reliable start-stop performance from an existing technology, like AGM or EFB, and a significant increase in charging performance from a lithium-titanate battery, according to Craig Rigby, Advanced Market Technology Strategist for Johnson Controls Power Solutions.
“This dual battery start-stop system enables both an increase in fuel economy and a reduction in greenhouse gas creation because capturing energy from braking events will allow the engine to stay off for longer periods.
“We believe this will lead to an additional fuel efficiency gain of 3 to 5% when compared to existing start-stop systems. This is the same as an up to 8% gain over baseline non-start-stop vehicles,” Rigby said in an Automotive Engineering interview.
Johnson Controls and Toshiba are collaborating on the lithium-titanate cell design. “Lithium titanate is used in production automotive applications today, including 12-volt start-stop applications in Japan,” said Rigby, adding that the lithium-titanate chemistry is effective for fast recharging under a wide range of temperatures.
The specific roles of the lithium-titanate and lead-acid batteries in the Johnson Controls’ advanced start-stop system will depend on an automaker’s vehicle implementation strategy.
“Generally, the lead-acid EFB or AGM battery will be responsible for starting the engine and supporting electrical loads (such as headlamps, navigation, and audio) after the key has been turned off. The lithium-titanate battery, on the other hand, will perform functions that require high power, like capturing electrical energy from vehicle deceleration (i.e., brake regeneration), something that is challenging for traditional battery technologies,” said Rigby.
The lithium-titanate battery will be in a hard case, prismatic shape. Overall package size for the dual battery configuration is approximately 7.4 in (190 mm) high, 9.4 in (240 mm) long, and 3.5 in (90 mm) wide.
Start of production for Johnson Controls’ next-generation advanced start-stop system is expected in 2018.
“We are in the prototype phase right now and are evaluating the design against several sets of customer requirements,” said Rigby, “We are also testing the battery system in our own test vehicles that we’ve modified to include the advanced start-stop function.”