Better batteries help make cars greener--and not all of them are large lithium-ion packs for plug-in vehicles.

Johnson Controls, the world's largest maker of lead-acid 12-Volt starter batteries, thinks small batteries can have a big impact on fuel efficiency too.

The company has developed a compact battery pack that's no larger than a shoebox, the Detroit News reports.

It's intended for use in "micro-hybrids", which are essentially cars equipped with engine-start stop systems.

Johnson Controls uses the term more broadly, to describe vehicles with some amount of powertrain electrification that cannot drive on electric power alone. The small battery pack is tailor-made for these types of vehicles.

The system consists of a 48-volt lithium-ion battery pack and a low-voltage lead-acid battery designed to work with regenerative braking and to support higher power loads.

2013 Ram 1500 - engine start-stop system

2013 Ram 1500 - engine start-stop system

Johnson Controls says adding a micro-hybrid system can improve average fuel economy by 15 percent over a comparable vehicle without start-stop.

The company also says its smaller pack will be relatively cheap--less than $1,000 per vehicle--which will make it more attractive to car makers. Its small size makes it easy to build into a wide range of vehicles with minimal design change.

Carmakers will need all the help they can get to help their combustion-engined vehicles meet increasingly stricter Corporate Average Fuel Economy (CAFE) targets from now through 2025.

There's a catch, though: The smaller lithium-ion battery packs only have an expected life of four years, against 10 years for the full-size lithium-ion packs used in hybrids and plug-in cars.

Replacing the small lithium-ion pack shouldn't be more difficult than replacing a normal 12-volt starter battery, Johnson Controls says, but will probably be more expensive.

Whether micro-hybrids take off or not, less-expensive start-stop systems are already proving their worth.

The 2014 Chevrolet Malibu features a stop-start system with two 12-Volt batteries, which--along with other improvements--allows it to equal the fuel economy of the old Malibu Eco mild hybrid.


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