As stop-start systems gain acceptance in North America, a range of technologies including lithium ion batteries, ultracapacitors, and 48-V "mild hybrid" systems are under consideration to handle the aggressive start cycles, typically more than 20 per day, that are required of these systems. Stop-start is aimed at reducing vehicle fuel consumption and emissions by reducing engine idling.
Even the venerable lead acid battery is evolving. Enhanced flooded batteries and absorbent glass mat (AGM) technologies with deep-cycling capability are slowly displacing batteries used for several decades. While the rapid expansion of electronics overall is a factor, a key reason is the rise of stop-start, which requires quick recharging and long lifetimes.
“Larger 12-volt AGM batteries, which deliver up to four times the typical life cycle of a conventional battery, are important to the current implementation of stop-start,” said Kathi Walker, GM Global Engineering Lead for Stop-Start Systems. “Lithium-ion batteries could be used in the near future.”
While Li-ion batteries may someday take over, they’re currently too expensive to displace the primary storage source for starting, lighting and ignition. However, Li-ion may expand beyond its role in electrified powertrains.
“Lead-acid and advanced lead-acid batteries continue to be the best technology for internal combustion vehicles in terms of performance and cost, and this will continue well into the next decade,” said Craig Rigby, Advanced Market & Technology Strategist at Johnson Controls Power Solutions, the largest global supplier of lead-acid batteries. “That said, fuel efficiency can be found by supplementing the lead-acid battery with other technologies such as Li-ion to deliver brake regeneration and support more electrified functions in the vehicle.”
At present, stop-start is the driving force for these changes. Research published in 2015 by Argonne National Laboratory examined the impact of stop-start systems on vehicle starter system component life, including the battery. The study revealed that the expected lifespan of a conventional flooded lead-acid starter battery is impacted minimally from the number of starting events. Rather, battery life is mostly impacted by limited charge times between frequent engine start events and from excessive discharge during engine-off events from accessory loads. The length of and the cumulative accessory power draw during each engine shutdown event has a direct and strong effect on battery longevity because of the depth of discharge. If the battery is returned to a full charge between engine starts, the effect on battery life is negligible or nonexistent.
Conversely, the ANL research showed that battery failure will occur more quickly if a full charge is never reached regardless of the number of engine start cycles. Also, idling was determined to not be an effective method of recharging the battery because of low alternator power output; driving is best. (See http://www.anl.gov/energy-systems/publication/stop-and-restart-effects-modern-vehicle-starting-system-components.)
Simply providing the power to regularly restart engines is just one challenge for electrical system designers. Keeping radios and interior lights at constant levels—a "buffering" role played by the 12V battery—is a critical factor for consumer acceptance of stop-start vehicles.
“During an auto start, there is a voltage dip in the vehicle’s electrical system,” Walker said. “In order to maintain functionality of cabin systems such as interior lights, there are technologies that can be added to a vehicle to protect the components against a voltage dip and maintain customer satisfaction during an auto start. These technologies include dual batteries, DC/DC converters and ultracapacitors.”
A handful of vehicles already use dual battery systems to support large numbers of power-hungry features and functions. It may become more common as more safety critical technologies are combined on vehicles that use complex fuel-saving techniques.
The 2016 Chevrolet Malibu 2.5-L's stop-start system, for example, uses two batteries: a 12-V battery under the hood and a second one mounted in the rear. The system (equipped with a tandem-solenoid starter for faster starts) is calibrated so that when the ICE shuts off, the second battery is signaled to handle the car's hotel loads—i.e., power the lights, climate control, audio, windows and door locks. Also, with the engine off the car's climate control remains on, albeit operating on reduced power.
“Dual battery systems will become more common as they provide value in several ways,” Rigby said. “Having two batteries provides better voltage stability during start-stop events as well as redundancy to support the high degree of reliability necessary with advanced safety systems. In addition, a dual battery, dual chemistry system allows the optimization of performance and cost to deliver the best value for automakers and consumers.”
Often, the second battery is a 48-V system. Engineering higher-voltage systems can make it simpler to power functions such as parking cameras and integrated sensors. Though going to 48V is beneficial, experts said the industry trend has been to retain the 12V systems. But at the 2016 Chicago auto show, Kia unveiled its 2017 Niro hybrid CUV, which uses its lithium battery pack to provide power for the headlights, windshield wipers, and other traditional tasks of the 12-V lead-acid battery, which has been eliminated (see http://articles.sae.org/14614/).
“Higher voltages help keeping the peak amperage down,” explained Stefano Zanella, Business Development Manager at Texas Instruments. “Batteries are sensitive to the ratio of their capacity expressed in amp hours and the load current, increasing their lifetime and reducing cable size, making them cheaper and lighter."
A 48V rail will require a DC/DC converter or a second 12V battery, he explained, because there are just too many 12V components in a car that are very cheap and cannot be effectively replaced by 48V components.
Some developers are turning to ultracapacitors to provide quick boosts for stop-start systems. They can provide power to turn the engine over without straining the battery.
“Since ultracapacitors do not rely on a chemical reaction to supply their energy, ultracapacitors can discharge their stored energy very quickly,” GM's Walker said. “The 2016 Cadillac ATS and CTS take advantage of ultracapacitors to provide a quick energy boost during the auto start. This burst of energy allows the engine to start faster and helps in providing a more seamless restart for the driver.”
This design technique can extend the lifetimes of lead acid batteries. The Cadillac system, co-developed with Continental, uses Maxwell Technologies ultracaps to augment battery power. It is not available on the ATS-V and CTS-V performance versions.
“Adding an ultracapacitor lets the battery deliver significantly lower starting currents, which leads to less degradation of the battery chemistry,” said Jens Keiser, Maxwell's senior product marketing manager. “Also, the battery sees fewer high-current peak demands.”