Stop-start systems have seen success in Europe, but development teams focused on the U.S. market are still working to get the right combination of fuel efficiency and comfort. Smooth restarts and radio volume are among for biggest challenges for garnering desired customer satisfaction ratings.

Stop-start is common in Europe and Japan, but acceptance in North America has been limited. The 4-10% fuel savings hasn’t been a big lure given fairly low fuel prices and noticeable noise and shuddering during restarts. That may change as regulations take hold and systems become more sophisticated.

“Adoption has been much slower than people anticipated back in 2007-08, but it should go forward quicker because of the CAFE credits available,” said Devin Lindsay, Powertrain Forecasting Director at IHS Automotive. “Systems have reduced the feeling when the engine engages, and many will have a button that lets the driver turn the function off.”

U.S. carmakers are increasing their offerings. General Motors is making stop-start a standard feature on many 2016 Cadillac models. Customer education and technology are both critical factors in the future of the technology.

“The most challenging part of the stop-start technology is educating the customers so they understand how the system works and they become more accepting/comfortable with the technology,” said Kathi Walker, GM Global Engineering Lead for Stop/Start Systems. “From a design perspective, repeatability of quick and seamless auto starts across vehicle platforms is very challenging.”

GM uses Continental Automotive Systems’ voltage stabilization system, which employs Maxwell Technologies’ ultracapacitors. The ultracapacitor helps maintain current to keep radios and air conditioners running when the starter is cranking. It also lessens engine vibration to smooth out restarts while reducing repeated cycling that can shorten battery life.

The ultracapacitor also helps GM meet requirements in cold environments. These components have equivalent series resistance (ESR) capabilities that give them better winter performance.

“In cold temperatures, the battery ESR is increasing and therefore, cranking power for the starter is below what is required for a reliable restart,” said Jens Keiser, Senior Product Marketing Manager at Maxwell. “The ESR of ultracapacitors is stable until -40 degrees Celsius.”

Regardless of the temperature, voltage levels often fall to around 4-6 V when the engine is started. That can impact systems that have higher voltage and current requirements such as infotainment, where some components don’t have the low 3.3 or 1.8 V levels of many digital devices.

“One of the trickiest areas is the audio system,” said Bill Cleaver, Principal Engineer, Market Development at STMicroelectronics. “A lot of sensitive analog parts in the audio systems operate at 5V—things like the AM-FM tuner and power amps, which have a fixed impedance so they need higher voltages to maintain sound levels. If music is loud, volume will be reduced unless you have boost support built into the power system.”

Concerns like these prompted GM to include ultracapacitors in the Cadillac system. That helps reduce battery size and weight.

“The energy demand of a cranking event is very low, but the power demand is very high, therefore the battery has to be oversized to meet these high power requirements,” Keiser said. “By adding an ultracapacitor to address the high power requirements, the battery can be downsized and focus on the energy demands. Ultracapacitors have approximately six to 10 times the power density of the typical lead-acid battery.”

The challenges are greatest in the mainstream of the U.S. market: internal combustion engines with automatic transmissions. The systems needed to ensure that restarts occur smoothly are a fair cost-adder.

“Any hybrid vehicle has the controls, so it’s easy to keep everything running,” said Roger Forchhammer, Senior Manager, Automotive Product Group (APG) MCU Marketing at STMicroelectronics. “If it’s not a hybrid, there’s a significant cost bump.”

Some battery makers are ramping up for solid growth. Restarting engines during city driving will require longer lifetimes and quicker recharging. Absorbent glass mat and enhanced flooded battery technologies are expected to see solid growth in vehicles with stop-start.

We expect it to reach 40% of new vehicle production by 2020,” said Craig Rigby, Advanced Market & Technology Strategist at Johnson Controls Power Solutions. “All the major automakers have indicated the move to start-stop as the new baseline powertrain strategy. We have already committed to expanding the capacity for AGM and EFB batteries to meet the increasing demand.”