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Design simplicity is key to new 48-V systems

The ubiquitous 12-volt battery may soon have a partner. Engineers are ironing out the kinks in 48-volt architectures being employed to power demanding systems, sparking expectations that deployment will skyrocket over the next few years.

“This is mainly a question of how rapidly it will be adopted,” said Kevin Mak, a Senior Analyst for Automotive Electronics at Strategy Analytics. “It’s not much of a technical issue, it’s more the OEM product cycles. The only issue in the U.S. is that it’s difficult to justify the cost premium for additional fuel economy.”

Higher voltage systems enable power-hungry functions like start-stop, with potential to drive many more like air conditioning and electrically heated catalysts. A key benefit is that the extra energy, which can be used to help fuel economy, comes with far fewer design challenges compared to high voltage systems found on today’s electrified powertrains.

“One significant benefit on the cost side is that this is below the 60-V threshold for isolation and protection for service personnel and first responders,” said Jason McConnell, Business Unit Director at IAV Automotive Engineering. “A typical 12-V system is limited to a maximum of around 3 kilowatts, when you go to 48-V, power increases by a factor of four to around 12 kW.”

Packaging challenges

Proponents say the technology’s benefits extend beyond its initial role of powering start-stop systems. Regenerative braking conserves energy. High power systems like air conditioning can be turned off, conserving energy. 48-V components including motors and wiring can be smaller.

“To get six kW from a 12-V machine, you need around 500 amps,” said Tomasz Salamon, Engineering Operations Manager for Hybrid and Electric Systems at Ricardo. “That’s a lot of current, you need very thick wires for continuous operation. When you upscale to 48 V, 500 amps becomes 125 amps, so you can significantly reduce the size of components. Eventually we’ll see more components going to 48-V, which gives you smaller and more-efficient electrical components—and more power capability.”

Getting the biggest bang for the buck can be a real challenge. Most developers say that many subtleties must be examined to gain all the available benefits. Optimization requires taking a broad view that includes many different systems and parameters.

“If you don’t focus on vehicle architecture optimization and an energy management system, you’re leaving a lot on the table,” said Mary Gustanski, Vice President of Engineering at Delphi Automotive.

While the complexity of implementing a 48-V system may be comparatively minor, the devil’s in the details. One of the first hurdles is figuring out where to put all the hardware. There’s not much room under the hood, under seats or in the trunk.

“The main challenge is integrating components, finding space for the dc-dc converter and the battery,” said Brian McKay, Head of Powertrain Technology and Innovation at Continental Automotive. “Packaging and mechanical integration are very important.”

Smaller 12-V batteries?

Although there will be a new power source, the basic power architecture won’t change dramatically. Conventional alternators will typically be replaced by a belt-starter-generator that will recharge both batteries via the dc-dc converter. Many existing components will be powered by lead acid batteries—for a while.

“We expect near term 48-V vehicles to retain the 12-V starter for cold engine cranking while the 48-V motor/generator will manage the restarts with a warm engine,” said Tom Watson, Technical Fellow, Powertrain, at Johnson Controls Power Solutions.

“This means the 12-V battery still needs to meet cold-crank loads as well," Watson said. "As the architecture and technology matures, the 48-V motor/generator may become the sole source of cranking the engine in all conditions, in which case the 12-V battery may reduce in capacity and size.”

Devising and implementing these architectures requires plenty of attention to detail. Though safety concerns are nowhere near those for high voltage hybrids and electric vehicles, some issues must be examined.

“The increased risk of arcing inherent in a 48-V system, and ensuring that a regenerative braking strategy meets all torque safety requirements, are some of the safety considerations that should be addressed before widespread adoption of 48-V systems,” said Soumendu Chanda, Manager, Electrified Powertrain Development & Testing at FEV.

One bright spot for development teams is that systems can often be used on a range of vehicle lines. Packaging will be among the design challenges facing engineers who are striving to meet stringent pricing goals.

“People are looking to integrate technologies, putting the battery, power electronics from the inverter and the dc-dc converter in one box so you’ve got less cabling,” McConnell said. “48V technologies can be adopted over a large number of vehicles; there’s definitely reusability and scalability in most designs.”

Future architectures

Cost considerations will become more important as the technology migrates from luxury to mainstream vehicles. Projected U.S. adoption has paled compared to Europe, largely because the fuel savings are less important in North America.

“The biggest hurdle is making feasible solutions at price points that are appealing to customers,” Salamon said. “It will take time until components hit lower prices so they’re accessible to lower price vehicles.”

Performance is perhaps more critical than pricing. Early start-stop systems drew plenty of criticism for rough, noticeable restarts. Most design teams are focused on making sure that electric motors and engines work smoothly together so drivers barely notice transitions.

“Calibration needs to be precise—you need to get dialed in to ensure that customers are pleased with the response times when they go from engine-off coasting to restarting the engine,” McKay said. “There can’t be more than a 400-millisecond delay for the belt- starter-generator to turn on.”

Keeping the batteries charged is another important parameter for system designers. Regenerative braking is an important factor when engineers are making design tradeoffs. Many developers feel that as power architectures evolve, the starter generator may migrate to a spot that regains more energy while reducing drag.

“Current 48-V systems typically integrate the motor/generator on the belt,” Watson said. “With a belt-driven motor/generator, the engine will always need to spin to drive the motor/generator. This means a tradeoff is needed between the fuel savings of 'sailing' or coasting and regen braking which will inherently bring with it engine friction even if the engine is not running.

"You may see a shift over time to motor/generators that are embedded in the transmission or driveline so that regen braking can be performed no matter what the engine state is,” he observed.

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