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Despite the advancements in lithium-ion chemistry that makes these battery cells smaller, lighter, and more power-dense, the lead-acid box that occasionally strands you before a job interview is not leaving your car. Not even in the 2018 Audi A8, which features a 48-volt electrical system. Similar to the systems found in the SQ7 TDI, Porsche Panamera, and Bentley Bentayga, the new A8 employs both 12-volt and 48-volt systems connected by a DC-DC inverter, only their operation is essentially reversed in the A8’s application. Here, the 12-volt system is the subsystem; powered by a conventional battery, it runs lighter-load accessories such as the lighting, stereo, and interior power outlets. It’s the primary 48-volt system that gives the car some hybrid functionality.
In principle, a hybrid like the A8 is old news. General Motors has continued refining its setup, called eAssist, since 2006. A secondary high-voltage battery powers a belt-driven motor-generator attached to the engine, which replaces the traditional alternator and acts as a starter when the engine is warm. Honda’s now-defunct Integrated Motor Assist, first seen on the 1999 Insight, put an electric machine in the place of a torque converter. Neither of those systems could propel a car on electricity alone. Instead of a split-power hybrid as in the Toyota Prius, where a traction motor coupled to the transmission can drive the wheels on full electric power or in tandem with a gasoline engine, hybrids like the eAssist or this A8 can only reduce the engine’s workload under acceleration and very light loads, such as a rolling start. They also help run the vehicle’s 12-volt accessories and allow the engine to shut down when coasting with some light regenerative braking. But the A8 advances this proven technology. A small 48-volt lithium-ion battery (roughly the same size as the car’s 12-volt battery) powers a water-cooled, belt-driven motor-generator that replaces a conventional alternator (like GM’s eAssist). As in the SQ7 TDI, a conventional 12-volt starter motor is still on board, but thanks to higher voltage, Audi says the A8’s belt-driven machine lets the engine start quicker and more smoothly. When coasting between 34 and 99 mph, the engine shuts off for up to 40 seconds. The system also activates the stop-start feature more aggressively when rolling at speeds below 14 mph. Audi won’t say how much its 48-volt system contributes to fuel economy and power improvements other that it can save seven-tenths of a liter for every 100 kilometers. In American, that’s one gallon of gas over 336 miles. GM’s latest LaCrosse eAssist increases city and highway fuel economy by 4 mpg versus the standard car, no small figure. Together with a lighter curb weight, the next A8 also should deliver higher EPA figures.
The 48-volt hybrid is slated for all A8 variants, the W-12 included. (That’s in stark contrast to the last A8 hybrid sold in China and Europe, which went front-drive and was saddled with a weak four-cylinder engine.) Future four-cylinder Audi models will use the A8’s hybrid setup on a 12-volt scale, with less impressive coasting and fuel-saving results but a lower manufacturing cost. Eventually, Audi plans to put 48-volt primary systems into every car it makes. Before that happens, secondary 48-volt systems will start running stereos, window heating, and various pumps and compressors as new models and refreshes arrive. The company, however, admits that electronic control units and lights will stick to 12-volt power for “well into the future.”