BAS is an acronym for belted alternator starter. It is a category of automotive parallel hybrid technology that uses an electric motor to contribute power to the internal combustion engine's crankshaft via a serpentine belt. By mounting this motor generator unit in the conventional location traditionally used for the standard automotive alternator, it permits a low-cost method of adding mild hybrid capabilities such as start-stop, power assist, and mild levels of regenerative braking. BAS differ from other mild hybrid systems as they are not run off the vehicle's crankshaft.
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Generation I BAS
General Motors introduced a mild hybrid system called belt alternator starter (or BAS) in the 2007 Saturn Vue Green Line. The BAS system is also used in the Chevrolet Malibu Hybrid and Saturn Aura Green Line. It operates similarly to other mild hybrids with a start-stop system, in that it shuts down the engine as the vehicle comes to a stop and instantly restarts it when the brake pedal is released.
The BAS system is capable of providing modest levels of power assist during launch/acceleration and similarly modest levels of "blended" regenerative braking during deceleration. Although unable to operate in pure all-electric EV mode, the BAS system provides improvements to both city and highway fuel efficiency over similar non-hybrid versions. In more powerful eAssist versions, a BAS system can add power to prevent excessive gear changes under variable load. According to the EPA the 2009 Saturn Vue BAS hybrid garners an improvement of 32% city (19>25mpg) and 24% highway (26>32mpg) making the combined economy improvement 27% (22>28mpg) over the base 4cyl FWD version. The system is reasonably simple and inexpensive, making BAS equipped vehicles some of the least expensive hybrids available.
A 36 volt electrical system (operating at 42-45 Volts) is used to operate a permanent magnet motor/generator unit mounted to the engine in a similar fashion to a conventional alternator. Then through a high-tension drive belt, the BAS system is capable of starting or assisting the 2.4L Ecotec engine. A conventional 12V starter motor is retained and used whenever the engine is cold as during initial start-up. The air conditioning compressor continues to be operated through a belt-driven pulley, but for fuel economy improvement it can be disabled in auto-stop mode if the "ECO" A/C mode has been selected by the operator.
Vehicles with the BAS system use a conventional 4T45-E automatic transmission which has been modified to include a more efficient final-drive ratio and includes an electrically driven pump to provide pressure in auto-stop mode.
One perceived benefit of the BAS technology is that it fits in the same space as a conventional engine. No significant modifications were required to the vehicle chassis to accommodate the BAS system, with the 36V battery pack housed in the trunk or spare tire well. This allows the vehicles to be produced on the same assembly line as the non-hybrid versions, producing substantial cost savings and allowing the company to adjust production more easily.
Cobasys which supplies the BAS system's battery pack had to conduct a recall in 2008 apparently due to internal leakage of the battery modules.
Citing supplier issues and slow sales GM has relegated the 2010 model year BAS hybrids (i.e. Malibu hybrid) to "fleet only" status.
Generation II BAS (eAssist)
At the LA Auto Show, on November 15, 2010, General Motors announced that it would be releasing an all-new version of the BAS system available in the 2012 Buick LaCrosse. While still a Belted Alternator Starter system, the system is named eAssist and includes a larger more powerful Hitachi-supplied 115 Volt Lithium Ion battery and a 15 kW (20 hp) motor-generator that delivers 79 lb·ft (107 N·m) of torque. The additional power provided by the more powerful battery and motor provides the ability to contribute more power, and more often able to electrically start and assist the 2.4L engine. The eAssist system also includes a specially modified GM 6T40 6-speed automatic FWD transaxle.