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.
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 2008–2009 Chevrolet Malibu Hybrid. 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.
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 11.2 kW (15.0 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.
Generation III (BAS3)
A third generation of the eAssist system was introduced in the 2016 Chevrolet Silverado/GMC Sierra and can improve fuel efficiency by about 13%. This adds about 100 lb (45 kg) to the total weight of the truck but provides an additional 13 hp (10 kW) and 44 lb⋅ft (60 N⋅m). The eAssist system includes a modified GM 8L90 automatic transmission. It uses the same battery cells from the Chevrolet Malibu Hybrid and also the software is a modified version of the Chevrolet Volt's.
- BSG: Belt-Driven Starter Generator
- BISG or B-ISG: Belt-Driven Integrated Starter Generator or Belt Mounted Integrated Starter Generator
- BSA: Belt-Driven Starter Alternator or Belted Starter Alternator
- eAssist: Electric Assist
- 2007–2009 Saturn Aura Green Line
- 2007–2009 Saturn Vue Green Line
- 2008–2010 Chevrolet Malibu Hybrid
- 2012–2014 Buick LaCrosse w/ eAssist
- 2012–2014 Buick Regal w/ eAssist
- 2013-14 Chevrolet Malibu w/ eAssist
- 2014 Chevrolet Impala w/ eAssist
- 2016 Chevrolet Silverado/GMC Sierra w/ eAssist
- 2017 Audi A8
- 2017 Audi A7
-  Archived December 18, 2007, at the Wayback Machine
- "Side-by-Side Comparison". Fueleconomy.gov. Archived from the original on 6 December 2010. Retrieved 2010-12-04.
-  Archived June 22, 2007, at the Wayback Machine
- "Saturn Aura Green Line - Review". Hybrid Cars. Retrieved 2012-01-27.
- Sam Abuelsamid RSS feed. "Saturn Aura Green Line approved for tax credit". Green.autoblog.com. Retrieved 2012-01-27.
- Korzeniewski, Jeremy (2008-06-03). "Doh! Battery leakage hurts GM hybrid production — Autoblog". Autoblog.com. Retrieved 2010-12-04.
- Stoll, John D.; Terlep, Sharon (June 11, 2009). "GM Pulls Plug on Hybrid Model". The Wall Street Journal.
- Migliore, Greg; Vaughn, Mark (2011-10-15). "L.A. auto show: Buick gives the LaCrosse an electric assist". AutoWeek. Retrieved 2011-08-25.
- "2012 LaCrosse Comes Standard With New eAssist™ Fuel-Saving Technology, For 37 Mpg Highway Fuel Economy In A Full-Size Sedan" (Press release). General Motors. 2010-11-15. Retrieved 2010-12-04.
- "2012 Buick LaCrosse eAssist achieves up to 37 mpg highway". Autoblog Green. 2010-11-16. Retrieved 2011-08-25.
- Smirnov, Andre. "2017 GMC Sierra 1500 eAssist Hybrid: Is There Future in Hybrid Pickup Trucks? [Video] - The Fast Lane Truck". www.tfltruck.com. Retrieved 5 April 2017.
- "Chevrolet Introduces 2016 Silverado with eAssist". media.gm.com. Retrieved 5 April 2017.
- "Versatile and efficient: mild-hybrid technology". Audi MediaCenter. Retrieved 13 March 2018.
- "Second-generation Audi A7 Sportback features standard 48V MHEV system with gasoline or diesel engines". Green Car Congress. Retrieved 13 March 2018.
- "Saturn Vue Greenline". Motor Trend. Archived from the original on 14 October 2007. Retrieved October 12, 2007.
- "Going Green". AutoWeek. Retrieved January 9, 2005.
- "Saturn Vue Green Line Hybrid Delivers Great Fuel Economy Through New, Lower-Cost Hybrid System". General Motors press release. Archived from the original on February 3, 2006. Retrieved January 9, 2006.
- "Saturn Vue Green Line Uses New, Less Complex Hybrid System". General Motors press release. Archived from the original on March 15, 2006. Retrieved January 9, 2006.