From a (now old) press rel:
http://www.bmwreport.com/bmwmininews...w-5-series.asp
Quote:
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Brake-energy regeneration is new to the 5 Series, which recycles previously lost energy by using a smart alternator and special battery. Whereas a conventional alternator constantly draws energy from the engine, the smart alternator only does so when the battery needs it — as well, engine energy generated while braking or descending a hill goes to charge the battery. This set-up achieves an overall three-percent improvement in fuel economy.
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The 1-series is getting this too.