This type of trickle-down technology makes perfect sense in the real world driving conditions that most of us encounter each and every day of our lives. Hybrids have been using regenerative braking for quite some time now as a fuel saving measure. In those applications, the electric motor or motors that help provide motive functions for the vehicle work as generators when braking force, or "drag" is applied to them, which charges the batteries. In vehicles like the new BMW 3 Series which does not use any electric motors, the alternator, which has long been used in vehicles to keep the battery charged, is selectively engaged or disengaged, and acts as both a "brake" and as a method of charging the batteries and saving gas. As a bonus, when the alternator is disengaged from the engine, the normal "drag" on the engine is eliminated, allowing for more power from the engine going to the wheels. Hooray for trickle-down hybrid technology!

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[Source: Motor Authority]