While I decelerated in regen mode, the unwanted momentum was converted to electrical power and dispatched to the battery by the drive motor, now serving as a generator. Reversing the current drain is the engine's signal to take a break, so it slacks off during diminished electrical demand. The engineers' goal is to maintain consistent performance whether the drive motor's electricity is coming from the battery or the generator. During peak need, both pitch in. But when the battery charge dips below what Posawatz terms the minimum buffer level and all the drive juice is supplied by the engine-driven generator, performance is reduced. Simple math says why. In Sport mode, the electric motor turns the front wheels with 149 horsepower through a single-speed gear reducer (versus 121 horsepower in the Regular mode). The 71-horsepower engine doesn't match that so the battery is allowed to drain a bit below the normal minimal state of charge threshold to assist. When regen is available, that 'borrowed' charge is restored. However, the battery is never fully replenished during driving because it's cheaper and greener to draw that power from the electrical grid by plugging in the charger. In pursuit of a 10-year, 150,000 mile service life, the strategy is to treat the 400 or so lithium-ion cells with kid gloves. That means charging the battery only after it's heated to room temperature by a system that circulates warm anti-freeze through its confines. Also, only half of the battery's full 16 kilowatts of energy is ever intentionally used. While GM has not specified the exact limits, the guess is that the state of the charge is never allowed to drop below 30 percent or rise above 80-percent in the interests of battery longevity.
What's Under The Hood
Overall, the electric-drive powertrain impressed me as highly refined but hardly sparkling with enthusiasm for intense acceleration. Under the hood, there are four components bolted together in one assembly for noise, vibration, and collision performance optimization. The electric motor is mechanically attached to a gear reducer and differential that spins the front axle shafts. Adjacent to the motor is the gasoline engine which drives a generator. To save weight and cost the electric drive motor and the generator share a common housing. They are electrically but not mechanically connected. Also, there is no mechanical tie between the gasoline engine and the front wheels. While this arrangement sounds like a series hybrid, GM prefers to characterize the Volt as an extended range electric vehicle.
A large box of electrical gear mounts atop the propulsion and generating components. A few orange cables snake hither and yon but, according to Posawatz, the final underhood appearance is still not set. A charging port mounted in the left-front fender allows connecting to an electrical outlet for recharging the battery pack. Expect 8 or more hours for a full recharge on 120 volts or less than half that with the optional 240-volt charger.