The Volt’s battery is big — far bigger than it needs to be:
The anticipated energy capacity of the Volt’s 375 lb (170 kg) 220-cell lithium-ion battery pack is estimated at 16 kWh, but is only charged to 85% full when charged up, and is discharged to 30% SoC approximately, before the engine cuts in and maintains the charge at around this level. When the vehicle is plugged into a charger the battery SoC is restored to 85%. Hence the battery has an effective capacity in use of 8.8 kWh.A recent Carnegie Mellon University study indicates that a PHEV-40 [plug-in hybrid vehicle with 40 miles of battery capacity] will be less cost effective than a HEV [Prius-style hybrid-electric vehicle] or a PHEV-7 [plug-in with just 7 miles' capacity] in all of the scenarios considered, due to the cost and weight of the battery. Jon Lauckner, a Vice President at GM, responded that the study did not consider the inconvenience of a 7-mile electric range and that the study’s cost estimate for the Volt’s battery pack was “many hundreds of dollars per kWh higher” than what it costs to make today.
The weight of the battery pack in the Volt which finally comes to market will reportedly be approximately 375 lb (170 kg),[71] primarily because the Volt will use lithium-ion (Li-ion) batteries while the EV1 used heavier lead-acid and nickel metal hydride (NiMH) batteries. Li-ion batteries are expected to become cheaper to manufacture, as economies of scale take effect.
Electric motors are very, very efficient, but electric batteries are not; they’re both heavy and expensive for the amount of energy they store. Gasoline engines are not efficient, but gasoline is; it stores a tremendous amount of energy in a light and still remarkably cheap form. Complicating this is the fact that electricity off the grid, especially at night, is extremely cheap, even if carting it around in batteries isn’t.
An extended-range electric vehicle, like the Volt, stores some energy in its electric batteries and some energy in its gas tank. (It’s on-board gasoline engine is a generator. It’s not hooked up to the wheels through a mechanical transmission.) The trick is to balance expensive and heavy batteries, which use cheap electricity from the grid, with fairly cheap and light gasoline from the gas station — and it looks like 40 miles of battery capacity is too much.