2011 Chevrolet Volt
2011 Chevrolet Volt. Click image to enlarge

Join Autos’s Facebook group
Follow Autos on Twitter

By Grant Yoxon

The electric extended-range drive system General Motors developed for the Chevrolet Volt is unique in the industry. At the heart of the system is a T-shaped, 435-pound (198.1 kg), 16-kWh lithium-ion battery pack. Combined with an advanced, 111-kW (149-hp) electric drive unit, it delivers 25 to 50 miles of pure electric operation (depending on terrain, driving techniques and temperature) on a fully charged battery.

In extended-range mode, which activates when the battery’s energy is depleted, power is seamlessly inverted from a technically advanced, 1.4-litre, 63-kW (84-hp) engine to the electric drive.

Developed by GM in conjunction with battery cell supplier LG Chem, the Volt battery pack is one of the first lithium-ion battery packs offered on a high-volume production vehicle. It is assembled at GM’s battery assembly facility in Brownstown Township, near Detroit, the first such facility in the U.S. built by a major automaker.

Major features of the battery pack include:

2011 Chevrolet Volt
2011 Chevrolet Volt. Click image to enlarge

Thermal management for durability and reliability: Because batteries can be sensitive to temperature changes, the Volt pack is climate-controlled via an exclusive active liquid control system that continually monitors and maintains the battery pack temperature for optimum performance and durability. Circulating liquid (consisting of coolant and distilled water) passes through a series of internal heat exchangers in the battery modules. The Volt features the only mass-produced battery that can be warmed or cooled. It is designed to provide reliable operation, when plugged in, at temperatures as low as -13 degrees Fahrenheit (-25 C) and as high as 122 degrees Fahrenheit (+50 C). In cold weather, the battery is preheated during charging to provide full power capability. In hot weather – the most challenging environment for a battery – the Volt’s battery can be chilled during charging. The Volt’s thermal management system can also be powered during driving either by the battery or engine.

Diagnostics for safety and performance: The Volt’s battery management system continuously monitors the battery in real-time for optimum operations. More than 500 diagnostics run at 10 times per second, keeping track of the Volt’s battery pack; 85 per cent of the diagnostics ensure the battery pack is operating safely, while the remaining 15 per cent keep track of battery performance and life.

Cell design and chemistry for performance and efficiency: GM’s selection of a prismatic cell design and LG Chem’s manganese spinel lithium-ion chemistry is designed to provide long life and high power output, with a properly maintained temperature. This enables better vehicle acceleration and increased regenerative braking capability for improved vehicle efficiency.

2011 Chevrolet Volt
2011 Chevrolet Volt. Click image to enlarge

Energy management for durability: Fully charging or fully depleting a battery shortens its life. The Volt’s energy management system never fully charges or depletes the battery. The Volt battery is controlled, or buffered, so it operates within a safe state-of-charge window of 65 per cent. In demanding situations (such as driving in Mountain mode), the battery state of charge will raise the lower limit to ensure there is adequate power when needed. The battery’s top and bottom “buffer zones” help ensure long life.

The battery pack is positioned in the centre tunnel of the vehicle, and is protected by ultra high-strength steel. It comprises nine linked battery modules that contain 288 prismatic lithium-ion battery cells, each of which functions as a building block. Each cell is the size of a 5 x 7-inch photo frame, less than a quarter-inch thick, and weighs about a pound. It contains a carbon anode (negative electrode), manganese cathode (positive electrode) and safety-reinforced separator, the medium that isolates the electrodes, while still allowing the transfer of electrically charged ions between the elements in each cell.

Lithium-ion was chosen over nickel metal hydride for the Volt battery because it packs two to three times the power of a nickel metal hydride battery in a much smaller package. It also is more efficient, offers more configurability, suffers little loss of charge when not in use and is less prone to “lazy battery” syndrome, when batteries gradually lose their maximum energy capacity from repeated recharging after being partially discharged.

The Volt battery cells are encased in polymer-coated aluminum housings. They are thermally efficient, safe and designed to withstand extreme heat, cold or dramatic temperature shifts.

The Volt battery carries an eight-year/100,000-mile warranty.

The Volt drive unit consists of two motors, three clutches and a planetary gear set that improve overall efficiency by reducing the combined rotational speed of the electric motors. This configuration reduces battery drain at highway speeds, adding up to two miles of additional EV range.

2011 Chevrolet Volt
2011 Chevrolet Volt
2011 Chevrolet Volt
2011 Chevrolet Volt
2011 Chevrolet Volt
2011 Chevrolet Volt. Click image to enlarge

The Volt’s motors and gear-set are mounted in-line with the range-extending internal combustion engine. Two of the clutches are used to either lock the ring gear of the planetary gear-set or connect it to the generator/motor depending on the mode. The third clutch connects the internal combustion engine to the generator/motor to provide range extension capability.

This innovative new drive unit has four distinct operating modes:

Single-Motor EV Driving

In this mode, the primary traction motor provides all propulsion at lower vehicle speeds and hard accelerations, drawing all of its energy from the battery. The ring gear is locked and the generator/motor is decoupled from both the engine and gear-set. The traction motor can use up to 111 kilowatts of power and deliver 273 pound-feet of direct torque to provide spirited off-the-line acceleration.

Two-Motor EV Driving

As vehicle speed increases, the ring gear is unlocked and coupled to the generator/motor. This allows the two motors to work in tandem to provide blended output with higher electrical efficiency.

Single-Motor Extended-Range Driving

Once the battery has reached its minimum state of charge, the 1.4-litre engine is coupled to the motor/generator via the third clutch. At lower speeds and hard accelerations the Volt is propelled by the traction motor alone with the ring gear locked. The engine-driven generator and battery provides electricity to the traction motor via the inverter. On an average basis, the engine-driven generator will maintain the battery at a minimum state of charge for extended range operation.

Since the most efficient way to charge the Volt’s battery is to plug it in, the generator is only used to maintain minimum battery state of charge. If the battery is drawn down below the minimum level during acceleration or when mountain mode is engaged below about 45 per cent charge, the generator will charge the battery up to its minimum state of charge and then maintain it there.

Two-Motor Extended-Range Combined Driving

The blended two-motor electric propulsion strategy used at higher speeds in EV driving has also been adapted for extended-range driving. The clutches that connect the generator/motor to both the engine and the ring gear are engaged, combining the engine and both motors to drive the Volt via the planetary gear set. All of the propulsion energy is seamlessly blended via the planetary gear and sent to the final drive.

Regardless of which drive mode is used, the Volt is always propelled with electric power delivered by the traction motor. The engine cannot propel the Volt unless the traction motor is also running. In order for a planetary gear-set to transmit torque, at least one of the three main elements (ring gear, sun gear, or planet carrier) must be able to be locked or held. Since there is no clutch to lock the sun gear, the traction motor must be used to provide the reaction torque needed for propulsion.

This unique propulsion architecture allows the Volt to achieve 10-15 percent better efficiency at highway speeds than would have been possible by using only the single traction motor. At the same time, the Volt always delivers a solid electric driving experience, even in extended range driving, utilizing the large battery for full electric launches even during brisk accelerations.

Connect with Autos.ca