Teardown Report: Chevy Volt's electronic secrets

Charging system and 'central brain'
Feeding the battery: Volt charging system
Besides regenerative braking, energy is stored in the battery pack by charging off the power grid using the 110V charger supplied with the car or an optional 220V charging station (for faster recharging) installed by a licensed electrician. (Author's Note: For experience with charging and driving the Volt for a week, check out my report, Driving Impressions: Chevy Volt, on the Automotive Designline website.)

The 110V home charger is made by Lear Corp and is a "straight forward" component, but its power electronics and software are sophisticated enough that charging will not occur if the circuit it is plugged into is not sufficiently grounded. The charger's relays and monitoring-electronics board communicates with the battery pack and onboard monitoring systems. And as noted previously, the onboard module for changing power mains AC into DC to charge the battery has its own cooling loop.

The charger plugs into a standardized receptacle behind a door on the left front fender. John Scott-Thomas states that taking apart this interface unit showed GM's attention to design detail. The high-voltage components (i.e. capacitors and common mode chock) in this high-vibration environment were taped and foam isolated for robustness and protection, and windings were "robust, stable, and mechanically redundant."

Al Steier found one puzzling feature with the charger configuration. While the charger receptacle is on the left fender, the charging module it feeds is under the right headlight. Likewise, the gas engine controller is on the left side, while the engine is on the right. Such an arrangement has greater wiring weight than if these components were reversed.

Brains of the Volt
On the electric motor/generator housing, which looks like the transmission case of a gas engine car, is a liquid cooled inverter module that takes battery power and feeds the traction motor. The high voltage orange cables leading to this module have disconnects with relays for safety—and Steier notes the module cover itself is also a safety circuit disconnect. Inside is what Scott-Thomas says is the closest to a "central brain" in the car.

The motor/generator inverter module also contains the "brains" of the Volt power train, where one supervisory MCU and three others define the operating state of the drive/regenerative-braking system. (Courtesy Munro & Associates)

Here are on a Hitachi board are mounted four Freescale Qorivva 32-bit MCUs. One of the four controllers functions as the supervisor taking inputs, including vehicle and wheel speeds, acceleration (throttle), braking, and battery state, and then deciding on the most efficient operating state. This could include, for instance, what combination of outputs from the traction motor and combustion-engine generator motor to use or when to activate regenerative braking and to what extent to recover energy.

The supervisory controller (below) is the largest of the four MCUs, with 3 MB of flash memory taking half the area of the die. The controller also endeavors to run the electric motors at lower rotation rates for more efficiency. But what first strikes John Scott-Thomas is the large amount of "real estate" available, "Such a large area layout can be modified readily in the future, either by changing circuits or adding them."

The inverter supervisory controller is the central brain of the Volt's hybrid architecture and determines current power train state. (Courtesy Munro & Associates)

The other three Freescale MCUs are dedicated to controlling the traction motor, the combustion engine driven generator, and the clutched planetary gear set that can be engaged by the IC engine if needed.