Sometimes engineers get an assignment that's not only a challenge and a learning experience but just plain fun as well. That was the case when UBM TechInsights' Product Marketing Manager John Scott-Thomas and Munro & Associates Senior Associate and "Design Prophet" Al Steier were tasked this past January with taking a Chevy Volt plug-in hybrid car apart to see what makes it tick (whir, and hum)—and how all the technology embodied in the car was put together.
Here is what they learned over the three days it took to creatively disassemble the Volt. (You can view a time-lapse video of the teardown and videos focused on specific aspects of the car's systems by clicking here
.)Energy lifeblood: The battery pack
The Volt Li-ion battery pack is made up of four modules arranged into a T-shape that fits below the rear seat and in the "tunnel" between the front seats. Bus bars connect the four modules and there is a service disconnect bar to "safe" the pack contacts.
The 375-lb (170-kg) Li-ion battery pack is the heart of the Volt. The vehicle's systems and software are geared to maintaining its health for a long service life. (Courtesy Munro & Associates)
A total of 288 individual cells make up the complete battery pack. John Scott-Thomas notes the pack is physically divided into plastic-encased "slices" or "blades", each of which is made up of two cells separated by a cooling fin that carries five channels of coolant. Electrically, groups of three cells are connected in parallel and 96 of these groups are wired in series, for the total of 288 cells producing 360V with a capacity of 16 kW-hr. To prolong battery life, the battery is never fully charged or discharged, so only the "middle" 9.4 kW-h of the battery energy is used.
Scott-Thomas notes the battery, made by LG Chem, uses lithium-manganese-spinel chemistry, but adds that GM has licensed battery cobalt chemistry from the U.S. Argonne National Lab so that a switch to a nickel-manganese-cobalt battery could be in the offing.
The battery cooling-fluid circuit is one of four in the Volt, each with its own controller and radiator module. The other three loops are for the internal combustion engine, the two electric motor/generators inverters, and the power line, plug-in charger's power converter.
Al Steier says when the battery is cold (below optimum operating temperature) the fluid will be used to heat the battery to operating conditions and then cool it to avoid over-temperature. Even if the car is not "running" the control electronics will activate the coolant loop to avoid having the battery become too hot during extremely hot weather or too cold in deep winter—thus keeping the Volt on its external charger when parked avoids drawing down the battery under such conditions.
Scott-Thomas adds that the battery-pack coolant loop is connected using hose clamps—which indicates the car is a limited production vehicle because higher production volumes would allow use of brazed joints. He also reports the bolts clamping the pack together each have three inspector's "paint marks," showing that the assembly is inspected very carefully to ensure quality and function for this costly ($8,000 for replacement) component at the heart of the Volt.
Battery electronics: Control and monitoring
The complex Volt battery pack, as the teardown revealed, has equally sophisticated control and monitoring—typical of the entire car. John Scott Thomas observes that 40% of the value of the vehicle is in electronics, typified by the nearly 100 MCUs on board. To control this electronic suite are nearly 10 million lines of code, more than in the Boeing 787 Dreamliner (8 million lines).
As for the battery pack itself, Scott-Thomas notes that long battery life is a key objective. To this end, pack temperature is regulated within 2°C and cell charge is balanced between cells so each ages at the same rate. Differences in manufacturing individual cells are another variable in aging, which the control software factors in.
For example, voltage on each cell is monitored during charging. To ensure the same, maximum charge on each, if one cell reaches capacity early, a resistive shunt across the cell will be connected to prevent it from being overcharged while the other cells come up to full charge.
Scott-Thomas says, "The level of control and software is hard to appreciate. The battery pack voltage and temperature are monitored with 500 diagnostics, 10 times every second—with control activity even with the car at rest."
The battery interface/monitoring module is mounted on top of the pack front. This unit has four monitoring circuit boards (one for each pack section) colored orange to indicate high voltage and populated by Freescale and LG Chem/STMicroelectronics chips (with the latter using BCD (bipolar CMOS DMOS (diffused metal oxide semiconductor)) technology). (Middle voltage boards are colored blue and low voltage PCBs are green.) Al Steier again notes the quality checks in place throughout the manufacturing process being evidenced again with each cell connector bearing multiple inspectors' marks.
On the battery interface/monitor circuit board, sensors on each cell monitor temperature and voltage. Their data are routed in clusters where readouts for 10 cells are on a single circuit, digitized to an MCU. These are connected by an Avago electro-optic coupler feeding a common bus to the main controller located in the inverter module. (Courtesy Munro & Associates)
John Scott-Thomas appreciates the difficulty in getting battery electronics correct because the system has to measure cell voltages to within several millivolts while each cell can be offset from ground by hundreds of volts—requiring attention to board layout, trace design, proper ground planes (virtual grounds at different voltages), and voltage isolation techniques. Scott-Thomas likewise observes the safety and quality checks, adding the car's design is a "work in progress" with flexibility and modularity to easily introduce new cells, battery packs, electronics, and controls into the vehicle.
Finally, the teardown team also discovered one unexpected battery-related module in the Volt. In addition to the standard OBDI diagnostic port under the driver's side dash, a sealed and potted module was found under the front passenger seat. This it turns out stores battery and hybrid operation diagnostic codes and has a connection for an appropriate cable for a technician to access them.