24 April 2012
By John Scott-Thomas, TechInsights
The Chevrolet Volt is GM’s atonement for the cancellation of its first modern electric vehicle, the EV1, and a potent attempt to create a bold new technical future. The Volt’s control systems are among the most elaborate ever created. Close to 100 microprocessors are in the car, and over 10 million lines of software are used; a Boeing 787 Dreamliner requires 6.5 million lines. However, success of the vehicle will hinge on the battery pack, so we decided to open up a 2012 Volt to see what was inside.
GM subcontracts the battery technology to the South Korean-based company LG Chem. The lithium ion (Li-ion) cells that store the energy are currently made in Ochong, South Korea, but manufacturing is slated to be moved to a plant in Holland, Mich., in 2012. The final assembly of the battery pack is done by GM in Brownstown, Mich.
The battery chemistry is based on a Li-ion polymer technology. This was chosen over a nickel metal hydride chemistry (used in the Toyota Prius) because the energy storage is two to three times higher, and the battery is safer, cheaper, and more durable. The Li-ion cells used a carbon anode and manganese spinel cathode. LG Chem and GM recently licensed Intellectual Property from Argonne National Labs. The chemistry covered by these patents is based on a nickel-manganese-cobalt cathode, to which GM hopes to migrate. This novel chemistry of the cells allows them to be charged to a higher voltage, resulting in increased charge density. UBM TechInsights will be looking at the battery chemistry in the Volt at a later date to see what Chevrolet is using in the 2012 model.
The Volt uses a total of 288 prismatic 5×7 inch Li-ion cells. Prismatic cells are rectangular; the other common shape is cylindrical. Three cells are connected in parallel, for a total of 96 series connect groups of cells. The target output is 360 V. The battery pack can store 16 kiloWatt-hours of energy, but only 10 kWh or 65% of the full range is utilized. This is done to reduce electrical stress on the cells and increase the lifetime. Once the battery energy drops below a minimum threshold, the 1.4L Austrian made gasoline engine kicks in and runs a generator motor. This provides electrical energy to both run the car and recharge the battery.
Battery Pack Design
We started our analysis by dropping the battery pack from the chassis of the Volt. The pack is T-shaped, sitting under the central tunnel of the vehicle with the cross-member under the rear seats. A protective covering houses and insulates the pack. The first thing we noticed is the large number of quality checkmarks on each screw and bolt in the pack housing. Many of the fasteners had been checked two or three times. Clearly safety and quality is a big concern. The pack is 5 ½ feet long and weighs 435 lbs. The cells of the pack are arranged in nine modules; five along the central tunnel and four on the cross member. Each module consists of a collection of cream colored blades, seen along the side of the pack in Figure 1. We counted a total of 135 of these blades; an unusual number since it doesn’t mesh well with the 288 cells (or 96 cell groups) in the pack. Likely each blade contains one cooling fin and two cells, with additional cells being contained in the walls separating the modules. We’ll complete more analysis in the future to confirm this. The cells are series connected using large orange busses that can be seen running along the side of the pack in Figure 1.
The battery pack is temperature regulated using a liquid coolant consisting of glycol and water that is circulated through fins placed adjacent to every cell in the pack. There are a total of 144 fins. Each cooling fin is 1 millimeter thick and uses five channels to distribute heat evenly. Tight thermal regulation is essential to maintain the targeted 10 year lifetime of the pack. The target maximum temperature variation is 2 degrees Centigrade across the battery pack. The temperature of the pack is regulated during both charging and discharging. This means that even when the Volt is sitting in the driveway, coolant can be circulating through the battery. The coolant can use a dedicated radiator loop (one of four in the Volt) to reduce temperature in hot environments, or alternatively be heated by an 1800 W heater when the temperature is low.
The control systems on the battery pack are notable. Consider some of the things the electronics must do. Every cell in the battery pack is monitored for voltage and current, and temperature probes are placed at 16 points along the battery. Five hundred diagnostic tests are done ten times a second. The processors must decide whether the cells are safe, overcharged or undercharged, and take corrective action as required. The temperature of the cells must be continuously monitored and the coolant must be chilled or heated as required. When the car is charging from the wall, the power for this is taken from the AC to DC converter, but the battery will cannibalize its own charge to temperature control the pack when the car is unplugged. During regenerative braking, the controls systems must capture as much of the kinetic energy of the vehicle as possible; this has to be done in a few seconds whereas the Li-ion cells like to be charged over a period of hours. All this must be done by the control electronics flawlessly and safely for a decade.
Control is achieved with five boards mounted on the battery pack. At the front of the tunnel is the main battery control board. It features a Freescale dual core microprocessor (Part # MPC5516EAMLQ48). This board acts as the main interface between the pack and the rest of the vehicle. It also monitors the input and output temperature of the coolant.
Interface Module Boards
Sitting on top of the battery cells are four battery interface module boards that monitor the voltage, current and temperature of the Li-ion cells. The PCB insulator for each board is conveniently color coded; the board monitoring the cells close to the high voltage end of the pack is orange (shown in Figure 2), the intermediate board is blue, and the two boards closest to ground are conventional green. Each of the boards has a similar layout. As shown in Figure 2, a set of four identical circuits performs the monitoring of the cells and this information is fed to a communications circuit that serially transmits diagnostic information between the modules. Optoelectronic couplers separate the voltage levels of the sensing and communication circuits. High voltage connectors (orange) at the edge of the board bring the sensing signals to the sensing circuits. The black connector carries the data from the sensing circuits.
On the orange board there are a total of four sensing circuits. Each sensing circuit features a Freescale microcontroller and a custom LG Chem ASIC; the L9763. The L9763 is branded with LG Chem markings, but the design was co-developed with STMicroelectronics. The intellectual property is shared between the two companies. Each L9763 has ten sensing channels and is fabricated using the proprietary BCD (Bipolar, CMOS, DMOS) process developed by STMicroelectronics. The L9763 appears to be using a battery management approach that ST calls “Life Time Management” (LTM). LTM can use cell balancing to maximize the charge and lifetime of the cell; proper cell balancing can improve the maximum charge and/or lifetime of the battery by up to 10%. LTM can also progressively disable ageing cells in the battery back. GM claims cell balancing is performed on the Volt, but whether progressive disabling is used requires further analysis. The monitor boards use a total of 13 sensing circuits (and 13 L9763 ICs).
When looking at the overall battery pack, it seems to be very much a work in progress. The blades holding the cells, the PCB layout, and the arrangement of the sensors appear to be designed with change in mind. The design is not so tight that it would be difficult to modify in the future. This possibility is perhaps not so surprising, since the battery pack is likely to evolve rapidly as GM learns more.
Figure 1: The Chevrolet Volt Battery Pack
Figure 2: Battery Monitor Board
See our complete slide-deck presentation from Design West here
Watch video excerpts from the Design West presentation (15:05)
Take a look at a partial parts list here
(John Scott-Thomas is an analog engineer with the research and electronics-analysis firm TechInsights, a UBM LLC company).