While we were at the General Motors battery plant in Brownstown, MI yesterday, we learned more about the Chevrolet Volt battery pack from newly promoted director of global battery systems engineering Bill Wallace. Over the past three years, we've learned a lot about how GM is controlling the battery pack in order to maximize both the electric driving range of the Volt as well as the lifespan of the pack itself.
For the first time ever, we got to see the internals of the Volt pack and how GM is building it up. The 400 pound, T-shaped pack contains 288 of LG Chem's lithium polymer cells. The cells are arranged into three primary modules, one forming the bar of the T with the other two running down the center tunnel of the car. The gap between the two central modules provides clearance for the main under-floor cross-member of the Volt structure.
[Source: General Motors]
Each of the primary modules is made up of several sub-modules which contain either 18 or 36 cells. There are two 18-cell sub-modules and seven 36-cell sub-modules. The cells are assembled in pairs into plastic frames with an aluminum separator between the cells. The separator is actually made up of two stamped plates braised together to form coolant passages that cool one side of each cell. The passages have been carefully designed to ensure even temperature control over the entire surface of the cell.
The separator plate has a flange with a hole in it that sticks out on either side of the cell. When the plastic frames containing two cells and a separator are stacked up into sub-modules and then modules, the holes on the sides form a coolant manifold that runs the length of the module. Maintaining even and consistent temperatures are absolutely critical to making the pack last the life of the car. The assembled pack contains 16 thermal sensors and the temperature is maintained within two degrees of the optimal level across the entire pack.
There are three separate thermal management systems that maintain the pack. An electric heater is located inside the pack directly behind the bulkhead at the front of the pack. When the car is plugged in and the ambient temperature is low, the battery is pre-heated to ensure full performance. The cooling system contains the same type of coolant used in the engine but the battery has its own independent circuit. Depending on the ambient temperature, the coolant can either be passed through a chiller that is part of the air-conditioning circuit or through a simple liquid-to-air heat exchanger up by the engine radiator.
The battery management electronics are mounted directly on top of the primary modules with the high-voltage electrical bus and coolant lines all being routed along the length of the pack and exiting out through the front bulkhead. Since Brownstown started building packs in early January it has produced about 200 packs some of which have gone to the Warren, MI battery lab with the rest going into the production process verification cars that have been built at the Detroit-Hamtramck assembly plant.
The Brownstown battery plant has three main assembly areas. Containers of cells from LG Chem come to the loading docks on the south side of the 160,000-square foot plant. The first section is the almost fully automated stacking system that installs the cells into the plastic frames. The stackers are capable of handling a cell about every 1-1.5 seconds. The electrical properties of each cell are tested before they are installed into the frames.
In the second section, which is about 80 percent automated, the frames are stacked into the sub-and primary modules. At various stages in the process the modules are tested to verify the mechanical and electrical integrity of the components.
The completed modules are transferred to the pack main line for final assembly. Each pack is assembled on an automated guided cart (AGC) that moves through the assembly area. Each incoming AGC is kitted out with all of the parts required to finish a pack before the modules are installed on the base plate. Because the finished pack is completely environmentally sealed, the complete pack is electrically verified and leak tested before installing the top cover. The finished packs are placed on racks in pairs for shipment to the assembly plant.
Since GM received its first prototype pack from Compact Power on October 31, 2007, it has accumulated more than 4 million hours of lab testing and over 1 million miles of vehicle testing. Based on this extensive testing, Wallace is confident that the packs will last the target ten years and 150,000 miles in the Volt. Of the 160 unique part numbers in the pack, GM has designed and developed 95 percent of them in-house; the main exception is the cells.
No matter how much you test and prepare, there will always be parts that don't work as expected or fail prematurely and GM has a plan for this. Over time, GM will be training its dealers to service the battery packs. The pack has been designed to be dealer-serviceable to the primary module level. However, until the dealers are trained to service the packs, any problematic units will be completely replaced.
Two-thirds of the floor space at the Brownstown facility is dedicated to pack assembly. A walled-off area on the west side of the building is being prepared as a dedicated pack refurbishment area. Faulty packs and later modules will be shipped back to the plant where they will be tested, repaired and then used as service replacement parts.