by Jack Rosebro
|Components of the Q5 hybrid system. Click to enlarge.|
In a presentation at the 2011 Advanced Automotive Battery Conference (AABC) in Pasadena, California last week, Daniel Andree, a battery development engineer at Audi AG, outlined the implementation of a Sanyo lithium-ion battery pack in the upcoming 2012 Audi Q5 compact crossover hybrid. (Earlier post.)
The Q5 has an all-electric range of about 3 km (a little less than two miles) with a maximum all-electric speed of 100km/h (62 mph). The vehicle’s electric drive is mated to a 2.0L, turbocharged, direct injected four-cylinder gasoline engine. Combined weight of all hybrid components is less than 130 kg (286 pounds).
|SoC window for the Q5 Hybrid battery pack. Click to enlarge.|
Battery pack performance. The 266V, 1.3 kWh battery pack weighs about 35 kg, yielding a power density ratio of 1143 W/kg and an energy density ratio of 37 Wh/kg. The cells themselves account for slightly more than half of the pack’s total weight (ratio of cell weight/system weight of 0.52).
The battery pack, which supplies 40 kW of a 180 kW total powertrain output, utilizes a broad maximum state-of-charge (SOC) window ranging between 20% to 80% pack charge, with full pack performance available between 30% and 70% SOC. (This is a much wider SoC window than some other automakers, such as GM, are currently utilizing for hybrid applications. GM, for example, is looking at a pack SOC window for a full hybrid of less than 20 percentage points. Earlier post.) Engine cranking is possible at temperatures as low as -30ºC (-22ºF) as long as battery pack SOC is 30% or greater.
|Overview of the pack cooling design. Click to enlarge.|
Cooling the battery pack. The Q5’s battery pack is air-cooled, with cooling air volume and temperature controlled as needed. The pack is split into two symmetrical parts, each with its own inlet and outlet interfaces.
Particular attention was given to preventing turbulence within the air passages, which could lead to uneven pressures and cooling. Audi also found that a battery pack air conditioning system could significantly shorten the time to reach a specific cooling target. For example, an active battery pack could be cooled from 50 ºC (122 ºF) down to 40 ºC (104 ºF) by an air conditioning system in approximately sixteen seconds, as compared to approximately six seconds via forced-air cooling alone.
The battery management System (BMS) can switch between passive and active modes. If the battery pack temperature rises above 34.5 ºC (94.1 ºF), fan cooling is activated. If pack temperature rises above 37 ºC (98.6 ºF), the vehicle’s air conditioning system is activated, cooling interior air via the front evaporator. At 42 ºC (107.6 ºF), a dedicated rear evaporator provides additional cooling capacity.
|“The electrification of a conventional vehicle concept has a very big impact on nearly all parts of the vehicle. For example, to merge the conventional Q5 with the q5 hybrid, you will find differences in nearly all parts of the vehicle.”|
Performance monitoring. The Q5’s battery management system stores historical stress data-cell resistance and capacity, pack temperature and current distribution, and violations of parameter limits—which can be displayed as histograms for evaluation. Audi’s testing indicates that in a worst-case scenario, battery pack capacity will be reduced to about 60%, with cell resistance increasing by 30%, by the end of the battery pack’s usable life. Audi expects to see no impact on battery performance during the battery pack’s first ten years of service, and intends to offer at least seven years of battery warranty, depending on vehicle miles driven.
Safety protection. The Q5’s battery pack protection is divided into three levels:
Protection Level 1: The vehicle’s battery management system detects cell imbalance (reduced cell capacity, increased cell resistance) and uses an algorithm to modify battery management strategy without affecting perceived vehicle performance.
Protection Level 2: The vehicle’s control system can isolate the battery pack from the rest of the hybrid system, shutting off current flow, if the pack exceeds given voltage, current, and/or temperature thresholds, or if a collision is detected. Battery function may be restored following a crash, depending on the intensity of the crash.
Protection Level 3: Mechanical and functional countermeasures are in place to prevent a chain reaction of thermal runaway in the event of a severe defect within one cell. Pack tests have been conducted in which one cell was purposely subjected to thermal runaway, to see if the effect would propagate across the pack. In all tests, the thermal event was confined to the original cell.
Q5 battery pack testing was conducted in accordance with UN lithium battery testing requirements, as well as those developed by Sandia Laboratories Test 2005-3123, which is used by Sandia itself in support of USABC FreedomCar testing contracts. Audi also added a "foreign particle" test to the abuse testing protocol for the pack. A nickel particle was built into to a cell to create a fault in which a cell’s separator would rupture under stress. No fire or explosion occurred during testing, according to Andree.
The 2012 Audi Q5 Hybrid Quattro is scheduled to be released in Europe this spring, and in North America later this year.