Audi Q5 Hybrid Hybrid System Structure and Technology Analysis

The Advanced Automotive Battery Conference (AABC) was held in Pasadena, California, USA, from January 24 to 28, 2011. In the AABTAM (Advanced Automotive Battery Technology, Application and Market) battery technology, application and market discussion, Daniel Andree, an engineer from Audi's battery department, gave a speech titled "Battery System for Audi's Q5 Hybrid Vehicle", and gave a detailed introduction to the lithium-ion battery system of the 2012 Audi Q5 Hybrid hybrid vehicle.

Figure 1: 2012 Audi Q5 Hybrid

Earlier reports suggested that the Audi Q5 hybrid would use the nickel-metal hydride batteries used in the Volkswagen Touareg hybrid and Porsche Cayenne hybrid, but in November 2010 it was confirmed that the car would use lithium-ion batteries produced by Sanyo Electric.

1. The hybrid system gives the Audi Q5 Hybrid sportiness and environmental performance

The 2012 Audi Q5 Hybrid has a range of approximately 3 km (1.86 mi) in pure electric propulsion mode and a top speed of 100 km/h (62 mph). The pure electric propulsion speed is controlled at around 60 km/h (37 mph) to reach the 3 km range, and the range will be reduced at the top speed. The car is equipped with a 2.0-liter direct-injection four-cylinder gasoline engine with turbocharging.

Figure 2 shows the configuration and performance of the Audi Q5 Hybrid hybrid system. As can be seen from the figure, the maximum power output of the power system is 180 kW (245 hp) and the maximum torque output is 480 Nm. According to the new European driving cycle standard NEFZ (Neuer Europäischer Fahrzyklus) or NEDC (New European Driving Cycle), the carbon dioxide emission level is less than 160 g/km. The time required to accelerate from 0 to 100 km/h is 7.1 seconds. The total weight of the entire parallel hybrid component system is less than 130 kg (286 lbs), and the volume is compact, avoiding occupying the interior space of the cabin.

In the structural description text on the figure, from top to bottom: the left column is the electronic air conditioning compressor and the disconnect clutch; the middle column is the pulse inverter and DC-DC converter, gearbox, and engine; the right column is the battery system with cooling function.

2. Battery Performance

The lithium-ion battery pack consists of 72 battery cells, with a rated operating voltage of 266 volts, an energy storage capacity of 1.3 kWh, a weight of approximately 35 kg, a power output density of 1143 W/kg, and an energy density of 37 W•h/kg. The total weight of the battery cells accounts for approximately 52% of the total weight of the battery system.

Figure 3: Cooling mechanism of the Audi Q5 Hybrid hybrid system

Figure 4: Cooling design of the Audi Q5 Hybrid system

With a peak power output of 40 kW, the new battery pack contributes a significant portion of the 180 kW power output of the Audi Q5 Hybrid's entire powertrain. The battery is designed with a wide state-of-charge (SOC) window, ranging from 20% to 80%, and can fully perform in the range of 30%-70%. This SOC window is significantly wider than the current level of batteries from other automakers. For example, General Motors' full hybrid models are equipped with batteries with a SOC window spanning only 20%. Because the Audi Q5 Hybrid battery pack fully performs with a SOC window spanning more than 30%, it can even start the engine at low temperatures of -30ºC (-22ºF).

In comparison, the nickel-metal hydride battery pack used in the Volkswagen Touareg Hybrid and the Porsche Cayenne Hybrid has a total of 240 battery cells, a rated operating voltage of 288 volts, a battery storage capacity of 75 ampere hours, an energy storage capacity of 1.7 kWh, and a peak power output of 38 kW. It can be seen that the lithium-ion battery system of the Audi Q5 hybrid is lighter and has fewer battery cells.

3. Battery cooling function

Figure 3 shows the cooling system of the 2012 Audi Q5 Hybrid. The upper, middle and lower parts are the electric motor water cooling system, the high-voltage battery module cooling system and the power electronics cooling system. The battery system uses air cooling, which requires control of the cooling air volume and temperature. The battery pack can be divided into two symmetrical parts, each with independent intake and exhaust interfaces (as shown in Figure 4).

The new battery is designed to prevent turbulence in the air flow channel, which can cause instability in internal pressure and cooling effect. The left side of Figure 4 shows the inhomogeneous pressure deviation caused by turbulence. The temperature regulation accuracy error is 5 Kelvin (equal to 5 degrees Celsius).

Audi has found that adding an air conditioning system to the battery pack can significantly reduce the time it takes to reach a specific cooling target. For example, a battery pack with active air conditioning can be cooled from 50 ºC (122 ºF) to 40 ºC (104 ºF) in just 16 seconds, whereas it might take six minutes if it relied solely on compressed air.

The battery management system BMS (Battery Management System) can switch between active and passive working modes. The cooling of the battery pack is controlled in three levels. If the battery pack temperature rises above 34.5 ºC (94.1 ºF), the fan cooling function will start; if the battery pack temperature rises above 37 ºC (98.6 ºF), the air conditioning system will start and blow the cooling airflow to the battery through the front evaporator; if the temperature reaches 42 ºC (107.6 ºF), the rear dedicated evaporative air cooling device will provide additional cooling effect.

Figure 5: Audi Q5 Hybrid hybrid system battery state of charge SOC window

4. Performance Monitoring

The battery management system stores a series of important historical data, including battery cell resistance and capacity, battery temperature and current distribution, and parameter limit violations, all of which are displayed in bar graphs for evaluation. Audi's tests show that in the worst-case scenario, the battery pack's energy storage capacity is reduced by about 60%, and at the end of the battery pack's service life, the internal resistance of the battery cell increases by 30%. Audi expects that the performance of the battery pack will not be affected by degradation in the first ten years of use, and intends to provide at least seven years of performance guarantee based on vehicle mileage.

5. Safety protection

The 2012 Audi Q5 Hybrid battery system adopts three-level protection measures. With these multiple protection measures, the battery pack will not catch fire or explode even if the battery cell is short-circuited by internal foreign matter.

First level of protection: The vehicle battery management system monitors battery cell imbalances, such as reduced battery cell energy storage capacity and increased internal resistance, and uses algorithms to correct management strategies to monitor the battery without affecting vehicle performance.

Second level protection: When the battery pack exceeds the rated operating voltage, current, and/or temperature limit, or the system detects a collision, the vehicle control system can isolate the battery pack from other components of the hybrid system and cut off the current. In this way, depending on the severity of the collision, the battery function will be preserved in some cases.

Level 3 protection: The system sets up countermeasures at the mechanical and functional levels to discharge the gas generated by the unit out of the exhaust port of the battery pack to prevent thermal disorder from causing a chain reaction when a single battery unit is seriously exhausted. During the test, a battery unit was deliberately regulated to enter a thermal disorder state to observe whether it would affect other battery units in the battery pack. All test results show that the thermal runaway event was controlled in the battery unit where the accident occurred initially.

The battery pack test of Audi Q5 Hybrid implements the United Nations UN 38.3 lithium battery test requirements and complies with the U.S. Department of Energy Sandia National Laboratory Test Standard 2005-3123. The latter is used by Sandia National Laboratory to support the FreedomCar test protocol of the Advanced Battery Alliance USABC (United States Advanced Battery Consortium). Audi also added the "foreign particles" test content to the battery pack "abuse" test clause to observe the effectiveness of the protective measures when the battery cell is short-circuited by internal foreign matter. During the test, nickel metal particles were implanted into the battery cell, causing a fault, and the battery cell disconnector immediately started to disconnect after being pressurized. According to Daniel Andree, there was no fire or explosion in the battery during the test.