When Audi competes in the Dakar Rally, this part will be the key focus: The high-voltage battery of the Audi RS Q e-tron. It embodies the heart of the innovative electric drive with energy converter. With it, Audi intends to set the next milestone in the world’s toughest rally and prove “Vorsprung durch Technik” in the desert as well.
“With our drive configuration in the RS Q e-tron, Audi is a pioneer in the Dakar Rally,” says Lukas Folie, the engineer for the high-voltage battery. “Defining the challenges for this type of competition was very demanding. There is simply no empirical data in motorsport for such a concept and for this kind of endurance competition.”
Designed for unknown requirements
Compared to the Formula E World Championship, which Audi last contested with a battery-electric powertrain, the standards in the Dakar Rally are different: daily stages of many hundreds of kilometers, the enormous driving resistance in the soft desert sand, plus high outside temperatures and a minimum vehicle weight set by the regulations at two tons are extreme values in motorsport. “It is not possible with today’s battery technology to realize an all-electric BEV off-road vehicle for the Dakar Rally under these conditions,” says Lukas Folie. The engineering team led by Axel Löffler, Chief Designer of the RS Q e-tron, therefore had to define basic benchmarks for the overall concept of the vehicle with electric drive and energy converter without any previous empirical values. Due to the short project development time, Audi relied on proven cell technology. The capacity of the high-voltage battery is 52 kWh and is therefore sufficient for the maximum expected requirements on each rally stage. The weight of the high-voltage battery including the cooling medium is around 370 kilograms.
The required energy capacity and performance, as well as control and safety mechanisms, led Audi to use proven round cells as the basis for the high-voltage battery. The battery system is designed in such a way that Audi Sport drivers Mattias Ekström, Stéphane Peterhansel and Carlos Sainz do not feel any difference between a new and a used battery.
Challenging off-road charging
When the rally drivers leave the bivouac in electric drive on the morning of each stage with a charged high-voltage battery, a highly complex control system begins. Only a few minutes before the start of the stage do the teams learn any details at all about the route when the roadbooks are handed out. The Audi RS Q e-tron with its innovative drive must always be prepared for all conditions in terms of distances, speeds, difficulty of the terrain and other factors. The engineers and electronic technicians have programmed algorithms to keep the State of Charge (SoC), i.e. the charge level, within defined ranges depending on the energy demand. Energy extraction and battery recharging are always in balance over defined distances. If, for example, a difficult dune passage with high driving resistance requires maximum energy for a short time, the state of charge drops within a controlled range. The reason: The drive power of the motor-generator units on the front and rear axles is limited to a maximum of 288 kW in total under the regulations. However, the energy converter can produce a maximum charging power of 220 kW. In extreme cases, therefore, consumption is briefly higher than energy generation. “Something like this is possible for a limited time,” says Lukas Folie. “But over a longer distance, it always results in a zero-sum game: We then have to regulate the power consumption down so that the battery’s state of charge remains within a corridor. The absolute amount of energy available on board must be sufficient to cover the day’s leg.”
Energy recovery as an important factor
In order to realize maximum efficiency, Audi is also relying in the desert on a principle that has already been used in the Le Mans sports cars and Formula E: The RS Q e-tron recovers energy during braking. The MGU units on the front and rear axles can convert the rotational movement of the wheels into electrical energy. The aim is to recuperate the maximum energy. The power flow in this reverse direction is not subject to the same power limitations as acceleration. What sounds so simple requires a complex Intelligent Brake System (IBS). It combines the hydraulic braking function with the electric regenerative brake.