ELECTRIC VEHICLES


Balance of Power


Simulation software


assists EV optimisation engineers to find the right balance of power


and cooling to meet range needs in diverse climatic conditions.


O


ne of the major concerns about battery systems used in electric vehicles is that their performance and durability can be


diminished by extreme temperatures. In cold temperatures, the need for cabin and battery heating and the increased rolling resistance of winter tires are the most adverse conditions for any electric vehicle. The key is to determine how to


allocate battery energy to make the most efficient use of the vehicle. On cold days, warming a battery gradually brings the system’s temperature to a level that allows it to provide enough discharge power for expected vehicle performance. Like humans, a battery functions


best at room temperature, and any deviations in temperature change the battery performance and/or longevity. Battery heating systems and optimal thermal management are critical for effective operation in all climates. The objective is to deliver a battery pack at an optimum average temperature with even temperature distribution. The question is: how do you determine the best heating architecture? The Department of System Integration


and Vehicle Test at vehicle architecture and powertrain specialist Continental Engineering Services (CES) has been working on electrified drivetrain performance studies and optimisation of heating systems specifically to solve this problem.


6 /// Automotive Test & Validation Vol 2 No. 2


❱ ❱ Simulation software helps find the right balance of power for all driving styles in different climatic conditions


FINDING THE BALANCE During the design, the layout of the high-voltage battery and the thermal and energy management strategies have to be specified. Despite space limitations, weight, complexity and cost limits, the requirements of driving range and performance have to be met. Finding the best possible balance takes numerous optimisation loops, testing, experience and time. To help achieve this, CES used Simcenter Amesim software from Siemens PLM Software. CES used Amesim to investigate cooling systems so it could understand the driving range implications by modelling the complete system of mission profile, virtual driver and vehicle, including the electric drivetrain containing electric motor and inverter with integrated DC/DC, LiOn battery and the cooling circuit as well as the electric vehicle controller. The objective was to test three different battery-heating approaches in winter conditions to get the best performance in terms of range and power.


ACTIVE ELECTRIC HEATING The first approach that was modelled was the internal energy battery-heating


method. As the internal resistance is high at low temperatures, it dissipates energy that will naturally heat the battery. The second approach uses the wasted


heat from the drivetrain to heat the battery through more complex circuits. The third option relies on active


electric heating in which additional circuits are designed that will use the energy of the battery for its own heating. Simulation conditions were the same for all three approaches. They started at minus 30°C and ran two Worldwide Harmonised Light Vehicle Test Cycles (WLTC) with a simulation being performed that showed conclusive results that were then confirmed by tests. The battery cells reached optimal


temperature much faster with the third approach, active electric heating. It reached 0°C in 14 minutes and 40 seconds and 6.5 kilometers driven, whereas it took about 28 minutes and 21 kilometers for the other two approaches to reach 0°C and the same final state of charge of the battery (around 55 percent of capacity) at the end of the driving cycle. The active electric heating strategy was selected to be used to optimise the driving range and vehicle performance. “We were able to quickly select the right architecture with the best performance and focus on the next steps of the project by using this approach,” says Sebastian Brixner of CES.


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