Researchers in Germany have developed a battery-in-the-loop testing environment for electric vehicle batteries that combines physical components with mathematical simulations of vehicles.
The team at the Fraunhofer Institute for Structural Durability and System Reliability LBF in Darmstadt say this is the first to enable lab trials under real-world conditions.
The current and voltage characteristics of the a battery pack used in electric vehicles is just one factor that needs to be tested. Other key elements to test include the wear and tear of electrical, mechanical and thermal stress when negotiating hairpin bends, bouncing over gravel roads strewn with potholes and motoring in the sweltering summer heat. This is why new battery systems have to be comprehensively tested in real world environments before installation in vehicles.
However, conventional lab tests are a far cry from reality, and real-world trials have to wait until engineers deliver a drivable prototype of the vehicle. If undetected problems surface at that late a stage, then the necessary modifications can cost a lot of time and money.
The Battery-in-the-Loop @ LBF (MEF-BILL) project breaks down the loads placed on batteries can be broken down into three domains – the electrical loads primarily attributable to current flows, the vehicular motion, and the climatic aspects. The conventional approach has been to test these three factors separately in lab with trials that have standard runtimes. I
n the real world, however, these factors are interdependent and affect each other in complex ways so the project allows these loads and their interaction to be evaluated simultaneously in the Fraunhofer LBF testing environment, alongside a real-time-enabled, computerized model of the vehicle.
This battery-in-the-loop approach allows the researchers to simulate the vehicle and its performance on very different types of roads. This simulation enables them to determine the loads that would also affect the battery in the actual conditions prevailing out there in the real world.
“We are now bringing the road into the laboratory and combining our multi-physical testing rig with a computational vehicle simulation. This means we can test batteries under realistic conditions before a prototype vehicle physically exists,” said Dr. Riccardo Bartolozzi, the resident expert on numerical system simulation at Fraunhofer LBF. “This way, we gain a lot of time in the development process and significantly improve the quality of results.”
In the past, lab tests have usually been carried out with a battery current profile that follows an idealized curve. This curve looks a lot different in reality. Its trajectory is highly dynamic with random variations, spiking unpredictably as the load peaks. The simulation factors a wide range of factors into the model to determine the load and current of the battery under test. For example, the amount of initially required power can vary as the temperature in the battery or other parameters change. The researchers constantly track the battery’s actual parameters and feed these readings back into the simulation.
The input data does not remain static throughout the duration of the test and is adjusted on the fly based on data sourced from the simulation and readings taken from the battery. “We can reproduce realistic driving manoeuvres in our test scenarios, for example driving uphill or downhill or around sharp bends,” said Bartolozzi.
The battery-in-the-loop approach allows researchers can investigate how other variables affect performance, for example, to determine what happens when an added load increases the vehicle’s mass by 20 percent. Shake tests are also performed, using a vibration table actuated by six hydraulic cylinders that can move it in any direction, to mimic the impact on the battery of movements of the vehicle chassis.
One of the great challenges for hardware-in-the-loop tests is that the simulation has to run in real time. For example, if a test is conducted to investigate ten seconds of operation, the entire simulation may not take a moment longer than ten seconds. After all, this is a loop where the results of the simulation have to be plugged right back into the test to update the simulation on the fly as the trial progresses. The researchers have fine-tuned the calculation’s complexity for this to work.
“We ran the simulations at varying levels of complexity to strike the best balance between complexity and computing time,” said Bartolozzi. The system is ready for use and preparations for the final demonstration are underway