New simulation software for modeling Li-ion batteries provides engineers with a realistic method for testing and validating the system that oversees the high-voltage batteries on electrified vehicles.
Conventional testing routines—such as those using static and/or limited capacity battery cell simulation with programmable analog voltage sources—essentially do not replicate dynamic usage conditions for battery cells.
"That's why dSPACE has developed a solution with both hardware and software components that provides the capability to simulate the behavior of Li-ion batteries accurately and in real time. The solution includes a user interface that simplifies the parameterization of the battery model," Mahendra Muli, dSPACE Inc.'s Manager of New Business Development, told AEI.
dSPACE's new battery models facilitate the development of Battery Management System software, a critical component for the safe and efficient operation of hybrid-electric vehicles and all-electric vehicles.
In addition to cell balancing, an electrified vehicle's Battery Management System is responsible for predicting the battery's state of charge, managing thermal functionality by monitoring cell/module temperature, handling charge- and discharge-functions, performing safety functions as well as handling diagnostics by monitoring fault conditions, according to Muli.
"It is necessary to validate the Battery Management System software over the entire operating range as well as validate for abnormal behavior—such as unexpected fault conditions—in order to guarantee safety and performance. Since every cell's operation can have an implication on the battery modules and battery pack operation, it is critical to have capability for dynamic simulation of batteries at the cell level," said Muli.
The simulation models from dSPACE enable each battery cell to be individually represented with cell-specific charges, voltages, and currents as well as the ability to calculate the state of charge dependent back-electromotive force (EMF) individually for each cell. And physical parameters—such as internal resistance, diffusion, charge-transfer as well as double-layer capacities—also can be defined for each battery cell.
Cell balancing functionality of the battery management system can be exercised and tested using the dSPACE solution.
"In a series configuration, each cell can be at a different state of charge and that can affect both the battery life as well as performance, which is why the Battery Management System tries to balance the individual cells by appropriately draining current from the cells. For engineers, it is critical to be able to simulate and monitor cell current which is being sunk and sourced from simulated hardware," explained Muli.
dSPACE's scalable hardware provides high-resolution voltage outputs with precision of ±1.5 mV that can include up to 128 cells with galvanic isolation up to 1000 V. Various fault conditions—such as an open wire to the BMS' electronic control unit—can be introduced via the hardware to the testing scenario.
"The hardware solution is an extension of our existing hardware-in-the-loop systems that are used for overall vehicle or system-level simulations," Muli said, noting that the company's controllable EV1077 buffer amplifier modules can be combined with the simulation battery models to build complete battery pack emulation.
Recently completed beta testing by dSPACE customers provided a framework for further refinement of the battery simulation models.
"Based on the studies, we've arrived at the best approach using the linear approximation for each physical effect. This approach allows for easier identification of modeling parameters and maintains the real-time execution capabilities. The results have been validated by excellent correlation of the simulation to real battery behavior," said Muli.