Battery tests in HiL system
For tests in the HiL system, the battery is substituted at the physical and electrical level by a real-time simulation. This makes it possible to safely carry out tests even under extreme conditions, such as short circuits, deep discharge, and overheating. In addition, aging scenarios and scatter in the behavior of individual cells can be precisely reproduced with a few clicks of a mouse – without the additional time needed to condition real batteries.
LABCAR-MODEL-BAT: Simulation model of lithium-ion batteries for HiL tests
Characteristics of the model
ETAS’s LABCAR-MODEL-BAT simulation model describes a lithium-ion battery composed of individual cells with explicit simulation of the state of each individual battery cell. The number of cells can be freely selected.
Strengths of the model
The real-time capable LABCAR-MODEL-BAT contains all the principal electrical characteristics of a battery. It models the static and transient processes of each individual cell in lithium-ion batteries very precisely. The ability to parameterize each individual cell allows users to test modern balancing techniques with their high-precision estimation of the state of charge (SoC) of individual cells. In addition, LABCAR-MODEL-BAT is able to model critical operating states in order to test functions such as crash recognition, emergency-off, and leak detection.
The model is implemented in Simulink®, offering free access to all its parameters. Because the underlying function model is realized via Simulink S-functions only at the lowest level, add-ons and modifications are possible.
If measurement data exists for the battery you want to model, it can be used for the automatic parameterization of the simulation model. Based on this data, the model parameters are automatically set so as to create maximum correspondence between the real-life component and the model.
Structure of the model
The stationary behavior of each individual cell of the battery is described through the relationship between the cell voltage (without load) and the state of charge. This characteristic curve is typically specified by the cell manufacturers, which makes parameterization easier.
LABCAR-MODEL-BAT models the dynamics of cell voltages while a cell is charging or discharging. In the process, it takes into account capacitive and resistive effects within the individual cells. The characteristics of each single cell are modeled individually to cover different charging and discharging characteristics of each cell - just like in real vehicles. This allows the comprehensive testing of balancing algorithms and SoC estimation algorithms (such as the Kalman filter), which incorporate manufacturing variations and distribution in the behavior of individual cells.
The model captures even complex relationships. Variations in the strength of charging and discharging currents cause the cells to heat up, which, because the internal resistance is temperature dependent, causes the cell behavior to change. To map this influence, the model contains thermodynamic effects for different battery geometries and a power-consumption model. This power-consumption model, which also takes into account self-discharge currents and chemistry-based cell behavior, is founded on an empirical approach that makes it much easier for users to parameterize the model and includes all the important effects.
Tests at different levels
In general, we distinguish between two types of tests: tests of the BMS controller software and tests at the cell supervisory circuit (CSC) level.
In tests of the BMS controller software (BMS: battery management system), all electrical BMS interfaces apart from the CSC contacts are connected to the HiL system. The high currents and voltages of a real battery can be avoided entirely, as the communication between the BMS and CSC takes place via a data interface (such as CAN or I2C) and the BMS’s other current and voltage sensors can be easily manipulated. The battery and CSCs are simulated by a model that also takes into account the limited resolution of CSC measurement technology.
In tests at the CSC level, the real CSCs are connected to the HiL. Accordingly, the real battery’s voltages and balancing currents (charge equalization) are present at the HiL system’s battery cell simulator (BCS). ETAS LABCAR simulates each of the battery’s individual cells by coupling high-precision current and voltage sources with LABCAR-MODEL-BAT. Tests at the CSC level put high demands on a HiL system:
- High parallel computational throughput with rapid input and output interfaces for real-time simulation of over 200 cells
- Ability to display voltage differences of up to 1000 V by means of cell emulator for a battery with 200 cells of 5 V each
- Providing of currents up to 8 A for active balancing test
- Ability to set individual cell voltages under all conditions better than 1 mV
- Accuracy of current measurements in μA range in order to detect leakage currents and quiescent currents
- Measuring of substituted charge quantities (Coulomb counting) for verification of balancing algorithms
With its modular approach and BCS cell simulator, the LABCAR-HiL system fulfills these requirements. Thanks to its PC-based simulation target LABCAR-RTPC and its openness, ETAS LABCAR is also ready to meet future requirements. This technology safely unlocks the performance potential of lithium-ion batteries for use in motor vehicles.