3.5 BMS Algorithms & Electrical Simulation

A typical electrical modelling and simulation workflow consists of several key steps: cell modelling, system-level modelling, algorithm integration, and validation through SIL and HIL environments.

The process begins with the development of an equivalent circuit model (ECM), which represents the electrical behavior of a battery cell. This model is parameterized using laboratory data such as open circuit voltage (OCV) mapping, capacity testing, and hybrid pulse power characterization (HPPC).

1. Cell-level modelling

The ECM serves as a digital representation of the cell and captures voltage response, internal resistance, and dynamic behavior under different operating conditions. Accurate parameterization is critical, as this model forms the foundation for all further simulations.

2. System-level modelling

Once validated, the cell model is scaled to the full battery system:

• Topology: Cells are arranged into modules and packs through series and parallel configurations
• Interconnects: Electrical resistance of busbars, welds, and connectors is included, introducing voltage drops under high current
• Electro-thermal coupling: Electrical and thermal models are linked. Heat generated by current flow (Joule heating) influences resistance, creating a feedback loop that must be captured for accurate system behavior

3. BMS algorithm integration

With the system model established, BMS algorithms are integrated. These algorithms perform key functions such as:

• Voltage and temperature monitoring
• State estimation (SOC, SOH, SOP) using methods like Kalman filtering
• Cell balancing
• Safety monitoring and contactor control

4. Software-in-the-Loop (SIL) validation

In the SIL phase, BMS software is tested in a fully virtual environment without physical hardware. The focus is on software robustness, logic validation, and edge-case handling.

Typical tests include:

• Triggering fault conditions slightly beyond limits (e.g., voltage thresholds)
• Executing all logical branches in safety-critical code
• Feeding unrealistic or inconsistent sensor data to validate fault detection

5. Hardware-in-the-Loop (HIL) validation

In the HIL phase, the real BMS hardware is connected to a real-time simulator running the battery model. This step verifies functional safety and real-world system behavior.

Typical tests include:

• Fault tolerant time interval (FTTI): Measuring response time to faults such as overvoltage
• Contactor diagnostics: Simulating failure modes such as welded contactors
• Thermal derating: Testing system response to elevated temperatures and validating power limitation strategies

Additional process considerations

Electrical modelling and simulation are essential for reducing development time, minimizing physical testing, and improving system safety early in the design process.

• Short circuit and fault simulation are critical to ensure that protective elements such as fuses and contactors behave correctly under extreme conditions.
• EMI/EMC modelling is used to assess electromagnetic compatibility and prevent interference with other vehicle systems.
• Model accuracy and validation are key challenges. The quality of simulation results depends heavily on input data, parameterization, and the correct coupling of electrical and thermal effects.
• Scalability and complexity increase significantly when moving from cell-level to pack-level models, requiring efficient simulation tools and computing resources.