Multi-Scale Modelling of Localized Battery Degradation in Heavy-Duty Series Hybrids: From Electrochemical Impedance Spectroscopy to Powertrain and Battery Pack Performance

The operational efficiency of heavy-duty hybrid electric vehicles (HEVs) is intrinsically linked to the health and power capability of the battery pack. Traditional battery management systems often rely on bulk State-of-Health (SOH) metrics, failing to account for localized degradation and the resulting current imbalances within parallel-connected modules. Recent literature emphasizes that cell-to-cell variations and thermal gradients can lead to a 6% reduction in accessible energy and a 5.2% acceleration in degradation rates due to heterogeneous current distribution. This study addresses these gaps by presenting a multi-scale modelling framework that translates cell-level electrochemical degradation features into system-level powertrain optimization. Initially, a thorough experimental campaign was conducted on LFP cells, using accelerated cyclic and thermal aging at 25°C and 60°C. High-fidelity battery parameters were extracted from EIS data using fractional-order equivalent circuit models (FO-ECM) in both the rest state and the dynamic load condition. To bridge the gap to real-time simulation, these frequency-dependent parameters were discretized via Z-transforms into time-domain transfer functions. This "Smart Battery Pack" model was integrated into a Simulink-GT-Suite co-simulation environment for a heavy-duty series hybrid vehicle. The framework enables the simulation of heterogeneous SOH levels at 10% intervals, evaluating scenarios where a single module possesses a distinct SOH, either significantly lower (faulty) or higher, than the remaining pack modules (at 100%, 90%, 80%, or 70% SOH). The research evaluates the impact of localized module degradation on terminal voltage and State-of-Power (SoP). Preliminary results indicate that as module-level SOH deviates from the pack average, traditional power management strategies lead to premature voltage clipping and reduced fuel economy. To mitigate this, the study proposes an optimized engine operation strategy for charge-sustaining and charge-depleting scenarios. By adjusting Internal Combustion Engine (ICE) operating points, the system can mitigate the electrical stress on degraded modules. This framework provides a robust tool for designing degradation-resilient control strategies that extend the service life of hybrid heavy-duty assets while maintaining vehicle performance targets.