Traction Battery Retrofit Using CATL Prismatic NMC Cells: On-Road Analysis of Internal Resistance, Voltage Sag, and Cell Voltage Delta

The first generation of mass-produced electric vehicles (EVs) is now experiencing significant traction-battery degradation, expressed as capacity loss and a nonlinear increase in internal resistance (Rint). In passively cooled architectures such as the early Nissan Leaf, this degradation raises internal Joule heating and contributes to the “Rapidgate” thermal bottleneck during fast charging and high-current operation. This study evaluates the electromechanical and thermal safety of retrofitting a degraded 24 kWh Nissan Leaf AZE0 pack with modern CATL prismatic NMC cells to approximately 50 kWh. The assessment combines 1/20 C bench testing with 1 Hz on-board diagnostics (OBD) telemetry acquired during severe on-road driving, and the pack response is interpreted with a first-order Thevenin equivalent circuit model. The retrofit reduced pack-level dynamic internal resistance from 143.1 mΩ to 25.3 mΩ (82.3%), limiting maximum voltage sag at 237 A to 6.0 V (from 33.9 V). Consequently, peak internal Joule heating fell from 8.0 kW to 1.4 kW (82.5%), providing effective passive thermal mitigation. The retrofitted pack also maintained a median cell-voltage imbalance (ΔV) of approximately 17 mV under peak torque demand, compared with persistent fluctuations above 35 mV in the degraded pack. UL94 V-0-rated insulation and a >500 V dielectric barrier, together with thermally derated 75 mm² busbars coordinated with the retained 225 A fuse, supported safe integration. The results validate in-vehicle life extension of legacy EVs, while dedicated fast-charge thermal mapping and multi-vehicle testing remain necessary.