Electro-Thermal Modeling and Evaluation of Self-Heating System for Samsung INR21700-50E Cells

Abstract

Lithium-ion batteries based on nickel-containing chemistries, including Nickel Manganese Cobalt (NMC) and Nickel Cobalt Aluminum (NCA), account for the majority of EV battery deployment outside China. At −20 °C, these batteries experience a significant increase in internal resistance, sharp capacity fade, and accelerated degradation during charging — a critical challenge for EV operation in cold climates. This thesis investigates the feasibility of internal resistance heating of Samsung INR21700-50E NCA cylindrical cells via bidirectional buck-boost pulse current excitation at −20 °C, based on the topology described in BYD (Build Your Dreams) Company Limited patent EP 4516579 A1.

A two-cell bidirectional buck-boost circuit was implemented in MATLAB/Simulink using Simscape physical network blocks, with battery parameters represented as a first-order Thevenin equivalent circuit model using 5 × 5 lookup tables for R0, R1, and τ1 over SoC and temperature. The simulation was run for 2,000 s starting at −20 °C with SoC = 50%. The confirmed results show that both cells heated from −20 °C to +0.92 °C, reaching the 0 °C target at t = 1,839 s (30.6 min) at an average rate of 0.628 °C/min, consuming 9.01 % SoC and 0.954 Wh of energy to reach the target temperature. The thermal asymmetry ratio (defined as the ratio of temperature rise between the two cells) was confirmed at 1.000× throughout the entire simulation, consistent with the identical cell parameters and control inputs used in the model, validating the simultaneous self-heating principle of the BYD topology. The minimum safe inductance was determined to be L = 15 mH — a hard physical constraint imposed by the single-cell voltage window at −20 °C. The heating rate decreased progressively from 0.966 °C/min in the first 100 s to 0.340 °C/min in the final 100 s, driven primarily by the temperature-dependent reduction of R0 as the cell temperature was raised.

Analytical scaling of the two-cell results to the Battery Workforce Challenge 400 V pack architecture led to derivation of the drive voltage and assembly resistance at the submodule (3s21p), module (6s21p), and half-pack (45s21p) scales, following the same first-principles voltage-window derivation validated at two-cell scale. A pack-scale feasibility verdict is not claimed, since it would require a convective heat loss model validated against pack enclosure geometry and cell-to-cell thermal coupling, neither of which is available from the two-cell result. Instead, the thermal mass equation and energy balance framework derived in this work provide a complete analytical foundation for next-stage pack-level circuit design. These results provide, to the author's knowledge, the first simulation-based feasibility assessment of the BYD bidirectional buck-boost topology applied to Samsung INR21700-50E cylindrical cells at −20 °C, and demonstrate that the self-contained internal pulse heating method is viable at single-cell scale with a practical preconditioning time of 30.6 min to reach the temperature of 0 °C.