Comparative Simulations of Thermal Management Strategies for EV LiFePO₄ Battery Modules in Mongolian Cold Climates

Abstract

Mongolia ranks among the world's coldest inhabited nations, with Ulaanbaatar averaging −25°C in January and the national record reaching −58°C. These conditions severely impair the performance of lithium iron phosphate (LFP) battery modules: at −40°C, available power may be reduced by up to 80% relative to room-temperature performance, and charging is not safely possible without prior thermal conditioning due to the risk of permanent lithium plating at the graphite anode. Standard battery qualification tests are oriented to major markets in the United States, China, and Europe and are typically conducted only down to −25°C, leaving the extreme cold end of the Mongolian operational range uncharacterised in the published literature. This thesis presents a comparative transient thermal finite element analysis of four individual thermal management strategies applied to a 12-series 1-parallel (12S1P) module of twelve CATL 100 Ah prismatic LFP cells, together with a fifth combined and optimisation system (System E) that evaluates five sub-simulations pairing active and passive strategies under cold-start and summer overheating conditions. The four individual strategies are: (A) passive aerogel insulation (thermal conductivity k = 0.016 W/(m·K), 7.5 mm thick); (B) external positive temperature coefficient (PTC) resistive heating at 150 W applied through a 404.14 mm × 200.33 mm × 10 mm heater plate; (C) internal alternating-current (AC) self-heating, modelled as a volumetric heat generation rate of 54,538 W/m³ applied uniformly to all twelve cell bodies; and (D) a passive RT35 phase change material (PCM) buffer, modelled as a 404.14 mm × 200.33 mm × 3 mm slab on the module face opposite the heater plate. Three-dimensional geometry was developed in Autodesk Inventor Professional 2026 and imported into ANSYS Workbench 2025 R2 for transient thermal analysis. Six Transient Thermal systems are defined: a Baseline representing the unprotected module, and Systems A through E. Two test types are used: heat retention tests (module starts at +20°C, cools in −40°C ambient) and heat-up tests (module starts at −40°C, warmed by active heat sources). Key results are as follows. System A (aerogel alone) extended the time before the minimum cell temperature crossed 0°C from 100 s (Baseline) to 3,140 s, a 31-fold improvement providing approximately 50 minutes of additional passive protection. System B (PTC heater alone) failed to bring any cell to 0°C within 4 hours; the minimum temperature after 14,400 s was −14°C. System C (AC self-heating alone) achieved 0°C minimum in 5,680 s. System D (PCM alone) extended minimum temperature protection by only 105 s, confirming aerogel outperforms PCM by a factor of 29 as a passive cold-weather strategy. System E identified the combination of AC self-heating with aerogel insulation (E2) as optimal: 0°C minimum reached in 2,200 s (36.7 minutes) consuming only 0.458 kWh, the best efficiency of all active strategies tested. Adding PCM to active cold-start systems was found to be detrimental, extending warm-up time in both E3 and E4 by absorbing heat that would otherwise warm the cells. System E5, a summer overheating scenario, showed that the aerogel-wrapped module reached the 60°C critical discharge temperature limit from 20°C in 4,680 s (78 minutes) under moderate driving heat generation at +35°C ambient, confirming that aerogel-based designs must incorporate provisions for warm-weather thermal management. The comparative results provide a quantitative engineering basis for selecting and designing battery thermal management systems suited to Mongolian operating conditions. This work represents the first systematic module-level finite element comparison of these four strategies under the specific thermal conditions of Mongolia, addressing a gap in the existing engineering literature.