Maintaining a safe and spatially uniform temperature field is critical for lithium-ion battery packs used in mobile platforms. This paper presents a three-dimensional, transient conjugate-heat-transfer model of a 13s3p pack built from 18650 cells to study how cell layout and operating regime shape thermal behavior and to derive practical guidance on temperature-sensor placement. The model couples laminar airflow with heat transfer in solids and fluids and uses a volumetric heat-generation term representative of electrothermal losses under charge-discharge cycling. Two archetypal layouts – aligned (rectangular grid) and staggered (chessboard) – are compared under 1C and 2C profiles with a relaxation phase, considering both natural and forced convection boundary conditions. We quantify peak temperature, spatial non-uniformity (ΔT), and hotspot migration, then evaluate candidate sensor locations against these fields to identify minimal sensor sets that capture pack-level maxima and gradients with low estimation error. Results show that layout choice materially alters hotspot location and ΔT, with higher C-rates amplifying non-uniformity; forced airflow mitigates peaks but can shift gradients, affecting optimal sensor placement. The study provides a reproducible FEM workflow and actionable placement recommendations that are directly transferable to pack design and monitoring for safety-critical applications.