Cell temperature uniformity inside most batteries is important, because temperature variation leads to cell resistance variation and thus cell voltage variation during discharge–charge cycling. Voltage variation among the cells leads to accelerated degradation of the overall battery. Goal of this work was to improve cell temperature uniformity of the General Electric Durathon TM E620 battery module (600 V class, 20 kWh, 280 °C nominal temperature), which uses the sodium metal halide chemistry and convection air cooling. Computation fluid dynamics (CFD) study and bench-top testing were used to evaluate multiple battery design options. The optimized battery design was prototyped and tested, which demonstrated 3.5× increase in cooling power and 30% reduction in cell temperature difference during discharge–charge cycling. Cell temperature difference during battery float was reduced 50%. The hardware design changes were implemented into production batteries, which showed 450% improvement in reliability performance during discharge–charge cycling.