Battery Temperature Effects and Cooling Architecture
Lithium-ion cells have a narrow optimal operating temperature range of approximately 15–35°C (59–95°F). Operating outside this range has serious consequences in both directions. At low temperatures (below 0°C), lithium-ion mobility through the electrolyte decreases dramatically — internal resistance rises, available power drops, and — most critically — charging causes lithium metal to plate onto the anode surface rather than intercalate into the graphite structure. Lithium plating is irreversible and produces sharp lithium dendrites that can penetrate the separator between anode and cathode, causing an internal short circuit and thermal runaway. Modern BMS systems prevent charging until the pack temperature is above a safe threshold (typically 5–10°C) and actively warm the pack using resistive heaters or the heat pump system before fast charging. At high temperatures (above 45°C), cell degradation accelerates exponentially — the rate of Solid Electrolyte Interphase (SEI) layer growth on the anode (which consumes lithium and reduces capacity) doubles roughly every 10°C temperature increase above 30°C. Sustained operation above 60°C risks thermal runaway — an uncontrolled exothermic reaction that propagates cell to cell. Liquid cooling systems pump a glycol-water coolant through aluminum cooling plates or channels in intimate contact with the battery modules. Tesla's design runs a cylindrical serpentine cooling tube between rows of cells; GM's Voltec system uses cooling plates sandwiched between cell layers. The coolant is chilled by the vehicle's refrigerant-based air conditioning system (through a chiller heat exchanger) during hot weather and can be warmed by waste heat from the drive unit or a dedicated PTC heater during cold weather. Target: keep all cells within 5°C of each other (temperature uniformity) and within the 15–35°C optimal range.