What is the biggest enemy of electric vehicle batteries? Extreme temperatures.

Lithium-ion batteries perform best in the 15-45°C temperature range. Temperatures above that can severely damage the battery, while lower temperatures can reduce the output of the battery cells, thus reducing range and available power.

The thermal management system always works to monitor or maintain the internal battery temperature, even when not in use (charging). While any temperature outside of the optimal comfort zone can affect the efficiency of the vehicle, the vehicle has intelligent systems that keep the system within its own comfort zone. In general, when discharging, batteries like to be kept below 45°C, and when fast charging, they like to be slightly above that temperature, around 55°C, to reduce the internal impedance of the battery and allow the electrons to fill the battery quickly.

Temperature above 45℃

Overheating can damage lithium-ion batteries, and extreme temperatures (e.g., above 60°C) can increase the risk to driver and passenger safety.

Above 45°C, the cells of an electric vehicle battery degrade rapidly. This requires that the system be controlled by a heat exchanger that both extracts heat from the battery and replenishes it when the system is too cold.

What causes EV batteries to overheat?

When cells are actively charged or discharged, they generate internal heat. Most of the heat moves through the metal current collector and is extracted by convection in the sink bar or by conduction from the cell to the cold plate under the cell to the coolant, which then leaves the battery pack to pass through the external heat dissipation exchanger. Care must be taken when charging quickly, as the battery generates heat as it charges. Great care must be taken to absorb the heat and remove it from the battery, as the battery must not exceed its maximum temperature.

Complex models in the battery management system determine the best strategy for controlling heater and coolant flow. Temperature sensors in the battery and throughout the cooling system need to provide real-time data to the model in order to function properly.

If the battery charges too quickly or gets too hot during vehicle use, the system must act quickly to reduce the battery temperature immediately. Otherwise, heat-induced battery degradation can initiate the thermal runaway process.

Regardless of the heat source, the temperature sensors in EV battery thermal management systems play a critical role in detecting overheating and taking mitigating measures.

Temperature below 15℃

Thermal management systems are not just about keeping EV batteries cool.

In colder climates, the thermal management of electric vehicle battery systems generates heat to keep the temperature above a minimum. They heat the battery before use - whether to power the vehicle, to draw power from charging, or to act as a power source.

At colder temperatures, the internal dynamics of the battery can lead to lower charging and discharging rates, which can reduce the amount of battery power available. Low temperatures slow down the chemical and physical reactions that make EV batteries work efficiently. Without intervention, this can increase impedance (leading to longer charging times) and reduce capacity (leading to reduced range).

When the battery is extremely cold, forcing too much charge into the cell can cause lithium to form dendrites. They can puncture the diaphragm between the anode and cathode, causing a short circuit inside the cell. Therefore, control the charge rate in extremely cold climates to carefully heat the battery, and increase the charge rate only when the battery is above the minimum operating temperature.

Internal combustion engine (ICE) vehicles seem to have an advantage in cold weather by generating a lot of waste heat to keep the vehicle warm in cold temperatures. Without this waste heat, electric vehicles would have to divert energy from the battery to support heating and cooling.

However, thanks to the efficient design of heat pump systems in EV applications, as well as heated/cooled seats and other technologies that heat and cool only when and where needed, they have proven themselves to be better vehicles than their ICE ancestors who were more likely to get caught in snowstorms or summer traffic jams.

While the BMS continuously monitors the voltage and current to and from the battery pack, it also controls systems external to the pack to manage temperature, such as refrigerant and coolant circuits.

To manage these systems, the BMS uses coolant temperature sensors inside and outside the battery pack cooling panels, as well as cell and bus temperatures inside the battery pack. This also extends to monitoring coolant temperatures in the external heat exchangers, as well as pressures and temperatures at key points in the expansion valves and refrigerant circuits. This high level of sensor monitoring provides critical data to control the precise amount of heating and cooling from these systems to optimize battery pack performance while minimizing parasitic energy losses from operating pumps, compressors, and auxiliary heating and cooling components.