EV Thermal Management: Get the Odds in your favor!
The adoption of Electric Vehicles (EV) is growing rapidly across the globe, a study estimates an annual growth rate of 8% by 2025. One of the drivers being lower cost with improved EV battery technology. Currently, 65% of the total EV cost is contributed by the battery, with the technology used predominantly being Lithium-ion due to high energy density and smaller size.
A key challenge with Lithium-ion batteries is the safe operation at higher temperatures and acceptable performance at lower temperatures. Automotive OEM’s across the globe are developing sophisticated battery thermal management techniques for safe and optimum battery performance. The higher the EV performance and safer it is compared to internal combustion vehicles, the faster it will drive the consumer adoption.
Structure of an EV battery
The typical structure of any EV battery consists of a cell, module, and a pack. At a very simplistic level, a module is a collection of cells in a specific arrangement and a pack is a collection of modules in a specific arrangement. Each pack has Battery Management System (BMS) which has proprietary algorithms that collects and analyses data using sensors to make decisions for safe operation.
Temperature impact on Li-ion battery
There are three important parameters that always tells us the state of the battery pack: current, voltage and temperature. Temperature is the critical parameter as it has significant impact on current and voltage. The ideal operating temperature range for high performance battery is 15 to 30°C. Higher the temperature from the desirable operating range, either caused by hot environment or improper cooling will cause significant battery power loss. This results in poor motor and vehicle acceleration, and hence battery should be oversized to compensate for the power loss, which is not ideal. On the other hand, lower temperature would increase the internal resistance of the battery negatively affecting the charge/discharge cycle.
The other impact is the thermal gradients introduced in the pack, which when equivalent to ~ 2°C inside the battery causes a phenomenon called thermal runaway, where the rate of propagation of heat increases exponentially resulting in the battery catching fire. The other gradual affects are non-uniform cell aging in the pack and overutilization of cooling system.
Why do you need battery (Li-ion) thermal management?
The pack temperature must be regulated to operate in the desired temperature 15°c to 30°c during the charge/discharge cycles for optimum performance. This is important as the overall battery temperature keeps changing based on the ambient temperature the vehicle is in, vehicle acceleration/deceleration, cooling system, and charge/discharge cycles. The other reason regulation is needed is to reduce uneven temp distribution among the cells in the battery pack. As we discussed before, temperature distribution must be managed to eliminate hazards due to thermal runaway.
The role of BMS
BMS was briefly mentioned in the structure of the battery and saw that it depends on data collected from sensors in the pack to maintain safe operation. The BMS control system is as only good as the raw data the temperature sensors collect. Currently, the BMS system is developed by extensively testing the battery pack and identifying temperature thresholds for various scenarios and using these thresholds as limiting values for safe operation. These thresholds determine how much power can be supplied to the motor without battery overheat, the driving range of an EV, the load on the cooling system and fast charging capability. So, the temperature sensor used during the BMS development to set threshold values are critical for optimal performance.
Fiber optic temperature sensors for battery thermal management
The fiber optic sensor is becoming the industry standard for battery pack testing. Several leading OEM’s have realized the inherent advantage of using fiber optic temperature sensors over K type thermocouples and have already implemented this technology in their battery validation or testing process.
The table below compares the features of fiber optic sensor with K type thermocouples:
The table below shows how each feature impacts the battery testing process. It is clear that fiber optic sensors mitigate all risks introduced by K type thermocouples and eventually reduces the time to release to production.