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Cell Core Temperature Monitoring in EV Cylindrical Battery Cells

posted by Rugged Monitoring on 2021-07-15

What is an EV Cylindrical Battery Cell and what are the latest trends in this field?

Cylindrical battery cells are the most popular type of battery cells used. This partially due to its mechanical stability and the ability to endure high pressures inside the cells without changing their shape. These batteries usually have good structural strength and a large capacity relative to its size. Additionally, the metal casing of the cell minimizes disruption caused due to thermal cycling or mechanical vibrations etc. Perhaps, the most famous cell would the 18650 lithium-ion cell which was optimized as it had a very low cost per Wh and is quite reliable. However, there are many new types of cylindrical cells being developed for EVs, some of which are working on new techniques to cool each battery cell and avoid the creation of hotspots. In 2017, with Tesla introduced its own cylindrical lithium battery made it become more accepted in the electric vehicle industry.

Why Temperature Measurement of Cylindrical cells inside an EV battery is crucial?

Thermal management of the cylindrical cells in an EV is vital as it helps the manufacturers and engineers understand the chemical analysis of the battery cell. This data can then be used by the engineers to improve their product for performance maximization. The goal for the engineers is to create a cell that sustains a high charging rate so thermal profiling with good quality sensors allows them to identify the root cause of heating and formation of hot spots within fast charging cells. Additionally, they want to find the optimal material that can be used for anode and cathode separator which can be used in Fast Charging batteries. The constant desire of having a higher capacity battery cell with superior performance but in a smaller size generates a lot of thermal stress. Another reason thermal stress builds up is due to the rapid charging technology that needs high current. So, to avoid thermal stress within the cell, thermal profiling which identifies hot spots and shows limitations of the product is very beneficial for the manufacturers and engineers who design and regulate the battery.  Finally, design engineers need an accurate battery management system that provides precise thermal profiling which increases the age estimation of the battery and eliminates the possibility of a thermal runaway between battery cells.

How fiber optics sensors prevents thermal runaway in Cylindrical EV battery cells?

Thermal runaway is a huge issue for battery manufacturers who want to develop fast-charging EV batteries but when cells are operating at high voltages, identifying the hot spots within the cell and checking the intracell temperature is mandatory. But most of the traditional sensors such as thermocouples produce unreliable and inaccurate data as they are too big to detect the temperature of the battery cell’s core. Also, using thermocouples to test at high voltages creates a safety hazard as they are made from metal. However, the engineers have a better option of sensors to use which are fiber optics sensors (FOS). These sensors are safe to use as there is no risk of fire from damage to the cell composition. Likewise, the sensors are about 0.4mm thick which allows them to fit into the cell’s core, and it has a lower installation cost than most traditional sensors. FO sensors are adjustable to fit at different locations inside the cell core and it is coated with polyimide which makes electrically, magnetically, and chemically stable unlike thermocouples which are insusceptible to results inaccuracies in exposure of electromagnetic interference. They deliver precisely and noise-free readings without creating pressure or strain with the cell itself. Another reason to choose FO sensors over thermocouples is that thermocouples are slow when compared to FO sensors as these sensors are very fast as they detect Thermal Runway situations and only allow for an intercell temperature difference of 2 degrees Celsius.

Rugged Monitoring Thermal Profiling Sensors

Rugged Monitoring’s Fiber Optic temperature sensors are being used by battery design/test engineers at each stage for battery testing:

  • Battery cell core and anode thermal profiling for fast charging
  • Intercell temperature monitoring in Battery Modules
  • Battery pack abuse testing and HV connectors temperature testing

The CANBUS is implemented to communication with Battery Management System or dataloggers. Also, RM sensors and monitors are easy to use as they don’t require calibration or complex inputs along with being rugged and suitable for all test conditions. Also, RM sensors have a higher immunity to electromagnetic interferences which increases the testing accuracy and reliability while being having a very fast response time of 0.1ms to 1ms that helps identify a sudden shift in the temperature within the cells. The state-of-the-art sensors of RM such as  LSENS-B, LSENS-T, LSENS-R are most reliable option that provides accurate, fast results while being easy to setup and use. The EV test rigs come with built in Fiber Optic temperature monitors, R501, O201 etc.