Thermal Characterization of High-Voltage Batteries

Background

The performance of high-voltage batteries based on lithium-ion is significantly affected by temperature, due to the chemical properties. The optimum working temperature for lithium-ion batteries ranges from 15 °C to 35 °C. At lower temperatures, the chemical processes in the battery are significantly slowed down, which reduces the energy and performance capability. Excessively high temperatures have the similar effects, but in extreme cases they can also lead to self-destruction (by so called thermal runaway) and fire in the battery. A major source of high internal temperatures is caused by heat generation inside the battery, which is caused by charge transfer and chemical reactions during charging and discharging. Over time, different temperatures occur during different load conditions.

The spatial distribution of temperatures is also by no means uniform: even within a single cell, the temperatures of different areas differ significantly from one another. Dangers can arise from locally limited areas with very high temperatures - so-called hot spots. These increase the risk of internal short circuits, which in turn can lead to a thermal runaway. These hazards affect all typical cell designs - cylindrical cells, prismatic cells or pouch cells - equally.

To avoid undesirable effects of thermal behaviour, high-voltage batteries are equipped with extensive temperature management and cooling systems to ensure operation in the optimum temperature range. In addition, it is thought to achieve an even temperature distribution within the battery as well as insulation against external influences.

For the development of suitable temperature management systems, the thermal behaviour of all components within the battery housing must be known. Simulations are often used in the development phase. However, simulations often cannot describe the complex chemical processes and their effects within the battery precisely enough for all situations, so that extensive measurements are necessary. Only with a precise investigation of the thermal behaviour of the individual battery cell as well as the complete high-voltage battery can a precise characterization be made and simulation models be validated. The findings will enable further optimization of the battery and the temperature management system.

The thermal characterization of high-voltage batteries places high demands on measurement technology with regard to the number of sensors required, the space needed and the avoidance of interference factors. Several hundred sensors are required to determine precise temperature profiles also at cell level. The sensors and their sensor cables must be small enough to be placed between the cells. The arrangement of the sensors should be as flexible as possible in order to be able to detect temperature profiles and hot spots. Furthermore, it must also be possible to install needed measurement equipment inside or outside the battery in confined space. The structural change of the battery caused by additional large objects inside the battery or a large number of openings in the battery housing for sensor cables should be as small as possible. Otherwise, the measurement results would be falsified too much to obtain a realistic picture of the temperature behaviour.

The following example shows how such a complex temperature measurement for the characterization of a high-voltage battery is performed with the HV DTemp measurement system.

Challenge

The large number of temperature sensors requires a correspondingly large amount of sensor cables and measurement modules. There is often no space in the battery blocks and housings for this measurement technology.

For placing the sensors between the battery cells, the sensors and sensor cables have to be made extremely thin. The application of the sensors should be easy and fast, otherwise too much time is lost for the application of hundreds of measurement points. For the verification of the temperature models, it should be possible to define the measurement points using the simulations and plan the layout in CAD. An exact transfer of the calculated measurement points and reproducible arrangement of the sensors ensures better measurement results.

The measurement object should be influenced by the measurement technology as little as possible in order not to falsify the measurement results. In addition, interferences on the sensor cables, which occur with large bundles of sensor cables, should be avoided.

Last but not least, the measurement technology must ensure safety for the user and the system during measurements in a high-voltage environment.