Thermal Management Data
A thermal management system holds a battery’s temperature constant and prevents thermal runaways. Investigating this requires the use of isothermal (as opposed to adiabatic or ARC) calorimetry. These isothermal calorimeters are widely used in the chemical industry for testing commercially important heat-generating reactions before they are scaled up. In batteries, these experiments could be performed with individual cells to obtain the basic properties of the cell. Larger packs will also need to be tested, as the packing and physical arrangement of the cells are important in governing the transfer of heat.
For this reason, the isothermal version of the BTC (iso-BTC) is available with a range of adaptors to enable batteries and packs of different sizes and shapes to be tested.
Basic principles of isothermal BTC
Isothermal heat measurement from batteries can be performed by balancing the heating and cooling duties necessary to hold the sample temperature constant. Thus, the iso-BTC consists of a chamber (which holds the battery sample) into which cold air or nitrogen can be blown from all sides. Provided the airflow and temperature are constant, the cooling effect on the battery will also be constant. At the same time, the battery surface is warmed by a thermal controller (fine heaters coated onto a conducting sheet) to counter the effect of cooling. The controller is software controlled and reports changes in thermal duty.
There are two temperature sensors, on either side the battery sample and the thermal controller keeps the average of these two temperatures, at the user-defined value. In a typical experiment, the cooling is started and the average temperature allowed to stabilize before any charging/discharging of the battery is started.
When the Cycler is operated, leading to either heating or cooling of the battery, the controller ensures that the average temperature of the battery remains constant. This directly provides a measure of the heat being generated by the battery. With Lithium-ion batteries, charging often leads to an endothermic reaction (cooling of the battery) and discharging results in an exotherm reaction (heating of the battery).
When charging starts, a small fall in battery temperature is observed but further change is prevented as the controller adds heat, maintaining isotherm conditions. When charging is complete, the temperature stabilizes and heaters return to a steady-state. During the discharge step, the battery temperature is seen to slightly rise. The controller reduces power input to the heaters to prevent a further rise in temperature to compensate for the heating (exotherm) within the battery. This is what is measured and displayed while the experiment is performed. These changes in the controller performance can be directly related to the heat effects within the battery and translated to the thermal duty needed to keep the battery temperature under control.