iso-BTC | Isothermal Conditions Battery Testing | HEL

iso-BTC | Isothermal Conditions Battery Testing

Performance Testing

Performance testing focuses on characterizing the thermal behavior and electrical performance of the cell under a range of operating conditions. The iso-BTC and the adiabatic BTC-130 and BTC-500 all support the full integration of a charge-discharge unit. This enables the automation of repeated cycling of battery cells under a range of operating conditions, while concurrently recording both the battery’s electrical performance and the heat evolved. The iso-BTC is an isothermal calorimeter and it enables cells to be characterized under normal or abnormal usage conditions. It also supports a range of adaptors which allows batteries and packs of different sizes and shapes to be tested. In contrast, the BTC-130 and BTC-500 are adiabatic calorimeters, facilitating the assessment of cell performance under extreme conditions.

Characterize differences in cell performance

Battery chemistry, electrode composition, type of battery cell, and battery age all influence battery performance. The iso-BTC enables the impact of these factors to be investigated during the development of new cells. The data generated on how battery efficiency, (dis)charging capacity, and heat evolved vary with temperature, and (dis)charging rate can be used to model battery performance and enhance understanding of battery behavior for cell development.

Characterize cell for Quality Control

Battery attributes, such as battery efficiency and heat evolved, at specified temperatures and C-rates, can be used to characterize the battery performance. These defined characteristics can be used in Quality Control for both cell manufacturers to demonstrate a stated performance, and for battery integrators to check cell performance downstream. The data from an iso-BTC can enable these characteristics to be determined.

Determine thermal management

Under normal use, heat is absorbed and evolved during the charging and discharging cycles of a battery cell. In addition to this, cells within a module may not exhibit uniform properties upon cycling. The potential imbalance this causes, may trigger a safety hazard and affect battery performance. Without careful management, this self-heating can result in overheating and trigger a thermal runaway. The packing and physical arrangement of cells within a module or pack are essential in governing heat transfer. Therefore, it is important to characterize the thermal behavior of the cells, modules, and packs over a range of temperatures and (dis)charge rates, as the data generated can be used to inform effective thermal management. The iso-BTC also supports thermal mapping during testing to highlight regions of the battery, which generate greater thermal energy levels. This information can also be utilized in the implementation of targeted thermal management strategies.

 

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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.

See our How to use an isothermal calorimeter to characterize a gel battery

 

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