How to determine the time for temperature equalisation in batteries and supercaps for reliable laboratory measurements

Temperature is an important impact factor for both battery performance and lifetime. As lifetime of a battery is typically smaller than that of the application it is used in and performance decreases significantly with ageing, lifetime and performance predictions are of great importance for almost all battery-powered applications. To achieve a good and reliable prediction, comprehensive ageing tests with a significant number of different operating conditions are necessary. From time to time, ageing tests are interrupted for capacity and resistance checkups. For comparable results, these checkups have to be performed under the same ambient conditions, typically room temperature. Thus, before the checkup is started, temperature equalisation is needed. The same is true for performance tests or parameterisation measurements for electrical models. Typically, they are not only performed at room temperature, but also at other temperatures to identify the influence of temperature on performance or electrical impedance. For this reason, a homogeneous temperature within the cell is essential for the test results as well. Therefore, a certain waiting time has to be inserted before starting the test, that on the one hand has to be long enough to assure that all parts of the battery have the same temperature, but on the other hand is as short as possible to prevent an unnecessary prolongation of the test, especially for already time consuming ageing tests. Temperature measurements within cells are typically impossible for commercial cells without destroying the cell. However, this paper shows that it is sufficient to measure the outside temperature to find out the necessary waiting time.