Heating Effect of Alternating Current on Lithium-IonBatteries at Subzero Temperatures
Abstract: Electrification of transport is a distinct goal of many authorities. Electric vehicles are locally emission-free and can help to improve the air quality in cities. Despite several stimuli to promote electric vehicles, customers do not accept them at the desired level. One reason is the poor ratio between driving range and purchase cost. One possibility to tackle this poor ratio is to reduce complexity and weight of a battery pack, which helps to reduce its costs while increasing the driving range at the same time. Recent batteries in electric vehicles use lithium-ion technology. This type of batteries have a limited operating temperature, which requires pre-heating in cold ambient. In many electric vehicles pre-heating is realized with a heating element in the battery pack. This heating element does not only bring along additional weight but requires extra energy for distributing the heat. The idea of this study is to propose a more efficient heating method which is called alternating current (AC) heating. This method uses the internal impedance of a battery to generate dissipative heat, heating it up from inside. This work consists of practical measurements and experiments on a cell level as well as simulations. First, a literature research is conducted to present the latest heating technologies. Then, an impedance measurement is performed. The result of the impedance measurement is used to define the operating points for the following experiment. The experiment shows how AC frequency and amplitude influence the heating effect. The results from the impedance measurement and the experiment are further used to construct a cell simulation model that connects the thermal properties with the electrical properties. After verification of the model, the simulation is expanded to a battery pack level. The conclusion of the study is that it is generally possible to heat lithium-ion batteries with alternating current. Lower frequencies provide better heating effect at the same current amplitude and a higher current amplitude gives better heating effect than lower amplitudes. Therefore, the study also shows that further research is necessary to determine how the AC frequency affects aging of lithium-ion batteries at low temperatures. Finally, a method for increasing the current amplitude is proposed.
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