LAUSR

In electric vehicles (EVs) and other applications, lithium-ion batteries experience variable load profiles with frequencies up to several kilohertz, as caused by power electronics. It is crucial to know if certain frequencies accelerate battery degradation and should be avoided. This applies in particular for EV batteries with an expected lifetime of more than ten years. This study investigates the influence of alternating current (ac) profiles on the lifetime of lithium-ion batteries. High-energy battery cells were tested for more than 1500 equivalent full cycles to practically check the influence of current ripples. The applied load profiles consisted of a constant current with superimposed ac frequencies. The frequencies were chosen with regards to the impedance spectra and ranged from 30 kHz to 1 Hz. The measurement results are compared to published results and to a second series of experiments conducted with high-power battery cells. The results of the experiments indicate that lithium-ion battery cells cycled with low frequencies experience a 1% to 2% higher impedance increase and capacity fade than battery cells cycled with high ac frequencies. These differences in aging characteristics established within the first 50 cycles of the brand-new battery cells and remained almost constant thereafter. For these experiments, the antithesis is disproved since no accelerated degradation occurred at higher frequencies. Therefore, with regards to battery lifetime, high frequencies can be tolerated as long as temperatures are considered as well. This new finding may help us to reduce the costs of products with complex battery systems, such as EVs.