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Influence of recuperation currents on battery lifetime in electric bus applications

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Recuperation is an important measure for extending the daily range of electric vehicles. It plays a major role in the transportation sector to return non-needed kinetic energy of the large moving masses back into the electrical systems during breaking.
For customers are reliable lifetime estimations very important. Their usage profiles must be analyzed. The aging effects need to be evaluated. The results are often the base of warranty discussion. Semi-empirical aging models are an important tool for indicating dependencies under specific usage conditions. They can become any complexity. Influences of calendar aging as well as temperature, DoDs and current rates during operation can be considered in case of constant current phases. For this model type, large-scale test series are often carried out, which include continuous cycling of single cells or packs. While many test series show low impact of discharge current rates on aging, greater attention must be paid to charge current rates. It is necessary to consider application specific properties separately. This study focuses on the aging effect of short-term charge current peaks in Li-ion cells cycle life. The considered scenarios are regional operating and city busses. Therefore, various synthetic customer profiles as well as real vehicle data are investigated. Based on this analysis cycle life test pattern are created and performed.
These tests designed as pairs consisting of reference constant current cycle life test and corresponding test with superposed charge peaks.
Afterwards, the different lifetime results should give an indication of how the aging is affected by short-cycled peak currents close to real life applications. To characterize the cells and analyze the aging progress, capacity and internal resistance measurements will be performed at periodic intervals. These measurements are supplemented by differential voltage analysis investigations which allow detailed conclusions to be drawn by comparing constant current and peak current loads.
These investigations are based on cylindrical cells in 21700 format with graphite anode and NMC cathode.

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