Abstract
Various commercial 18650 and 21700 lithium-ion high-energy cells were investigated experimentally in a wind tunnel for battery cells, regarding their fast-charging performance. Convective cooling with air stream velocities of 1ms−1, 5ms−1, and 10ms−1 were applied at different ambient temperatures ranging from 10°C to 40°C. For each combination of convective air stream velocity and ambient temperature, a constant current (CC) charge rate capability test, controlled by voltage and temperature, as well as a multistep constant current (MCC) protocol were investigated.An improved rate capability was achieved with stronger convective cooling when charging was limited to the risk of overheating. For 3C CC charging, the charged capacity was doubled for the 18650, as well as the 21700 format. Overcooling at low temperature deteriorated the rate capability. In general, the rate capability of the 18650 cells was superior to the 21700.Regarding the MCC protocol tests, the charging time, charging efficiency, maximum surface temperature and specific cooling power were evaluated. To minimize MCC charging time, an air stream velocity of 1ms−1 at 25°C is recommended to achieve 21min and 25min for the 18650 and the 21700 format, respectively. For this, a peak specific thermal cooling power of around 275W, normalized to a 1kWh battery pack for better format comparability, was required. The charging energy efficiency was above 91% for all tests.
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