Life cycle testing of lithium batteries for fast charging and second-use applications

Two aspects of the life cycle testing of lithium-ion batteries were studied-(1) the effect of fast charging (4C) on cycle life and (2) extended cycling of cells beyond first-use requirements for second-use applications. In the case of the fast charging studies, the test data indicate that the Ah capacity of the lithium titanate oxide (LTO) cells is essentially independent of charge rate up to 6C with no current taper at the end of charge. This means that the LTO cells can be fully charged at the fast charge rates. The life cycle tests of a 24V module (10 cells in series) at the 4C charge rate and C/2 discharge rate showed negligible degradation in Ah capacity or resistance (voltage response) in over 1000 cycles. The temperature response of the module without cooling showed a maximum interior temperature of 40C that remained unchanged over the 1000 cycles of the testing. The second-use studies involved the life cycle testing of new and used 20 Ah prismatic cells (Lithium Manganese Oxide) obtained from EIG, Korea. Testing was done at room temperature and 45 deg C. The new and used cells were cycled (4.15V to 3.0V) about 850 times at room temperature and about 350 times at 45 deg C. The way in which the new and used cells degraded in terms of the decrease in Ah capacity and increase in the resistance were compared with second-use of the cells in mind. As expected it was found that the resistance degraded (%/100 cycles) more rapidly with cycles than the Ah capacity and that for both new and used cells the degradation was much more rapid at 45 deg C than at room temperature. The test results indicate second-use batteries will be best suited for applications requiring relatively high energy density, but relatively low power capability. The degradation of the used cells was gradual (no sudden failures), but accelerated with increasing cycles especially at 45 deg C.