Fast Charging Tests (up to 6C) of Lithium Titanate Cells and Modules: Electrical and Thermal Response
There has been much discussion of fast charging of lithium-ion batteries as a means of extending the practical daily range of electric vehicles making them more competitive with engine-powered conventional vehicles in terms of range and refueling time. In the present study, fast charging tests were performed on cells of three lithium-ion chemistries to determine their characteristics for charging rates up to 6C. The test results showed that the lithium titanate oxide chemistry has a clear advantage over the other chemistries especially compared to the Nickel Cobalt Manganese chemistry for fast charging. In this paper, the results of extensive testing of 50Ah LTO cells and 24V modules from Altairnano are reported. The modules were instrumented so that the voltage of the individual cells could be tracked as well as three interior temperatures. Cooling of the modules was done via a cooling plate positioned on one end of the module. Life cycle testing of the 24V module is still underway. The cycling involves fast charging at the 4C rate and discharging at C/2. The voltage at the end of the charge corresponds to a stateof-charge of 90 % and the voltage at the end of the discharge corresponds to a state-of-charge of 24 % resulting in the use of 33.3 Ah (66%) from the module. The charging is done at 200A and the discharge at 25A. The charging time is 10 minutes and the discharge time is 80 minutes. The test cycle is meant to mimic the use of the module in a transit bus application with fast charging. To date the module has experienced 285 cycles without any apparent degradation in Ah capacity or voltage response. The maximum measured temperature inside the module stabilized at about 40 deg C without active fan cooling.