Implementation of a Junction Temperature Cycle Recorder in a Battery Electric Vehicle and Accuracy Assessment by Road Testing

Abstract:
Fleets of people mover, postal delivery cars or car-sharing vehicles are exposed to very different operational and environmental stresses. To improve availability, maintenance cost and planning efficiency a homogenous degradation of all vehicles within a fleet would be helpful. One approach to reach this is to exchange heavily loaded vehicles by vehicles that haven’t been stressed so much. For this purpose, this paper develops a recording device, that calculates and records the load histories of the traction inverter and the high-voltage battery during field operation. The device can be connected to the on-board diagnosis interface of a car, where it uses available data on the CAN. One special focus is put on the calculation of the junction temperature based on CAN data. The accuracy of this CAN based junction temperature estimation is discussed in detail and found to be suitable to determine and compare the load histories of different cars relative to each other. To reduce the required storage capacity, an online Rainflow cycle counting algorithm is implemented that identifies and parameterizes closed temperature cycles. All cycles are stored on a microcontroller in a three-dimen-sional frequency distribution, that can be read out via Bluetooth. In studies on a roller test bench the reproducibility of the recording device and the conformity to standardized ASTM Rainflow cycle counting is investigated. The overall relative deviation is found to be < 0,4%, so the developed concept is capable to compare load histories of different vehicles relative to each other. The absolute accuracy of the concept will be determined in future research activities.