Evaluation of Hitachi Electric Vehicle Combined Battery System Lifespan in India

We have developed a drive cycle (DC) to test Hitachi’s combined battery system (CBS) for electric vehicles (EVs) having battery lifespan enhancements. Conventionally EV batteries consist of high energy density cells, and we call them as energy cells (EC). A major issue with the EVs is high operational costs mainly due to short lifespan of the ECs. CBS almost doubles the EC and thus overall battery system lifespan, as per the evaluation over a WLTP based method. We want to test the CBS under Indian conditions which has predominantly hot weather, and traffic jam scenarios. Battery deterioration and thus its lifespan is sensitive to traffic conditions and ambient temperature. Hence, it was needed to evaluate the CBS over an Indian DC and use 40°C as ambient temperature. However, it was difficult to carry out the tests since there is no standard Indian DC for small / light weight four wheelers. Hence, we decided to synthesize a DC and collected over 1.5 million OBD data samples from four cars driven along different routes within Bangalore city. We propose a novel method to synthesize DC. We evaluate our method through statistical and frequency based similarity between the synthesized DC and OBD data. Finally, using the synthesized DC it was recognized that the battery lifespan could be enhanced by a factor of 2 through CBS over EC alone, for Indian conditions also. However, lifespan under the Indian conditions is reduced by a factor of 0.72 of the corresponding WLTP based lifespan estimates.