The performance of high voltage batteries is the key factor for further success of electric vehicles. The primary areas for battery development include high voltage (HV) and functional safety, maximum power and usable energy, battery life, packaging and weight reduction.This paper explains the development of the HV battery and the battery management system for the FEV Liona fleet, a retrofit of a pure electric powertrain into a FIAT 500. The multi-disciplinary process used to develop this program includes electrical, mechanical and functional aspects.The layout of the electrical system includes cell selection, layout of modules and the interconnection of twelve modules to a battery pack. The mechanical design of mounting the battery under the floor addresses the housing issues regarding robustness and sealing, the packaging into the vehicle as well as the positioning of the HV components inside the battery. The key functional aspects include safety-relevant functionalities, such as isolation monitoring or emergency disconnect. Additional functions of the battery controller include cell balancing, charging and secure communication with other vehicle controllers.The fleet tests conducted as a part of this program provided valuable bench and performance data for analyzing the battery and its components.