BMS optimised for high power density, low price

August 25, 2015 // By Christoph Hammerschmidt
In electric cars, batteries continue to be the most critical component as they determine the driving range of the vehicle. The Fraunhofer IISB research institute has developed a fully redundant, modular , low cost battery system for light EVs. It includes an advanced battery management system and meets the Japanese Kei Car specs.

Developed within the European AVTR (Optimal Powertrain via Adaptable Voltage and Transmission Ratio) project, the battery and its management system were introduced to the public at the Parco Valentino Show in Torino, Italy. Due to its high modularity and its simple design it can be manufactured at low cost and contributes to reducing the overall electronics complexity.

The AVTR project has addressed the development and the industrialization of complete powertrain systems for light electric vehicles. In contrast to available EVs, the international project consortium focused on a special electric vehicle that may fulfil the Japanese Kei-Car specification. The vehicle developed in this project by Italian companies IFEVS and Polimodel, strictly follows four main objectives: low cost, modularity, producibility, and high-end Italian product design.
The vehicle is designed with a total length of approximately 3 meters, thus making the vehicle ideal for crowded inner cities as and confined spaces, as they are just common in Italy.

Being part of the international consortium, Fraunhofer IISB (Erlangen, Germany) was responsible for the redundant battery system. The main objectives were attributed to the battery system, its battery management system, and battery monitoring.
The battery modules were designed in cooperation with automotive supplier Dräxlmaier Group and manufactured by this Bavarian company. Highest modularity and independence of the battery cell manufacturer was achieved by using automotive grade 3 Ah cylindrical lithium-ion battery cells type 18650 from an Asian battery manufacturer. Type 18650 battery cells are in highest mass production for years now, thus providing lowest costs as well as low manufacturing tolerances. Plus, they are available from most premium cell manufacturers. The eight battery modules provide 12 kWh of energy to the powertrain (15 kW nominal, 30 kW peak power). The modules feature a weight of just 9.4 kg, thus providing a gravimetric energy density on battery module level (i.e., with electronics included) as high as 160 Wh/kg.

The battery monitoring system followed the same objectives. The system has been optimized for small size and lowest bill of materials. Providing best voltage measurement accuracy, temperature sensing and passive balancing, the main board measures just 47 square centimetres. The small footprint could be realized by using a highly integrated battery monitoring IC with highest voltage measurement accuracy. Panasonic developed and provided novel prototypes of MOSFETs including protective elements for passive battery cell balancing. The use of these MOSFETs enabled the system designers to drastically reduce the bill of materials with positive effects on reliability.