Selective Shut-off Strategy in Distributed Battery System

Abstract

For the past years, the perspective of societies and governments on global warming and climate change have changed considerably. So, being in search of solutions to fight against global warming can be observed in many countries both in individual and collective ways. Using electricity as a source of energy instead of other harmful fuels is one of the known methods. Therefore, this increasing consciousness and demands of nature-friendly way outs have sped up the process in the arrival of electric vehicles, particularly in the mobility industry [1]. CO2 reduction laws, specially brought to cities, have enabled a forced conversion in public transport [2]. However, this rapid increase has brought a range anxiety to it. Improvement in energy density on cell basis is essential to prevent this perception. In xEV applications the smallest unit in the battery system is described as cells. In general, the chemistry is li-ion and it consists of four main parts which are cathode, anode, electrolyte, and separator [3]. The cells create the modules by connecting in series and (or) parallel. Moreover, modules form the battery system with a specific configuration. Furthermore, paralleling battery packs provides more energy content for the electrification of large vehicles such as buses and trucks [4]. This type of application provides flexibility during the design. Such systems also lead up to the usage of all unused areas in large vehicles. Using distributed battery systems (DBS) allows more capacity instead of one bulky pack. However, the system which is created by more than one battery pack shall also fulfill the functional safety requirements as a single standalone pack. A safety system is performed against fault currents in a battery pack through the alignment of the fuse and the contactor. However, since parallel packs will have a (PDU) power distribution unit where all packs are inter-connected to supply the required power to the traction, a functional safety concept shall be implemented to both the components in the (BDU) battery disconnect unit and the elements in the PDU. When the system is examined electrically through a single line diagram, the system is in the form of parallel conformity of more than one contactor-fuse pair and connecting these parallel strings to a larger contactor-fuse pair. While this is a method for general use in industrial applications [59] it is also required in automotive battery applications. However, in the literature, over current protection studies are made for only one pack system [10-14] and there is no noteworthy study done for distributed systems. In this study, a detailed analysis of the selective shut-off strategy for a parallel connected battery system is investigated. The main target is to maintain the vehicle operation properly in a failure case by cutting-off just the pack with fault instead of shutting down the whole system. Thus, the other packs can continue to provide energy and power to the traction. As a result, the alignment of contactor and fuse in the system shall be conducted properly on not only a pack basis but also a system basis.