Why PHET

Abstract

In the development process of electric vehicles, the design of battery systems has long been the most challenging problem due to high energy density and high power density requirements. To date, the problem has been alleviated in hybrid electric vehicles because the employment of internal combustion engines replenishes the energy deficiency of batteries available in today's market. However, the advent of lithium ion chemistry may circumvent the difficulties in design of battery systems for electric vehicles. The safety issues of lithium ion chemistry still concern the public in the past few years due to reported accidents either in the lab or on the road. In this paper, a fault-tolerant battery system consisting of small-format cylindrical cells based upon lithium iron phosphates chemistry is investigated with design details and experimental verifications.

Introduction

Since more than half a century ago, electric drive technologies have gradually gained significant attention from both government and industry because of the forecast for shortage in oil-production and global weather warming. Along with Toyota promoting her HEV, namely Prius, to the market, many auto manufacturers are showing their future cars based upon FCEV, HEV, or EV platform. However, the key decision for mass production of these future cars has yet to be made due to a lack of availability of mature energy storage technologies. Since secondary batteries are essential for both HEV and EV development, many types of chemistry have been proposed and tested in the past decades. Not until recently, lithium ion battery technology has been carefully investigated because of its characteristics include [1]:

• ● High gravimetric and volumetric energy densities
• ● Ambient temperature operation
• ● Long life cycle
• ● Good pulse power density

Although there are many "high voltage" cathode materials being tested continuously in the past years, LiCoO₂, LiNiO₂ and LiMn₂O₄ are the most commonly reported potential chemistries [2]. Among the various chemistries, Zaghib et al [3] recently reported that C-LiFePO₄ is one of the most promising technologies because its charge and discharge properties at high operating temperature could circumvent the problems of thermal runaway. Table 1 shows comparisons of the general characteristics among C-LiFePO₄, LiMn₂O₄ and LiCoO₂ available for the market today. Nonetheless, when battery cells are packed into battery systems, Performance Safety and Abuse Test must be conducted on the systems to characterize the responses of integrated battery system to expected and worse-case accidents and abuse situations. Under no circumstance, the battery systems under test should burst into flames or combustible fumes. Table 2 compares the mechanisms of decomposition between LiCoO₂ and C-LiFePO₄ chemistry which indicate potential hazards still existent due to internal short-circuit situations. Hence, it is noted that safety protection devices must be installed in the battery systems if higher safety standard is adopted for the battery systems in HEV or EV applications. In this paper, a patented DOSBAS^® safe battery system is introduced and investigated for illustration of protection mechanisms on thermal runaway or fire hazard [4]. The DOSBAS was named as DOnald Safe BAttery Systems first studied in 2005 and then proposed for EV battery packs by Dr. Wu in 2007. The main feature of the system is to protect each cell in the system individually by a serially connected quick-blown fuse with or without a LED indicator. The protected cells are then connected in serial or parallel configuration or encapsulated the parallel or serial battery cells by a prismatic plastics box or together with a voltage balancing circuit. The proposed system can further enhance the inherent safety of the C-LiFePO₄ chemistry or different battery chemistries to a road-worthy level. In addition to the robustness in safety of the DOSBAS^® battery system, the total cost of system is significantly reduced by replacing the isolated malfunction-cells in the battery system during maintenance services. As compared to the battery systems packed with large format cells, standard format cells, such as 18650 or 26650 cells, or even lithium polymer cells can be employed in the system for reducing costs and enhancing reliability. The DOSBAS^® safe battery system is devised as a trade-off of performance, safety, reliability and costs in the battery systems for EV or HEV applications.