38
Battery systems for hybrid and electric vehicles
There are many things that need careful consideration in the design and management of a battery pack for hybrid or electric vehicles To the uneducated, what could be simpler than connecting some cells together and putting them in a box with a + and – marked on the outside? If only it was that easy. Designing a battery starts with understanding the fundamental performance requirements of the vehicle. The key vehicle targets to start with are acceleration, top speed and range. Acceleration and top speed will give an indication of the power and torque requirements of the electric motor and factoring in any losses (efficiency etc) will give the levels of power and current the battery has to supply. Analysing the vehicle range will give what capacity the battery has to have – usually
measured in kilowatt hours (kWh). Fundamentally, cells can be divided into two kinds – primary cells (non-rechargeable) and secondary cells (rechargeable). For the purposes of this article, only secondary cells are considered. The current cell chemistry of choice will usually be something from the Lithium family of chemistries and identifying exactly which one can be a difficult task as there are so many variations in the cell chemistries, characteristics and technology maturity. There are three main architectures for lithium cells: cylindrical, prismatic and pouch. Cylindrical is a familiar cell format – for many years we have been used to primary and more recently secondary cells in sizes such as AAA, AA, C and D for home electronics and toys. A prismatic cell is a ‘flat’ format like that of a book but is enclosed in a hard casing. A pouch cell is another ‘flat’ format cell that has a foil enclosure rather than a hard
LEFT: Battery pack assembly for the Lotus Evora 414E Hybrid
LOTUS ENGINEERING