An account is given of the lithium-ion (Li-ion) battery pack used in the Northern Territory University's solar car, Fuji Xerox Desert Rose, which competed in the 1999 World Solar Challenge (WSC). The reasons for the choice of Li-ion batteries over silver-zinc batteries are outlined, and the construction techniques used, the management of the batteries, and the battery protection boards are described. Data from both pre-race trialling and race telemetry, and an analysis of both the coulombic and the energy efficiencies of the battery are presented. It is concluded that Li-ion batteries show a real advantage over other commercially available batteries for traction applications of this kind.