Lithium batteries are increasingly being considered for installation as power sources in electric and hybrid vehicles, because of their high specific energy and power. To effectively size the vehicle Rechargeable Energy Storage System, it is very important to be able to mathematically model their behaviour. Battery modelling is also very useful for on-line management of electric and hybrid vehicles. This paper presents a dynamic model of lithium batteries based on experimental tests on high power Lithium-polymer models. The results can be adapted, with suitable parameter evaluation, to other lithium batteries as well.