This paper investigates heat transfer with phase change materials (PCMs) in passive thermal management of electric and hybrid electric vehicles where the PCM is integrated with a Li-ion cell. When higher current is extracted from the Li-ion cells, heat is generated due to the ohmic law. Therefore, it is vital to design a successful thermal management system (TMS) to prevent excessive temperature increase and temperature excursion in the battery pack. During the phase change process, PCMs absorb heat and create a cooling effect. In the discharging (solidification) process, stored heat is released and it creates a heating effect. The case considered in this paper includes the use of PCMs with different thicknesses around the cells. Despite the small peripheral surface of the prismatic cell, the orthotropic property of Li-ion cells improves the planar heat transfer and effectiveness of the PCM around the cell. A numerical study is conducted using a finite volume-based method. The results show that the maximum temperature and temperature excursion in the cell are reduced when PCM is employed. The PCM with 12 mm thickness decreases the temperature by 3.0 K. The corresponding value for thinner layers of 3mm, 6mm and 9 mm are then obtained as 2.8 K, 2.9 K and 3.0 K respectively. Furthermore, the effect of the PCM on the cell temperature is more pronounced when the cooling system is under transient conditions. When a 3 mm-thick PCM is employed for the Li-ion cell, the temperature distribution becomes about 10% more uniform which is an important result in thermal management systems in electric vehicles. (c) 2014 Elsevier Ltd. All rights reserved.