In this paper a novel method to determine the specific heat capacity of lithium-ion cells is proposed. The specific heat capacity is an important parameter for the thermal modelling of lithium-ion batteries and is not generally stated on cell datasheets or available from cell manufacturers. To determine the specific heat capacity can require the use of an expensive (>£100 k) calorimeter or the deconstruction of the cell whereas the method proposed by the authors in this paper uses common equipment found in most battery laboratories. The method is shown to work for cylindrical, prismatic and pouch cells, with capacities between 2.5 Ah and 10 Ah. The results are validated by determining the specific heat capacity of the cells with use of a calorimeter and a maximum error of 3.9% found. Thermal modelling of batteries is important to ensure cell temperatures are kept within specified limits. This is especially true at rates over 1C, such as the fast charging of electric vehicles, where more heat is generated than lower rate applications. The paper ends by demonstrating how the thermal model that underpins the authors' methodology can be used to model the surface temperature of the cells at C-rates greater than 1C.