The external force applied on a li-ion cell when assembling it in a battery pack manifests as an important variable to consider in its design. This study investigates the influence of external pressure on automotive and industrial applications oriented high-capacity pouch and prismatic NMC/G lithium-ion cells in the behaviour and lifetime of the battery. Three different mechanical boundary conditions have been defined and its effect on the ageing of the cells is analysed in the same current and temperature conditions, having a big impact in the lifespan. Electrochemical Impedance Spectroscopy (EIS) has been used on the prismatic cells to understand the ageing mechanism that has decreased the capacity of the cell, showing big differences between the three cases. Impedance has been also measured with different force levels, showing also its effect on the cell’s efficiency. This study shows how mechanical pressure is a critical parameter to consider in the optimization of a battery pack design or in the prediction of its behaviour.