The electrification of vehicles is seen nowadays as a promising way to decarbonize the personal transportation. The assessment of environmental impacts of electric batteries is usually case-specific due to the complex modelling of such systems which present a large variability in designs, in user behavior or in geographic use conditions. A typical example is the battery lifespan which is arbitrarily chosen in most cases, even though it has a decisive influence on lifecycle emissions. Computing the battery lifespan in addition to a Life Cycle Assessment (LCA) would enable to highlight new hotspots and new parameters to reduce the environmental impacts of batteries. This paper introduces a new approach, based on a LCA conducted with the open-source software Brightway and built on primary data collected from a complete disassembly of a commercial electric vehicle. An original functional unit has been proposed that better represents the service offered by the battery over its lifetime and a semi-empirical ageing model has been integrated to predict more precisely the battery lifespan depending on design parameters and the use conditions. This innovative methodology is easily parameterized and aims to compare several eco-design strategies.