As the automotive industry goes through developing a new generation of fully electric or hybrid vehicles with a native electric platform, electrification of older generations of ICE cars as a retro-modification method might also be appealing to certain costumers, as it is not only more environmentally sustainable in terms of reutilizing the older vehicles and avoiding manufacturing, scrappage and new-car development cost, but it is also a more cost-effective solution for popularizing the electric mobility. The aim of this work is to develop a modular lithium-ion battery pack for an electrified classic Fiat model, based on the customer requests while applying the industry standards on the overall architecture of the pack as well as verifying the safety aspect of the design.