One challenge of fast charging for electric vehicles is the potential degradation caused by high charge currents on the battery. This article focuses on the numerical optimization of fast charging protocols and on their impact on battery cycle life. An optimization problem is formulated to define the parameters of a multi-stage of constant current charging protocol. The problem is based on a strongly coupled electro-thermal model and is developed to achieve fast charging while taking aging into account in an implicit manner. The proposed method is used to define optimized protocols in different operating conditions in terms of ambient temperature, charging time, and charged capacity. Experimental aging tests are then conducted to investigate their impact on the cycle life of a lithium-ion cell. Optimized protocols are compared with fast charging reference protocols in similar operating conditions. The results show that the optimized protocols can reduce the charging time and/or the degradation compared to the reference protocols. This indicates that there exist opportunities for significantly higher currents to reduce battery charging time while still maintaining a long cycle life.