This presentation will study the work done on the new hybrid powertrain of the K-Challenge Lab fast vessel, to optimize battery size and weight while keeping the same performance and increasing battery life. The battery requirements for the hybrid powertrain are 60kWh embedded energy, 180kW (3C) continuous discharge power, and marine type approval. Classical batteries with marine type approval present a maximum continuous current of 0.5C and peaks up to 1C. Wattalps offered a battery that met all the requirements, with some margins in power, leading to 70% weight savings and improved vessel efficiency.