A method of operating a drive train for a hybrid electric vehicle, and a drive train, is disclosed. The drive train includes an internal combustion engine, a first electrical machine and electrical energy storage. The engine is coupled to drive the first electrical machine as a generator and the first electrical machine connected to supply electrical energy to the electrical energy storage. The electrical energy storage is arranged for supplying electrical energy to at least a second electrical machine for driving wheels of a hybrid electric vehicle. The method includes sensing a parameter indicating the voltage across the electrical energy storage and, in response to this sensed parameter, controlling the internal combustion engine and/or the first electrical machine such that the first electrical machine gives rise to a voltage output of the first electrical machine such that the electrical energy storage is charged without the need for power electronics.