A hybrid electric vehicle drive system comprising a combustion engine, an electric motor and at least one nickel-metal hydride battery module forming a power source for providing electric power to the electric motor, the at least one nickel-metal hydride battery module having a peak power density in relation to energy density as defined by: P>1,375−15E where P is the peak power density as measured in Watts/kilogram and E is the energy density as measured in Watt-hours/kilogram.