NOVELTY - Storage battery system includes one or more, e.g. sodium nickel chloride, storage batteries and an engine cooling system to cool the vehicle engine and to regulate temperature of the batteries. Each battery generates an internal battery operating temperature between 270 and 350 degreesC. Engine cooling system includes a processor, a database that stores a thermal model for the batteries, and a temperature sensor monitoring a battery temperature. The processor selects a thermal model based on a parameter associated with the battery and compares this with the temperature sensor reading.    USE - For storage and transfer of electrical energy e.g. dynamic braking energy or excess prime mover power for a large hybrid energy off-highway vehicle e.g. locomotive, large excavator, dump truck and other large, off-highway vehicles driven by electric traction motors.    ADVANTAGE - The high temperature battery used by the system requires no pre-cooling and no deration even under the hottest ambient air temperature conditions. After extensive shutdown due to unscheduled events e.g. extensive maintenance, the batteries can be heated using external means e.g. external dc/ac power with appropriate control using the battery processor. During long periods of battery inactivity, the battery temperature may fall close to its internal electrolyte freezing temperature. The battery processor makes a decision whether to use the battery internal energy to heat the battery or to allow the battery to freeze based on acquired variables e.g. temperature sensors, or operator inputted information, e.g. time of shutdown. If the locomotive will not operate earlier than a specified time, such as 7 days, the battery processor allows the battery to freeze. If the locomotive is expected to operate earlier than a specified time, the battery processor enables the additional energy source to heat the batteries to keep them at operating temperature.    DESCRIPTION OF DRAWING(S) - The drawing shows a hybrid energy propulsion system.