A thermal management system for a high density power source is disclosed. The system includes a housing with an interior divided into first and second compartments. The first compartment is configured and adapted to house at least one electrical battery and the second compartment defines a coolant reservoir. A fluid release member connects the first and second compartments. Upon the first compartment reaching a temperature in excess of a predetermined limit, the fluid release member releases coolant form the second compartment into the first compartment to cool the at least one battery within the first compartment.