A battery unit is the main power source for under water vehicles such as torpedoes launched against enemy targets. Generally,magnesiumsea water activated batteries are employed in these vehicles. The advent of lithiumion battery technology offers a great promise to develop a suitable battery unit for these vehicles. However, themajor drawback of the lithiumbatteries is their thermal runaway characteristic.Aclose review of literature reveals that phase changematerials (PCM) are used inmany applications to contain temperature within limits. In this regard, investigations have been carried out experimentally to study the suitability of different PCMs for a lithium ion polymer (LiPo) battery obtained from trade. Three PCMs viz., OM35, OM37 and OM46 have been used in the present investigations. Preliminary experiments have been carried out to test whether they behave accordingly or not. The LiPo cell was casted on both sides with computed amounts of different PCMs and the temperature variation of the entire cell, its anode and cathode were studied at different discharge rates. It was found that the use of PCMwill definitely hold the temperature within the limits during the mission time.