Electric power storage is an important technology for all equipments of underwater vehicles however environmental pressure is high, the temperature is 5 degrees Celsius or less, and conditions are unsuitable for many chemical reactions in the deep sea. Battery capacity is mainly dependent on its mass; this means that the cruising range of underwater vehicles is proportional to the mass of the battery. To solve this problem, a high energy density battery and its enclosures are being developed. Concretely, batteries are enclosed with oil, to equate environmental pressure and to be insulated in seawater. This is called the oil compensated method, and is applied to various batteries. The oil-filled 29Ah Lithium-Ion battery for the vehicle was developed in 2010. And sea trial was carried out equipped autonomous underwater vehicle "MR-X1". The paper introduces Lithium-Ion secondary battery and result of sea trail.