The explosion on board the INS Sindhurakshak was possibly a result of the buildup of volatile hydrogen gas during a battery charging. Sources told India Today that the submarine had faced a similar explosion when she was docked in Visakhapatnam in February 2010 which killed one crew member.

The navy's Board of Inquiry in 2010 pinned the cause to a faulty battery valve that leaked hydrogen. The submarine was lightly manned at the time of the accident and later sent for a 2.5-year refit to Russia that year. It had returned to the naval dockyard on April 29 this year after the refit that cost approximately $80 million.Conventional submarines like the Sindhurakshak are powered by a combination of diesel generators and electric batteries. The 2300-ton Sindhurakshak has 500 batteries. These have to be 'over charged' once every few months during which process each cell is manually checked. The presence of a large crew early in the morning points to a supervised battery overcharge."The Kilo-class submarines do not have automatic monitoring systems which mean the overcharging is manually supervised," says a former Kilo-class submarine skipper. The built-up hydrogen during a battery charging is sucked out by two blowers. The performance of these blowers can be affected if a proper vacuum is not maintained in the exhaust pathway. If the vacuum as well as the exhaust pathway is not maintained, hydrogen settles in small pockets which can be triggered off by any small spark like a falling utility tool.Sindhurakshak is the ninth of a series of ten 'Sindhughosh' class submarines that were bought from the erstwhile Soviet Union beginning in 1985. India and China, with ten submarines each, are the world's largest operators of the Soviet-designed Kilo class submarines. Seven Indian Kilo-class submarines have been given mid-life refits in Russia. Refits of two other Kilo-class submarines, Sindhukirti and Sindhushastra, are underway at the naval dockyard in Vizag.