KDDI Corp., Japan’s second-largest mobile-phone operator, will replace the battery packs made by Sony Corp on two million handsets over concerns of the device overheating.

According to reports, KDDI said in a statement that the replacement involves two handsets of Casio and one by Hitachi. Further, the company added that the first overheating problem was reported in January last year but no decision was made to replace the battery packs at that time because the incident occurred after the battery had been damaged. However after two customer complaints in 2010 followed by six this year, wherein the damaged batteries emitted smoke while being recharged, KDDI announced replacing the batteries free of charge stating that the batteries may heat and melt.

Industry analysts say that while this expense is not of much significance to a large organization like Sony, it does raise concerns regarding the reliability of its products. In 2006, Sony recalled as many as 9.6 million lithium-ion batteries, the largest recall in consumer-electronics industry, over concerns that the batteries could overheat and become fire hazards.

KDDI fell 3.4 percent closing at $6,950.98 in Tokyo, marking a third straight day of declines. Sony dropped 0.5 percent while Casio and Hitachi fell by 2.4 percent and 0.8 percent respectively.