[Resolved]  Exide Battery — Battery premature failure

I bought 38B20RMF type Exide Battery from Exide Dealer M/S Camac at 2E Camac Street, Kolkata-700 016 on 28.04.2006. The serial No. of the battery is 321631 and Mfg Code is 406. The battery is regularly (at monthly interval) brought in to the dealer for servicing. On all occations till recently the battery on checking the cell gravities during servicing were all found to be perfectly in order. It is only recently (about 10 days before) that one cell of the battery on checking was found to have rendered dead with a dark mark appearing on the upper surface of the cell. I want to know if this could be regarded as out come of natural ware and tear of running the battery or the same could be branded as unusual for searching remedy?