It shouldn't read 12 volts after any sort of load condition, (thats why i said if it 'actually reads 12v' sorry if i wasn't clear), if a cell is dead. Each cell produces (ideally) 2.1 volts, no less than 2. To have a bad cell it'd have to simultaneously have a bad cell, and a single severely (over twice the voltage) sulfated cell, bringing the surface charge back up to twelve, even with a dead cell. That is to say thats just a surface charge, under any kind of load the sulfated cell would drop back to almost nothing, leaving you with a battery with 8.4 or 9 volts. A weak battery like he describes that reads an actual voltage of around 12 volts is almost always medium to heavy sulphation. Shorted cells aren't very common anyway, not to mention for such a young battery (10 months isn't long for a modern lead acid cell battery).