A quick sketch for you. Essentially each cell is 1.2 volts and you need to get to the positive (+ve) and negative (-ve) terminals. By making a hole in the side of the battery you get access to the -ve terminal of that cell which is also the +ve terminal of the cell below it. If you make a hole on all the cells you will be able to measure the voltage on any cell and determine which cell is not working properly.
