A nicad is normally made up of 19,20 or 21 individual cells. When it is competely flattened some cells will reach zero volts before others, and then go past zero to reverse polarity as the current continues to be drawn from the battery. Although some aircraft charging systems are becoming very sophisticated, they cannot normally account for the "reverse polarity" cells and continue to charge. These cells will then get hot, very hot, to the point where a thermal runaway could occur, and the battery could explode with devastating effect. (I've heard of the nose being blown off a Bell 206!).
Cells will vent during a charge and the battery shop will remove the vent caps to ensure no electrolyte spillage. They will control the current and check individual cell voltages and also check the electrolyte levels prior to returning the battery.
Basically, the battery shop will monitor every aspect of the charge and make sure you have a serviceable battery, not a bomb!
