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I just fixed my 18V NiCad lawn trimmer battery using Paul's method. Easy way to think of this is that there is a fine wire jumper connected across the positive and negative terminals of your bad battery. The charger current tries to push electrons onto the negative terminal of your battery to charge it but the electrons just take the easiest path through the jumper and nothing takes place in the battery. When you "boost" the shorted battery (exactly as you boost a dead car battery) you cause lots of current to travel through the little jumper and it heats up and melts. Now when you hook up the charger the electrons have to enter the battery and cause the chemical reaction that stores them there.