Almost certainly you will find if you take the battery pacj apart that a few of the cells have dies.There are various schemes for "revitalising" them but IMHO they don't work.After MUCH frustration I bought 2 x 12 volt lead acid battery 20MAh for about £4 and replace the battery pack with a wired cord to connect to the batteries. Works better than before. I can charge from a normal car battery charger & the charge lasts MUCH longer than the NmiH did even when new.
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Yes, without a proper tag welder, its very hard to rebuild a pack at home. There are companies that will do it for you though. Steve
Type "nicad battery tool hack" in the google search box on the top right of the page, then click graphical search on the page that opens. There you will see a heap of helpful instructables.