Kawasaki Drill problematic battery?
The problem has something to do with the batteries becoming over-charged if left on the charger too long. I think the charge current put out by the charger after the battery has become fully charged is higher than the battery loses resulting in overcharging the battery. Maybe the charger is at fault in that the 'trickle' charge mode after the battery has completed charging is too high, or the batteries are too good in that they do not lose much charge, and can be overcharged while left in the trickle mode.
Their solution to the problem (whatever it is) is to give everyone a free slow charger and make you promise to cut the wire on the original charger. So instead of having a 1-hour charger, you end up with a 12-hour charger. Not an acceptable replacement as far as I am concerned.
I have the same set, don't use it any more, but do charge the batteries every 6 months just in case it is ever needed (if a neighbor wants to borrow a drill). I still use the fast charger; the slow charger just doesn't cut it. Provided you remember to remove the battery from the charger after it is fully charged, you will be OK. If you think you might forget sometime, buy an inexpensive timer which cuts the power after an hour or so. Remember, if you do continue to use the fast charger, you do so at your own risk. Once the company has sent you the replacement charger they are absolved of any problem that might come up.