Get this... Say your cheap 18V cordless drill has one bad NiCd cell out of 15 in the pack...

Say, for whatever reason, you just can't revive it, but all the other cells are fine-enough...

What-say you just cut that cell out of the pack, tie the surrounding cells together for 16.8V instead of 18. Now add two diodes in series at the charger's output. 0.6-0.7V each makes for 1.2V-1.4V which is darn near perfect. 

Now you shouldn't have to worry about that cheap charger pushing in too much voltage...

Big friggin' whoop, your 18V drill is running off 16.8 now. Should Be Fine. At Least It Works Again!