I bought a used drill, and cordless screwdriver from a fella online, both dewalt, I already had a charger for them so when i put the battery from one of the tools in the charger and plugged it in, it started making a high pitched noise…??? The other battery did NOT do the same. Anybody know what the issue is???
I’m going to guess one of the cells is bad - either won’t take the charge or shorted across. What you are hearing is the voltage regulator in the charger over currenting the capacitors. It should have also starting blinking the red light indicating a fault.
It might have been within tollerance and it kept trying to charge it - but I bet it would fault out give some time.
I would chuck that battery to a recycle place.