CAUTION!! 16.5 at idle is too high. Could me much much higher at higher RPMs if the alternator/regulator is shorted "full field". . (14.5-14.7v max at any rpm is normal, I have seen 15v max but not crazy about it. )
Use safety glasses or face mask. Check the water in your battery , if you can see the top of the lead plates you could have a very dangerous situation -ventilate it well before you do anything (blowing compressed air works, don't use anything electrical) . The water should be covering the lead plates but below the plastic lip inside the holes. (Always have plenty of fresh water or water and bakeing soda mix close by to neutralrize the acid )
Overcharging , due to a stuck or poorly calibrated voltage regulator WILL. Boil your battery water dry and turn your battery into a 40lb lead and plastic bomb or overheat it if it's a gel cell. Disconnecting the battery may be the only spark needed to ignite it. Ventilate first and top it off with distilled water before you do anything. Maintanance free batteries still need water in them, you may have to work at it to get the caps off or they may be glued closed on some (not much you can do in that case).
I have seen batteries blow up first hand a few times. (Pushed a huge outward dent in the hoods of a 70's Volvo and a 68 Plym Satalite. Something you never forget. I aslo know a guy that had his lip torn off and a face full of acid (almost blinded him).
Be careful. Better safe than sorry.