It's normal that the Dell 19.5V ac adapter's voltage fluctuating between 16 and 19.5V? The fluctuating: the starting voltage is 19.5V, and decreasing 0.2V per second to 16.5V, and the voltage jump 19.5V, then decreasing again to 16.5V.
While you can still use it, for now, it will start to have trouble powering the device and charging the battery at the same time. Were it me, I would get a new one now, rather than later. Just be sure to get a good OEM one and not aftermarket.