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.
Can I use the laptop with this adapter still? This was the biggest question in my mind. For answer to this question I schedule geek squad appointment. There they suggested to replace the cable that is connected to the power socket. After doing so the problem was solved. Now it is working without any issue.