The battery meter is wrong. The battery is failing. Remove the battery and try using the adapter alone. If that works use it that way until you can buy a new computer. Very hard to find genuine new Dell replacement batteries from Dell or Dell Parts now. Beware of counterfeit batteries on the internet even from Marketplaces.  

https://www.dell.com/support/article/en-us/sln128667/dell-laptop-battery-frequently-asked-questions?lang=en

I'm using a DELL Latitude 7470 and also experiencing similar issues. Very annoying.

I noticed one phenomenon accompanying the onset of the auto and sudden shut down: even when the laptop is connected with power adapter, the battery percentage was decreasing, which is so strange. After such sudden and unexpected shut down, the laptop cannot be turned on in a normal way, the battery cannot be charged. Usually I have to uncover the back cover, and unplug the battery connection, then put it back, thus the laptop can work in normal for some time, maybe a few weeks.

Check Windows system logs for "red x" and review for "Thermal Event" if found your system is over heating. Have had this problem with a recent E7270, the fix is mainboard replacement if you have not noticing any thing blocking the cooling vents.

This is what happening today: connected to power adapter but the battery is decreasing. when it to a critical level, auto shutdown will happen:

How to paste a photo here?