Why does my phone keep shutting itself off/not charging?

[Windows Central Question]

My Lumia 950 keeps either shutting off in an instant, no warning, black screen like I've taken the battery out. Or it says goodbye and shuts down. The battery has plenty of life? I'm also finding after I unplug the phone after a night of charge (or any charge) the battery can drop from 100% whilst plugged in to 40% the instant it's disconnected. Sometimes it'll drop from there, or a minute later says it's out of battery?
I've soft reset a number of times & hard reset once still no change, any help appreciated.

 

[Hotvak123]

battery need to be replaced, my phone did the same thing now it will not take a charge. I just ordered a battery for it.