Dell E7470 sudden shutdown confusion. I'm in need of expert help - please!

My Latitude E7470 suddenly gives a warning about very low battery then shuts down. BUT, I always run on AC power! The battery symbol even shows 97% when powered up again!

Having an internally fitted battery, it's not easy to disconnect, but would it be better to remove it completely and only use AC power?

At other times the laptop shuts down with only this warning: 'Shutting down' - why is this happening?

I've checked the CPU temperatures with 'Core Temp' and they never get above 47, but typically below 40. I can't be sure of the GPU temperatures though.

I'd very much welcome your thoughts & ideas - thank you.

Niloc1017