Laptop Battery Replacement?

Have a Dell XPS 15 9520.  Had this laptop for over 2.5 years and am still using the same battery.  My laptop is plugged in almost always 100%.  It is connected to 2 external monitors.  Not only that, for quite some time, I have left my laptop.  I just lock it at night.  I have bitlocker as well.

The only time when my laptop is not plugged in is if I power off my laptop which is rare or if during the night, my UPS powers off by itself... the battery is bad but I'm using the UPS as a surge protector... so when in the morning, either my laptop is very low battery or it shut off by itself because the power went out of the UPS.