After using "B" + Power Button to use Service Mode on my Latitude 5520, to perform a fan replacement (*), I was stuck in Service Mode, with no ability to use the machine on battery power.

(*) per Dell Service Manual 5520-sp15-sm-en-us.pdf

On a wild guess, I unplugged the battery from the motherboard and plugged it back in again (about a minute later), and Service Mode turned back off at the next power-up. Without that step, I could only use the machine with a charger plugged in.