For what it's worth, I've been having this problem for a few days and came across this Reddit thread today:
https://www.reddit.com/r/Dell/comments/4zka9r/xps_15_9550_bios_1213_battery_plugged_in_not/?st=iskhbtf5&sh=4306d675
I tried the suggestion to roll back your BIOS to 1.1.19, and it...