Incorrect reading on laptop battery

Since last 3-4 days, my laptop's (Dell XPS 9370) battery is behaving quite erratically. I use my laptop mostly plugged in during the day, so the battery is charged to 100% at the end of the day. If I take the charger out at any point, it lasts for a couple of hours at the least. But if I switch it off with 100% battery, and then power it on after an hour, then the laptop thinks the battery is in single digits (6-7%), and warns me that it's about to turn off soon (and it does). If, however, I manage to connect my charger before that happens, then it quickly charges to 100% before it switches of

Question by taninamdar on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.