You can check the battery’s health/calibration level using the Power Statistics app. Check the “Capacity” field (this is calculating the ratio between the “Energy when full” and the “Energy (design)” fields). If it’s not 100%, try recalibrating your battery and seeing if that resets it to 100% or close to it. If not, then your battery is possibly starting to degrade or is degraded.
You could also use the upower command but I won’t cover that here.