Battery Charge Reporting in Windows
When the battery charge is fully charged to 100%, the Charge Field Effect Transistor (FET) will stop charging the battery. Afterwards, the battery will naturally discharge during use. When the battery charge drops to 95%, the Charge FET will begin to apply a trickle charge to the battery and again charge the battery to 100%. This is working as designed and is intended to extend the physical life of the battery.
Battery Charge Reporting in System Setup
If you enter the System Setup utility (BIOS), the battery charge should be reported at 100%. If it is not, charge the battery until it reaches 100% in the BIOS.
For information on entering the System Setup (BIOS), refer to Dell Knowledge Base Article: "What the BIOS Is and How to Access it on a Dell Computer" Article ID: SLN86546.