Slide a large piece of heat shrink tubing over the main cable, and smaller tubes over each individual wire.
Many Dell chargers use a center-negative topology. That means the inside of the barrel is negative, and the outside is positive. However, this is not universal across all models. The red wire is always the main positive voltage, and the black wire is the return path (ground). Always verify with a multimeter before soldering. 3 Wire Dell Laptop Charger Wire Diagram
If you’ve cut open a Dell DC cable (typically for a DIY repair or mod), you’ll likely see these three colors: White Wire (Positive / V+): Slide a large piece of heat shrink tubing