This is the main power line that carries the voltage (typically 19.5V) to power the laptop and charge the battery.
While colors can vary slightly by manufacturer, the standard configuration for many Dell DC output cables is as follows: Wire Color Description Plug Location Ground (-) Negative return path for power. Outer barrel sleeve Positive (+) Carries the 19.5V DC power. Inner barrel sleeve (or Green) Sends a 1-Wire signal to identify the charger. Center needle pin Connector Pinout Details
Plug the charger into the wall. Use your multimeter probes (red probe on one wire, black probe on another).
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