USB-C fast charging often feels mysterious. You plug in a charger, sometimes you get rapid charging, and sometimes it is slow or unstable. The reason is not just wattage. For USB-C Power Delivery (USB-C PD), charging speed depends on a short negotiation between the charger and the device, with the cable and port acting as critical gatekeepers.

USB-C PD is a fast charging standard that allows a charger and a device to agree on how much power should be delivered. Instead of pushing maximum power all the time, PD uses communication to select a safe, compatible power level. This is why a charger with a high watt rating does not automatically force your phone to charge at that wattage.

When you connect a USB-C charger, the charger usually begins at a basic safe level. Then the device requests a higher power profile that it can accept. If both sides support the same PD profiles, power ramps up. If not, charging falls back to a lower level.

This negotiation is designed to protect the battery and prevent unstable or unsafe power delivery. It also explains why the same charger can behave differently with different phones.

In USB-C to USB-C charging, the cable is not always just a wire. Higher-power PD charging often relies on cable capability recognition. If the cable cannot reliably support the required current, the system may limit power to protect against heat and voltage drop.

Some higher-current USB-C cables include an E-Marker identifier, which helps the charger and device understand what the cable can handle. When cable capability is unclear, PD negotiation may choose a lower power mode even if the charger and phone are both capable.

Even if the charger supports 65W or 100W, your phone may only accept a much lower level. The phone’s battery management system controls power intake based on battery temperature, state of charge, and long-term aging protection. Charging speed often slows down at higher battery percentages because the phone is actively protecting battery health.

PD charging can fluctuate due to heat, cable resistance, and port condition. If the connector is loose, dirty, or worn, voltage drop can increase and the system may reduce power to stay stable. In some cases, a phone may repeatedly renegotiate power when temperature changes, which can feel like charging cuts in and out.

When the charger, cable, and device are well matched, USB-C PD fast charging becomes predictable and stable. Understanding the negotiation process helps you troubleshoot speed issues and choose accessories that deliver consistent real-world performance.