Why Does My Laptop Show “Slow Charging” When Powered Through a USB Hub

If your laptop displays a "Slow Charging" notification while connected through a USB Hub, it does not necessarily indicate a problem with the hub or power adapter. This article explains how USB Power Delivery (PD) works, why the hub reserves some charging power, and how to maximize charging performance.

Key Takeaways

• USB Hubs reserve part of the incoming PD power to support their own operation and connected devices.
• The charging power available to your laptop is the remaining power after the hub's reserved power is deducted.
• A higher-wattage power adapter can provide more charging power to the laptop.
• The reserved power is a predefined power budget and is not the same as the hub's actual power consumption.
• A "Slow Charging" notification does not necessarily indicate a fault with the USB Hub.

Key Concepts

Upstream Device

The upstream device refers to the device connected to the USB Hub through the USB-C plug (upstream port), such as a laptop, tablet, or phone.

Downstream Device

Downstream device refers to all devices connected to the USB Hub through its other interfaces, excluding the USB-C plug. These include, but are not limited to, the USB-C port, USB-A ports, SD card slot, Micro SD card slot, RJ45 (Ethernet) port, HDMI port, and other available interfaces on the hub.

Charging Protocol

A charging protocol defines the voltage and current rules used during power delivery. Common charging protocols include PD 3.0 and PD 3.1.

Negotiated Power

Negotiated power refers to the power parameters (voltage and current combinations) agreed upon between the power adapter and the USB Hub or connected device through a charging protocol such as Power Delivery (PD).

It represents the maximum available power capability, not the actual power consumption.

Actual Input Power

Actual input power refers to the real-time power drawn by the USB Hub after power negotiation is completed.

The actual input power is typically less than or equal to the negotiated power, depending on system load and connected devices.

Pre-deducted Power

Pre-deducted power refers to the portion of power the USB Hub reserves from the incoming Power Delivery (PD) input. The remaining power is then available to the upstream device.

This reserved power does not equal the USB Hub's actual power consumption; it is a predefined power budget set aside to ensure stable operation of the hub and its downstream interfaces.

No-load Power Consumption

No-load power consumption refers to the power the USB Hub consumes when no downstream devices are connected.

Explanation

When a USB Hub is connected to a power adapter through its PD (Power Delivery) port, the input power is determined by the negotiated power between the power adapter and the USB Hub. TP-Link USB Hubs support the PD 3.0 protocol, with supported power profiles including 5V@3A, 9V@3A, 12V@3A, 15V@3A, and 20V@5A, allowing for a maximum input power of up to 100W.

Because the USB Hub must supply power to its downstream interfaces, a portion of the negotiated input power is pre-deducted to ensure stable operation of the hub and any connected downstream devices. This pre-deduction is not affected by whether downstream devices are currently connected, but depends solely on the negotiated power. The remaining portion of the input power, after the pre-deducted power is reserved, is available for charging the upstream device (such as a laptop). This is why the upstream device may display notifications such as “Slow Charging,” even when using the original power adapter. For detailed values of the pre-deducted power, please refer to the table below.

The pre-deducted power is not the actual power consumed by the USB Hub. In a no-load condition (with no downstream devices connected), the hub itself typically consumes less than 1W of power.

To provide more power to the upstream device, use a higher-rated power adapter. When using a power adapter rated above 100W, the USB Hub can supply up to approximately 98W to the upstream device after accounting for reserved power. This 98W represents the maximum achievable power after power reservation, not a guaranteed charging power. The actual charging power may vary depending on system conditions and connected devices.

Q&A

Q1: Can I Use a USB Hub to Charge Other Devices, Such as Phones or Tablets?

Yes, the Hub can charge various devices through its upstream port, but charging power depends on the power reserved for the Hub and connected downstream devices.

Q2: Why Does My Device Charge at Much Less Than the Rated Power of a High-wattage Adapter?

Please check the label on your adapter. For some chargers, especially multi-port chargers, the total rated power may be high, but the maximum power available per port may be much lower.