Why is my Mac Charging Slowly? Monitoring USB-C Power Delivery (PD)
Your Mac's charging speed depends on a negotiation between the charger, the cable, and the laptop. Here's how USB-C Power Delivery (PD) actually works, and how to see the profiles your charger advertises — no USB-C multimeter required.
If your MacBook seems to charge slowly — or only reports “Not Charging” when plugged in — the cause is almost always a Power Delivery (PD) negotiation mismatch. USB-C charging isn’t a simple “plug it in and go” affair. The charger advertises a menu of voltage/current pairs (called PD profiles), the laptop picks the best match it supports, and that picked profile determines the wattage you actually receive.
This guide explains what PD profiles are, why mismatches happen, and how to see exactly what your charger is advertising on macOS — without needing a standalone USB-C multimeter.
Platform note: The PD profile monitoring discussed here is a macOS-only feature of USB Connection Information. The Windows version reports USB speed and hardware specs but not charger PD profiles (a limitation of what Windows exposes to user-space apps).
How USB-C Power Delivery works
When you plug a USB-C charger into a Mac, a tiny handshake happens over the cable’s CC pin within milliseconds:
- The charger advertises its profiles. A profile is a voltage/current pair — for example, 9V at 3A (27W), 15V at 3A (45W), 20V at 5A (100W). A single charger typically advertises 3–6 profiles.
- The Mac picks the highest-wattage profile it supports. Modern MacBook Pros can accept up to 140W (28V / 5A) on USB-C Power Delivery 3.1.
- Power flows at the negotiated profile until something changes (the laptop gets close to full, thermal throttling kicks in, or you unplug).
If the charger doesn’t advertise a profile that matches what the Mac wants, the Mac picks the best it can — which may be far lower than the advertised headline wattage.
The common Mac charging problems
Problem 1: “96W charger” delivering only 60W
A lot of third-party “96W” chargers advertise only 20V / 3A (60W) in their PD profile list, even though their headline spec is 96W. The extra wattage only comes out of a second port, or only at specific voltages your Mac doesn’t request.
How to diagnose: Plug in the charger and look at the advertised profiles in USB Connection Information. If the highest profile is 20V / 3A, the “96W” is marketing — the Mac will never pull more than 60W from it.
Problem 2: Cable is the bottleneck
A cable rated for 60W (the default for most USB-C cables in the box) can’t carry a 100W profile regardless of what the charger advertises. The eMarker chip in a 100W+ cable tells the charger it’s safe to deliver the higher current. Without that chip, the charger caps at 3A.
How to diagnose: Swap in a known 100W+ cable (Apple’s USB-C Charge Cable, or any cable explicitly rated 100W / 240W). If the wattage jumps, it was the cable.
Problem 3: Dynamic Power Supply (DPS) chargers thermally derating
A Dynamic Power Supply port (DPS, defined in §3.3 of the USB Power Delivery Specification) is allowed to advertise a higher “Port Maximum PDP” than it can sustain indefinitely. The spec guarantees a DPS port will hold that maximum for at least tDpsColdStart (15 minutes) from a cold start at ~25°C ambient — after which the charger may send a new Source Capabilities message with a lower “Port Guaranteed PDP” to protect itself thermally.
This is why some chargers start out fast and then slow down after 10–20 minutes. It’s not broken; it’s a DPS port doing exactly what the spec permits.
How to diagnose: Plug in your charger from cold and note the advertised PD profile list in USB Connection Information. Charge under load for 15–20 minutes and look again. If the top profile dropped (e.g., 140W → 100W), you’re seeing thermal derating from a DPS port. Let the charger cool and it should re-advertise its max PDP.
Problem 4: Multi-port chargers sharing their power budget
Separate from DPS, many GaN chargers are shared-capability ports: a 100W brick might advertise 100W on Port 1 when nothing else is plugged in, then drop Port 1 to 60W the moment you add a phone to Port 2. The profile list updates live as you plug and unplug. The USB PD spec explicitly allows Source Capabilities updates when “the updated Source Capabilities are due to sharing power with another Port” (§3.3).
How to diagnose: Watch the PD profiles in USB Connection Information while you plug and unplug a second device on the same brick. If the laptop’s profile list shrinks when a second device arrives, that’s shared-capability behavior — and it means your laptop charges slower when your phone is on the same charger.
Reading PD profiles in USB Connection Information
Plug in a USB-C charger on your Mac and open the menu bar popup. You’ll see a block like:
Apple 96W USB-C Power Adapter
5V / 3A (15W)
9V / 3A (27W)
15V / 3A (45W)
20V / 4.7A (94W)
Each line is one profile the charger is advertising. The wattage in parentheses is simply V × A — useful for quick comparison.
If you don’t see a profile close to your Mac’s rated charging wattage, that’s your ceiling. No software or firmware trick will get you more.
Mac charging wattage reference
| Mac model | Max accepted PD wattage |
|---|---|
| MacBook Air (M-series) | 30–35W (M1/M2), 67W with fast-charge cable (M3) |
| MacBook Pro 13" / 14" (M1/M2/M3 base) | 67W |
| MacBook Pro 14" (Pro/Max) | 96W |
| MacBook Pro 16" | 140W (USB PD 3.1, requires MagSafe or a 140W PD 3.1 charger) |
If your charger’s profiles top out below this number, you’re charging slower than you could be — even if the charger’s label says otherwise.
Practical checklist
- Check the charger’s advertised profiles, not the label wattage.
- Use a cable rated for the target wattage (100W+ for MacBook Pro 14/16").
- Check for multi-port power sharing if multiple devices are on one brick.
- Let a DPS charger cool down if top-end wattage dropped after sustained use.
- Try the charger with only the Mac plugged in to isolate the profile.
- Verify in Battery preferences that macOS reports the expected “Power Source” wattage.
Sources & further reading
- USB Power Delivery Specification (usb.org) — primary specification. DPS is defined in §3.3; shared-capability ports in §3.2.4–3.2.5.
Related guides
- How to check USB-C cable speed
- Thunderbolt 4 vs USB4: Identifying Your Connection Type
- USB Reference Guide — versions, connectors, and power
For real-time visibility into other kinds of connections your Mac makes, see the rest of the Connection Information suite — audio, displays, and network.