MacBook external display disconnects when charging via USB-C

Your issue happens because your 16-inch MacBook Pro needs 96–140W, but most monitors give only 60–65W, causing USB-C power renegotiation and display resets.

To fix it, you should:

✔ Charge MBP using original charger (best fix)

✔ Turn off optimized charging

✔ Use high-quality E-Marked 100W cable or Thunderbolt 4 cable

✔ Lower refresh rate or resolution

✔ Try other ports

✔ Update monitor firmware

✔ Disable monitor USB-C PD if possible

✔ Restart USB-C controller logic (full shutdown)

irolley wrote:

Thank you very much for that explanation (USB-C renegotiation).

However even on slow charge I don't expect my computer to have an excuse for not being able to maintain a display connection. This seriously seem like a major issue. At a minimum, macOS should display an error message explaining it can't connect properly and needs a 90W charger. But ideally I simply expect my computer to work and just charge slowly.

90W should be adequate for charging your MacBook Pro 16" or 14" laptops. I have a 2019 MacBook Pro 16" that came with a 96W charger but my OWC dock charges it just fine and it provides only 85W. I have experimented with lower power chargers in fact and even lower power delivery ones will work but the charging is a bit slower than with the 85W or 96W power supplies, or if under extreme load the battery might even lose a little charge for a short time but it recovers. But the glitches you see are indicative (I think) of a different problem.

I think Grant may be on to something with his concern about the cables. That Dell monitor comes with 1.8m cables, I think for a Mac you would be better off with less than 1m in length, see Grant's post. This is easy and relatively cheap to test.

Also, there was a post on LG's customer feedback area on their site that when charging a laptop, the USB hub fell back to USB-2 speeds. I don't know if that also happens with the Dell unit but that would indicate a compromise in functionality in the USB hub that could have negative implications for video use with Macs when also charging through the hub. LG acknowledged that limitation for the monitor you have.

Keep in mind these monitors are designed for PCs and might have "features" with Macs, like that one.

I use an OWC hub with my Mac and an external Dell display connects through that hub and the OWC hub also charges the laptop. With just 85W power delivery. No issues. That charging works well and the OWC hub also provides Thunderbolt speeds at the sometime in its ports that I use for external drives (backup, etc.).

========

USB hub doesn't work on full speed

romtje

5 MONTHS AGO

Overall very good monitor except one major flaw - USB hub works with USB 2.0 speeds if you connect (laptop) using USB C and use it for display connection. Also this is not mentioned anywhere - not in the specification, nor in the manual. I was going to use 2.5 Gbit ethernet adapter, but with USB 2.0 it's useless.

Response from LG Electronics:

5 MONTHS AGO

LG Digital Care Team

Thanks for sharing your experience, this model supports USB-C for both display and data, but due to bandwidth allocation when using DisplayPort, the downstream USB ports may default to 2.0 speeds in certain configurations. If high-speed data transfer is critical, you may want to connect it directly to your laptop or use a dedicated USB 3.0 hub to bypass the bottleneck. We’ll share this feedback with the team! ^Jose

Cables "shipped in the box" are notorious for being "lowest bidder" cables. They tend to be good enough to get you a picture, but not adequate at highest resolutions.

USB cables used for displays need to be 20 G bits/sec rated, and ONE meter or less in length.

HDMI cables have their own different specs for different resolutions.

if your display does not stay connected, that suggests your cable is not up to spec or is longer than the maximum ONE meter length.

what Make&model display?

any other items in the display cable? dock, adapter?

what model-year MacBook Pro?