Charging a MacBook on the Wrong Side Can Significantly Lower Its Performance

Apparently, there’s a right way and a wrong way to charge a modern MacBook, and the wrong way can have significant performance consequences. It’s not clear exactly which MacBooks suffer from this flaw or how widespread the problem is — multiple different users on StackExchange refer to the issue hitting the new 16-inch MacBook as well as laptops from 2017.

Here’s how the original StackExchange author described the problem:

Image by StackExchange

Kernel_task is a process macOS uses to schedule no_ops — periods of time when the CPU is processing null tasks (which is to say, deliberately processing nothing). This reduces the amount of CPU performance available for other tasks, thereby reducing temperature. As the author notes, this can leave the system useless for doing anything else.

According to the chosen answer, the solution is not to charge your MacBook off the left-hand port if you’re using it for video. Attempting to use both Thunderbolt ports at the same time sends the thermal sensors for the TB ports skyrocketing. After a few moments, the CPU begins to throttle sharply as a way of reducing overall power consumption.

Further testing shows that plugging two devices in on the right side may send temperatures soaring, but apparently at least some MacBook laptops don’t respond in the same way. The selected correct answer provides graphs and charts demonstrating how the laptop throttles sharply when two peripherals are plugged in on the left side simultaneously.

The high power consumption of the Thunderbolt ports doesn’t appear to be driven by the actual bandwidth consumption of the connected devices; the author tested a USB-C hub with keyboard and mouse + power attached on one port and a USB-C HDMI 2.0 adapter in the other. According to his testing, simply having a device plugged into a TB port is enough to raise temperatures by about 10 degrees (even if the TB port isn’t used to provide power). Using both TB ports raises temperatures by about 15 degrees. Presumably, this is when the kernel_task function kicks in, to keep the overall temperature within a reasonable range.

It is not clear which laptop generations have this flaw, or how many systems it impacts. It’s a problem, however, that shouldn’t exist at all. Under no circumstances is it acceptable for a system to throttle to the point of unusability simply because the customer used two ports on the same side of the laptop. There’s nothing in Apple’s support documents about needing to drive multiple monitors off different ports. The MBP 13″ laptop manual states only that the MacBook Pro “may become very warm during normal use.” It says nothing about needing to use ports in a specific order to avoid overheating the TB controller(s).

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