Do USB-powered devices degrade MacBook’s battery while on the charger?

[kazkx]

If I leave USB-powered devices plugged into my MacBook while it’s charging, will it degrade the battery? What I’m trying to figure out is if the USB devices will drain the battery and then the charger recharges the MacBook repeatedly, or do the USB devices consume power directly from the MacBook charger without going through the MacBook’s battery?


My use case is charging my phone or using a high-performance drive in a thunderbolt enclosure. Using these when the MacBook is running on battery, these can significantly drain the battery life. If the MacBook is on the charger, will it mitigate the long-term wear on the MacBook’s battery? Is there anywhere I can find documentation on this?


If this is a potential problem, I think that maybe using a docking station that powers with it’s own cable to AC like a CalDigit. To plug in USB-powered devices there might be a safer way to use them than to use them directly from the MacBook.

 

[Mr. Awesome]

I don't know for certain, but I would assume that if your Mac says "Power Source: Power Adapter", that anything being powered by the USB ports is coming from the power adapter as well.

 

[Juicy Box]

I don’t think it will unless the power source cannot keep up with the demand.

 

[Fishrrman]

I'm going to GUESS that if you have USB devices (that require bus power) attached, it will slow down the charging a little, as there is "an increased draw" upon the battery even as it charges.

That is, the USB devices will still be drawing power directly "from the battery".
Not "from the charger".
I could be wrong.
Perhaps someone "in the know" will clarify this.

 

[dmccloud]

I'm going to GUESS that if you have USB devices (that require bus power) attached, it will slow down the charging a little, as there is "an increased draw" upon the battery even as it charges.

That is, the USB devices will still be drawing power directly "from the battery".
Not "from the charger".
I could be wrong.
Perhaps someone "in the know" will clarify this.

Per the USB spec itself, the ports and connected devices should be supplied with power by whatever is powering the system as a whole. In the case of a desktop (e.g., iMac or a Windows-based desktop), it would always pull from the power supply. For laptops, the USB ports would draw from whatever the power source is at that time. So when connected to the AC adapter, the USB ports are drawing power from there instead of the battery.