If I use a non-powered USB hub, the iPad Air or iPad Pro could be losing power over a few hours, because the charging power is not enough.

However, if I get a powered USB hub, would it be an issue, say, if the USB port on the MacBook, MacBook Air, iMac is at 5.2V, while the powered USB hub happens to be 5.3V, in which case the current could be going back into the Mac instead of out of it?

This may be more of an issue if the iPad Air and iPad Pro is unplugged, so the current has no where else to go.

  • 2
    I've used powered USB hubs with all my Apple devices as applicable and have never had any issues. Nov 30 '20 at 0:42
  • Yea, been doing that since 1998, I have not had any issues.
    – ErniePC12
    Nov 30 '20 at 1:51
  • It's not a garden hose, power does not just flow back, it is controlled by the electronics in all connected devices. And those connected devices only charge when the proper, set, conditions are present. Which they are not in the situation you describe, the power flows from the hub to the connected devices, never the other way. Dec 1 '20 at 14:40
  • somehow I am allergic to the argument like "it is not a garden hose, power does not just flow back" because it is easy to lead people to think "Oh yeah! It is not a garden hose, therefore what he said is correct." What was described in college physics was that the voltage is electromotive force. When force is present, then it gives the cause to push something in a direction. Whether there is electronics to prevent such things from happening, may it vary from design to design, and cannot be guaranteed. Dec 2 '20 at 1:28
  • So to prevent current from flowing back from the USB hub back to the Mac, maybe the Mac that was designed since 2015 has to feature, but before that year, there was no such feature (something like that), and current just flow back and since it is supposed to be tiny difference of voltage, maybe it didn't matter. Whether the USB hub has such a feature, maybe it depends on whether it is a well designed hub or not. They are connected, and if you say a 2mm thick rubber band is not going to affect a 1mm rubber band, it has got to be some good reason. Dec 2 '20 at 1:28

There's an isolation on the power connectors to prevent power from the hub flowing into the USB-A port on a computer. People have been plugging powered USB hubs into laptops for something like two decades, if this was an issue someone would have seen it by now and fixed it.

  • the reason is, there are so many third party makers of these powered USB hubs. So reviews on Amazon might say, the power is almost useless, and the power supplied might be at 4.8V or even lower voltage. I checked some adapter and was very surprised what they say 5V can be 3.7V or 6.8V, so they are not that accurate. And these powered USB hubs can be from FUON and the next one you see, looks very much alike or almost the same, but is made by BAREALY. So if the power that goes in needs to be 5.0V, hopefully it won't damage it if the voltage is 6.7V. Jan 12 at 17:39
  • you said "an isolation on the power connectors to prevent power from the hub flowing into the USB-A port on a computer"... is that a verifiable fact, and let's hope all models of MacBook have this feature since the very beginning... Jan 12 at 17:39

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