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I have a device with multiple USB inputs wired in parallel (Raspberry Pi 2B with Kedei Display, each of the units has an Micro USB input).

Are there any dangers of connecting both inputs at once, powered by different powerbanks/chargers?

I don't want to do this to increase the supplied current, but rather to swap power supplies without having to turn off the device. So e.g. the connected powerbank is about to run out of power, so I connect a wall charger to the second port and then disconnect the powerbank.

Are there any possible negative interactions between the two power supplies? (e.g. different ground potential (especially with two wall chargers), slightly different voltages that cause power to flow in the wrong direction, different voltage fluctuation frequencies from the regulators in the power supplies that cause problems, ...)

Related questions

While this question seems similar to other questions posted on electronics.stackexchange.com, those questions are quite different in some aspects.

Higher current draw from powerbank?

Parallel connecting USB power for higher currents

Both of these questions don't actually use different power supplies, but instead multiple ports on the same powerbank. Also both of them do that, to increase supplied current.

USB power in Parallel, increasing the maximum current using a Y splitter idea[POWER ONLY]

This question actually talks about two different power supplies, but also here the idea is to increase current.

Dakkaron
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    (there's really a lot of questions on "how do I switch between power supplies?" and that's the first one that popped up. Many of the answers to these many questions include tips on how to make it seamless.) – Marcus Müller Jun 29 '20 at 08:33

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First, it's a BAD idea to connect multiple power sources in parallel without any protection at all. Imagine one of your power sources give 5.05V, the other one gives 5.00V. One power source will send current to another one, and these power sources are not designed to sink current. And since no power source is perfect, they will always have slightly different voltage. So just no to that.

A common practice is to put two schottky diodes that meet on the line you feed. You will drop about 0.15V, but usually it's an OK price for such an easy solution. Of course you could design some circuit that switches the sources and makes sure only one power source is attached to power line at a time, there's a dozen ways to do that, you could use back-to-back MOSFET to detach lines from the circuit, it's a common practice in laptops when you choose between powering system from battery or from charger. You can find double schottky diode there too, by the way.

Picture 1: double diodes

Picture 2: double mosfet back-to-back in macbook air 2011-2017. Air also has double diode in another place.

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enter image description here

Ilya
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