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Will it work? Will it damage the power banks? What if I connect 10 banks or 100? Is it safe?

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  • In order to answer this question we'd need to know the make/model of the powerbanks and the device you're connecting to. However, it's unlikely that any powerbank would be designed to work this way, so at best it won't work, at worst it will damage everything. If you're able to provide significantly more detail, you may consider asking this question at Electronics.SE.
    – LShaver
    Sep 13, 2022 at 17:21
  • For i.e. this bank aliexpress.com/item/1005004225785453.html in PD version or this aliexpress.com/item/1005004127142508.html to connect 12V ports. I planned to connect few power banks to get a power source for all my telescope accesories in the wild field.
    – Robotex
    Sep 13, 2022 at 17:28
  • With PD and other smart protocols it will almost certainly not work, and it could easily damage the equipment you're connecting it to. PD means up to 48V so if you confuse one of the power supplies enough it might send 48V to what you're powering regardless of whether that can accept 48V. usb.org/usb-charger-pd
    – Móż
    Sep 16, 2022 at 0:21

1 Answer 1

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Firstly, if they are USB power banks, they have USB port outputs, USB-A or USB-C. You would need a paralleling cable for those. I'm sure you won't find any such paralleling cable for sale because paralleling USB ports on a computer for twice the performance simply isn't something that would work.

Now assuming you could build such a cable, it might even work! The USB power bank is simply a buck converter or boost converter depending on what voltage internal battery it has (less than 5 volts or more than 5 volts).

Buck and boost converters have the battery as the power source, plus an inductor to absorb voltage differences in a manner that doesn't lose any energy (unlike a resistor which would lose energy), plus an output capacitor. Power supplies that produce power through an inductor are excellent candidates for paralleling.

If you parallel the outputs, and draw less than 1 ampere, you would essentially have a system where the higher voltage power supply would dominate and the lower voltage power supply would be entirely off. By higher voltage and lower voltage I mean slight calibration differences, e.g. 5.001 V and 5.002 V (just drawing the numbers from my hat). So in this case the 5.002 V power supply would supply all power and the 5.001 V power supply would be entirely off.

However, when you will start to draw more than 1 ampere of current, the higher voltage power supply can't maintain the 5.002 V voltage anymore. So its voltage sags. In fact, it sags to 5.001 V, after which the lower voltage power supply would kick in and provide the rest of the current.

So as long as you don't expect both batteries to be depleted equally, it might even work!

If you draw less than 1 ampere, you would find one of the supplies is depleted first and only then the other starts to be depleted. But if you draw full 2 amperes, it'll probably deplete them reasonably equally.

If you draw something between 1 and 2 amperes, let's say 1.5 amperes, then in this case one supply would provide 1 ampere and the other would provide the missing 0.5 amperes.

And also to make it clear: this would only work for USB-A. If your device requires USB-C power delivery negotiation, none of this is going to work.

By the way, everything I said applies to all switched mode power supplies. If you have two AC-DC switched mode adapters of the same voltage, you can parallel them assuming their grounds aren't conflicting (for example one having negative ground and the other having positive ground would be conflicting).

However, theoretically it could be the case some switched mode adapter doesn't sag its voltage on overload, but rather turns entirely off on overload. If this is the case, the adapters / power banks can't be paralleled.

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  • "If your device requires USB-C power delivery negotiation, none of this is going to work." - Why? I planned to use USB PD ports.
    – Robotex
    Sep 14, 2022 at 10:28
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    USB PD negotiates the voltage between power supply and load. You can't negotiate with two supplies using a single wire. If you put the wires to only single supply, the second supply doesn't know what voltage should be used and uses incorrect voltage.
    – juhist
    Sep 14, 2022 at 17:53
  • I would focus on the PD negotiation, based on the OP comments. With same-model dumb USB 5V supplies it should be relatively safe to paralell them. As soon as you bring in PD the best outcome is 5V 1A, and the worst is 48V at 3A or more. But with multiple devices and multiple power banks why not just hook each power bank up to one device? Or buy one 5V/10A power supply, run it off a big 12V battery, and hook all your USB devices to that? My preference is one 12V battery and as many USB adapters as I need, but you don't have to to it that way.
    – Móż
    Sep 16, 2022 at 0:26

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