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I have a few USB-C devices with their own chargers and I'm not sure if I can interchange them. What's confusing me is the different output values listed on the charger. From what I understand too high voltage can damage the device that is being charged and too low current can damage the charger/cable, while current above required is always fine (please correct me if I'm wrong).

For example my laptop charger can output: 20V=3.25A, 15V=3A, 9V=2A and 5V=2A. If I were to charge my power bank with it which accepts 5V=3A, 9V=2A and 15V=1.2A as input, could something go wrong? In this case if I were to plug in the power bank to my laptop charger how do I know that it's charging at the 15V=3A that it should and not at 20V?

On the other hand, if I were to charge my laptop with the same power bank (output: 5V=3A, 9V=2A, 15V=1.2A, same as input) could the power bank be damaged?

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The USB-PD standard allows for multiple output voltages on USB-C devices, but every USB-C power supply must initially output 5V and communicate with the device on the other end. If the other device does not specifically ask the power supply to output more than 5V it will just continue to put out 5V and no more.

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  • \$\begingroup\$ Let's say that the charger I'm using can output 5V=3A or 9V=1.5A and I'm using it to charge the power bank I mentioned in the post. Is there any way to tell which voltage it will use? (the power bank can take 9V but requires more current that the charger can supply) Will it switch to 5V so that the charger can provide enough current? \$\endgroup\$ – Wojtek Wencel Jul 28 '19 at 15:07
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    \$\begingroup\$ It will use 5V unless the power bank requests more. The charger will tell the power bank that it can output up to 9V and the power bank will probably request more. Chances are it will output 9V. It's a bit of an edge case though because 9V at 1.5A is less power total than 5V at 3A, but I don't think USB-PD devices generally consider current rating, if they even announce current ratings at all. \$\endgroup\$ – Hearth Jul 28 '19 at 15:11
  • \$\begingroup\$ So in short if a charger has multiple outputs it will provide the highest voltage that the device can take (but not higher even if it can provide more), but then the current can be too low, correct? \$\endgroup\$ – Wojtek Wencel Jul 28 '19 at 15:15
  • \$\begingroup\$ @WojtekWencel In general. I don't know the minutiae of the specification so there might be different cases, and sometimes a device might not request a higher voltage even if it does support it, for one reason or another. The charger just outputs what the device asks it to. \$\endgroup\$ – Hearth Jul 28 '19 at 15:16
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A load can always take in less current than than there is available. A source must always provide more current than the load can take. Devices will negotiate over USB-C connection what voltages they should use. So powerbank can be charged with laptop charger. There is missing information how much the laptop can take in current so it either can or cannot be charged with the powerbank. It might work if laptop is off and just take longer to charge.

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