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I want to use a power bank as UPS for my Raspberry Pi but it doesn't has pass-through power, so whenever mains power gets connected or disconnected, the power bank cuts off power supply to the Pi for a second, which resets it and has corrupted the filesystem multiple times.

Will attaching a 5.5v 1F+ super cap parallel to USB power lines on the Pi keep it running for the second or two it takes the power bank to reset and is it safe?

This does assume that the power bank can be charged and dischaged at the same time. I don't know that for sure. Is there a way to find out without opening it?

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It is very likely it won't work.

Since Q=C×U, a 1F capactor takes 5C of charge to reach 5V, and since Q=I×t it means that it would need for example 5A for 1 second, or 1A for 5 seconds, etc, to raise the voltage linearly from 0 to 5V.

So if you plug in a 1F capacitor to your powerbank, it will most likely draw too much current while keeping voltage low, so any sane powerbank should detect this as a short circuit or overcurrent situation and shut down.

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Without a schematic of your power bank I cannot be 100% sure but it should be OK. Your power bank connects to electronic devices that have capacitors and they feed back as does your existing system when the charging power is removed. 5.5V is a good choice for a 5V system, you are at the high end of the voltage where you will be able to store the most energy in the capacitor. You will not get a lot of time which you have already alluded to. Go for it!

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  • \$\begingroup\$ You are correct that power banks do connect to equipment with capacitors and it works. However the maximum capacitance for a USB device, mandated by specs, is 10uF. And sure they work if there is a little bit of extra capacitance if it is a non-compliant USB device. However 1F would exceed that by a factor of 100000 (one hundred thousand). Are you sure you want to give an answer that it should be fine? \$\endgroup\$
    – Justme
    Jul 30, 2021 at 5:21
  • \$\begingroup\$ It is being used as a power plug not USB. I would feel better if the cap were outside the Pi or at the connector however the Raspberry Pi has reasonable size traces. The Pi Model B draws about 1.2 Amps through that connector. I found this: "All USB-C cables must be able to carry a minimum of 3 A current (at 20 V, 60 W) but can also carry high-power 5 A current (at 20 V, 100 W)." \$\endgroup\$
    – Gil
    Jul 30, 2021 at 21:17

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