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I have been working on a battery back up for my raspberry. The idea is classic: connect power bank to wall outlet and raspberry to power bank output. It works well with only battery power. But when the power bank is plugged to the wall at the same time it's supplying to raspberry, I get the "low power" lightning bolt signal on the right upper side of the screen, and even worse, if I unplug the power bank from the wall, my raspberry simply shuts down.... Any ideas on how to prevent this? I would like it work as a UPS where in case of a blackout, the power bank prevents the raspberry from shutting down.

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  • Do you have access to a voltmeter/DVM? If so, what voltages are you reading under plugged-in and unplugged conditions. Also - more info on the battery pack itself would help your cause: is it a commercial unit, DIY, what? If DIY, maybe a schematic?
    – Seamus
    Commented Mar 20, 2019 at 13:45
  • Hi Seamus, thank you for answer. Yes, I measured the tensions with the battery plugged and unplugged, and it does not change, fixed at 5.1 V. The powerbank I'm using is this one amazon.fr/gp/product/B01NBRGY7N/… The only difference is that I connected an output to the cells to use this to monitor the battery percentage, following this great tutorial: raspi.tv/2013/… But I didn't break the battery or anything, and the output tension is stable Commented Mar 21, 2019 at 14:15
  • I can see no reason that this would not work as advertised. Without studying the schematics in detail, it seems rational, and your plugged/unplugged measurement would seem to confirm that you've got it wired correctly. But the fact is that when you unplug, the Pi shuts down, and clearly that means something is amiss :P You will need to troubleshoot to find the cause. 1) Was your volt. measurement made with the Pi connected to the battery? 2) If you disconnect your monitor circuitry, does the Pi still shut down?
    – Seamus
    Commented Mar 21, 2019 at 16:36
  • 1) I'll tell you that tonight, I think that this information will be useful. 2) I already tried this using VNC viewer, it still shuts down. The problem however is that the lighning bolt symbol is only shown on monitor screens, not through a wireless visualisation. But it still shuts down so... Commented Mar 26, 2019 at 9:47
  • A UPS is designed to handle this kind of thing, but a power bank is not. A UPS has a certain amount of switching time when the power is cut and it designed to make sure that time is small enough.
    – T. M.
    Commented Mar 28, 2019 at 8:29

2 Answers 2

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Firstly, do you know the voltages being output by the UPS at the moments you described (only wall, only battery, wall and battery)? It may be that your UPS is not supplying the correct voltage the Pi needs, around 5V.

It could also be a power level problem, where your batteries are not "beefy" enough to supply the Pi by themselves. Remember the Pi need at least 500mA (https://electronics.stackexchange.com/questions/386361/how-to-estimate-usbs-power-consumption).

It would be interesting if you posted schematics and the UPS model name for us to help you.

The lightning bolt at the top right is (for me) a mistery, once had a cheap USB cable offering the Pi 4 around 4,91V and the lightning on; changed the cable (for a Baseus one, found on Aliexpress) solved the problem.

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It sounds like your power bank is not able to supply enough power to your Raspberry Pi while it is simultaneously charging. This is a common issue with many power banks, as they may not be designed to supply enough current to charge and power a device at the same time. I think a good solution it to use the DFRobot UPS HAT with a compatible battery. There are many more UPS HATs available which you can consider too.

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