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The Pi4B runs the latest 64-bits Raspberry OS Bullseye. By design the AC power supply is connected to the Raspberry Pi and not to the UPS HAT. The UPS gets its power from the Pi's GPIO pin carrying 5 V.

Documentation for this product contains errors and is unfinished and so, by admission in the manual, is the software. I would have a lot of questions, but let me try and start with this:

Whenever the power supply is disconnected and therefore the Pi powered from the UPS batteries, every now and then, think every minute or so, the Pi's red power LED goes off for several seconds. The Pi continues running. There are regular low voltage warnings in the upper right corner. Also entries are made in a log (I use journalctl to follow it) to the tune of:

aug 02 21:53:35 RPI-TST kernel: hwmon hwmon1: Undervoltage detected!
aug 02 21:53:39 RPI-TST kernel: hwmon hwmon1: Voltage normalised
aug 02 21:54:40 RPI-TST kernel: hwmon hwmon1: Undervoltage detected!
aug 02 21:54:44 RPI-TST kernel: hwmon hwmon1: Voltage normalised

I've been wondering how exactly the UPS detects if and when AC power returns and if it has something to do with the undervoltage events.

PiModules' current documentation (June 12th 2022) only describes how the UPS detects if the 5 V supply voltage on the appropriate GPIO pins sags and how fast the UPS makes the batteries (and 100F supercap) take over.

Now assume the batteries have taken over feeding 5 V to the GPIO pins to power the Pi and keep it running. If then AC power is reconnected, the USB power supply will again produce 5 V to the appropriate GPIO pins competing with the UPS. How is the UPS to know it can 'disconnect' the batteries?

Could it be a possible strategy to periodically cause a drop in the voltage supplied by the UPS and the check if the voltage actually drops. If it doesn't, AC power must have returned and the UPS can disconnect the batteries?

I have several other UPS HATs in use, but all need the USB supply to be connected to the HAT, so they work differently.

There's a lot more unclear and disfunctional, at least with my current settings, like the UPS not honoring the proclaimed lower voltage threshold to shut down and power off the Pi. In the case of a timed shutdown (like 30 min.) the UPS keeps restarting the Pi for a new period of 30 minutes.

I hope to find others who own this UPS or a dedicated forum. (The manufacturer's forum is closed because of malicious spammers.)

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