New to the Witty solution...

I guess you mean "power bank" when you say "5V battery pack". Power bank has its own DC/DC converter and it always outputs 5V until the battery runs out, so there is no way to monitor the voltage drop and do graceful shutdown when using a power bank. Witty Pi 4 Mini can be used to schedule shutdown and startup.

If you really need voltage monitoring, then you can NOT use any power bank. If you use 3.7V Li-Po battery, then Witty Pi 4 L3V7 is suitable.

For details please read the product user manuals.

So, a powerbank will just shut off without any "warning" - right? - So, if I used a L3V7 with a 3.7v LiPo backup battery, it would "take over" from the powerbank and then the Witty would measure its input voltage from that LiPo battery and be able to issue a graceful shutdown command before a "blackout" occurs?

I read the docs on the L3V7 version and I noted that the unit can continue to operate when the 5v supply fails (via UPS mode with the 3.7V battery) - this then CAN be monitored in terms of a voltage threshold and  a graceful shutdown can be actuated (even if the main 5V supply is from a PowerBank - Am I reading this correctly? - Thanks

@russpw Witty Pi 4 L3V7 can only monitor the voltage of connected battery and take action when that voltage drops too much. You can use any kind of 5V power source for the USB-C connector, including a power bank. The voltage on USB-C connector will not be monitored, but when that voltage is cut off directly, the 3.7V battery (if exists) will continue powering the device, until its voltage goes below the threshold and trigger a graceful shutdown.

Thanks - so that would work for my use case, I think 🙂 -  We are a volunteer group using a bird call monitoring application called "Birdnet-Pi" - We will need to install several Pi minis in woodland and operate them remotely, via battery. So, would need the system to gracefully shut down once the (20,000mAh) power banks are depleted - it seems that the LiPo battery would kick in (as a UPS) when a power bank  shuts off, then the L3V7 would monitor the LiPo voltage and allow  shutdown of the Pi in a controlled manner (based on the Voltage threshold value set). We would also schedule times when the system would be shutdown (during darkness when birds are not calling) to preserve the power bank battery life. Does this sound sensible - If so, I will buy a L3V7 now and test it with a Pi4B in my garden, before building "runtime" Pi-Mini units for deployment in the Woods.

Sorry, one more question... could I use a 16850 3.7V Lion battery for the L3v7 (instead of a LiPo pack)? - What mAh capacity do you suggest if all I want is for the system to run for enough time to gracefully shutdown (the RPi4b running the application and the microphone, is only drawing around .5A-1A max)

@russpw There are others who use power bank as the main power source, while using LiPo 3.7V battery as the UPS backup. Such solution works, but you need to make sure the power bank will not enter its "sleep" mode. Witty Pi has a functionality named "Dummy Load", which may be useful in the majority of cases.

I think these discussions may be helpful for you as well:

https://www.uugear.com/forums/technial-support-discussion/witty-pi-4-l3v7-dummy-load/

https://www.uugear.com/forums/technial-support-discussion/minimum-battery-level/

Also you may want to search "L3V7", "Dummy Load" in this forum.

Although you will find quite some useful information in this forum, it is always better to verify your idea with a real prototype.

Thanks - but what would cause a power bank to enter sleep mode? - I had one running on my pi4b for 10 hours (only lost one led out of 4 in that time) - Surely by putting a dummy resistive load on the 5v power bank output would cause current drain and therefore deplete the power bank life? My understanding was that with the L3V7, if the 5V supply (whether mains PSU or Power Bank supplied) was removed, the LiPo battery would kick in immediately, maintaining power to the Pi Mini - Then when the LiPo dips below a threshold voltage (which is settable), the "Shutdown" command could be issued, allowing for a graceful shutdown and then when I replace the Power bank, it would startup again (and the LiPo battery would be charged back up by the recharged Power Bank supply? - It seems simple to me - Maybe I am missing something, and as the L3V7 (plus battery) is quite expensive, I dont want to purchase until I know that it will work for my use case. 

Posted by: @russpw

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what would cause a power bank to enter sleep mode?

Most power banks in the market has such a feature, that it enters sleep mode when no load is detected for a while. When a power bank is in sleep mode, it outputs no voltage at all. You may force it to quit sleep mode by pressing the button on the power bank (if it has one), or physically unplug and re-plug the USB cable connects to it.

When Rasbperry Pi is off, Witty Pi only draws about 1mA from the power bank, so such a load is very small, and it may be too small to keep the power bank alive. As for how "dummy load" could help, please search "dummy load" in the user manual and in this forum. I'd like to remind you that sometimes it doesn't help, so it is always better to do some testings.