This is functionally a very simple project. It uses an off-of-shelf 18650 battery bank with an iTag Bluetooth Low Energy beacon combined.
The combination of these two devices provides multiple benefits. The beacon reports the battery status, I can have my phone alert me when it needs recharging. I don't have to deal with replacing the batteries in the iTag, just takes plugging it in. One of the coolest features is that I can use the Link Loss Alert to make sure that my phone and the battery bank don't get separated, that way I don't forget either of them somewhere.
For ease of use I also added a microswitch that allows turning the beacon on and off.
The project is very simple, I'm rather surprised this isn't a feature in commercial battery banks, costs a dollar to add.