Commercial power banks are designed for the use with smartphones and tablets, devices with a high power requirement. When the smartphones not connects to the power bank then the internal circuit shut down the power to the USB port. When connected again the circuit detects the high power consumption and switch on the USB power. Ultramics are very low power devices and the commercial power banks detect a very small current consumption. For this reason, the circuit of the power bank shut down the Ultramic with the obvious consequence.
You can download the guide here:
You can DIY a power bank with these components:
This is a STEP UP CONVERTER
And a BATTERY HOLDER
or you can use a similar battery holder with a total voltale lower than 5V
