Using micro controllers in the field requires battery power.  USB power banks are readily available, but these devices are not serviceable and cost substantially more than the components they are built from.  This post will cover battery power using a 18650 Li-ion cell.  It is based off of the work done here, https://circuitdigest.com/electronic-circuits/power-bank-circuit.  This diagram shows how these components are connected.

The post referenced above had one flaw that should be avoided, the on/off switch was placed on the negative line.  This prevents expanding the circuit to include other devices because they require a common ground connection, which uses the negative line.  The diagram above shows the switch on the battery’s positive line.  Doing this allows other devices to share a common ground with this circuit without any risk of an energized circuit when the switch is off.  These are the parts needed to build the power bank.

AITRIP 10 PCS PFM Control DC-DC Converter Step Up Boost Module 600MA USB Charger 0.9V-5V to 5V Power Supply Module
10 count for $8.39
https://www.amazon.com/dp/B08X1P227C

HiLetgo 10pcs 5V 1A 18650 Lithium Battery Charging Board TP4056 Lithium Battery Charging Board Micro USB Charge Module With Protect
10 count for $7.79
https://www.amazon.com/dp/B00LTQU2RK

Holder for 18650 Button Top Single Battery
8 count for $6.69
https://www.amazon.com/dp/B07KD9JLH3

100PCS 5mm High Knob Vertical Micro Mini Slide Switchs,3 Pin 2 Position SPDT Latching Toggle Switches Panel Mount DC 50V 0.5A SS12F15-G5
100 count for $7.99
https://www.amazon.com/dp/B099N3HFPG

3.7 Volt Rechargeable 18650 Battery 9900mAh 18650 3.7V Battery Button Top Battery for Flashlight, Headlamp, 8 Pack
8 count for $29.99
https://www.amazon.com/dp/B0CKHN95T6

The build was expanded to allow a micro controller to monitor the battery level using a voltage divider.  The resisters have a high ohm rating to reduce wasting energy.  The voltage divider is needed because the battery voltage has the potential to exceed what the micro controller can handle without being damaged.

Collecting the battery level values is useful to monitor the health of the power bank.  The following is a plot from a micro controller that was attached the power bank.  It’s easy to see that a voltage avalanche started shortly before the micro controller lost power.

The weight of the power bank is 52 grams (15 grams without the battery).  In comparison, an off the shelf unit weighs 74 grams.  This can make it suitable for use on UAVs or other applications where weight is a consideration.