Dual 16340 Rechargeable Lithium Battery Shield Power Bank 5V GND Micro USB Charger Output Board For Arduino R3 One Module : Amazon.co.uk: Electronics & Photo
Powering a pro micro board with li-ion battery and USB connector - Project Guidance - Arduino Forum
Rechargeable battery solution for your MCU Projects? - Motors, Mechanics, Power and CNC - Arduino Forum
Charge Batteries with an Arduino
Ideal 3.7v battery charged module for use with Arduino - Project Guidance - Arduino Forum