18650 Li-ion rechargeable battery is very popular because of it capacity and voltage. We encourage user to use rechargeable battery because it reduce battery waste and save Earth! We only have one Earth.
This is a 2 cell 18650 battery holder terminated with standard 2.1mm DC Barrel Jack. Connect two 18650 battery in series so the output voltage is rated at 7.4V (2 x 3.7V). This voltage is pretty good for Arduino that comes with 5V or 3.3V voltage regulator. Of course not to forget all IoT and Robotic project.