Using the VIN/GND pins to power the Arduino

The VIN and GND pins in the power and external reset header can be used to power the Arduino with an external battery. Powering the Arduino in this way is mainly used when we wish to connect a battery, in series, with a switch to turn the power to the Arduino on and off. The following photograph illustrates this:

It is not recommended that we power the Arduino in this manner unless we are looking for the most expensive and short-lived way to power the Arduino. We could use six AA batteries in series, which will provide the same voltage as the 9V battery in the preceding photograph but would give us approximately four times the capacity. It is still not recommended that we power the Arduino in this manner as it would be fairly expensive.

Unless there is a specific need to use a battery to power the Arduino, I would avoid using them.