Even though it sounds simple, making your Arduino battery-powered is a complicated subject. There are a lot of different types of batteries and multiple ways to connect them, each with their own requirements and effects on the power that is used.
This section gives you a very concise summary of the battery terminology, intended to be exactly enough to make sense of the rest of this chapter. For more detailed information about the batteries and an overview of some common types that are available, see https://learn.adafruit.com/all-about-batteries.
Batteries are devices that store energy and can be used to power electrical devices. Every battery has a nominal voltage (such as 1.5V for alkaline batteries), indicating an average voltage of the battery over its entire lifetime.
Battery capacity is measured in mAh, or milliampere-hour. A battery of 1,000 mAh can supply 1,000 mA for 1 hour, 100 mA for 10 hours, 10 mA for 100 hours, and so on. To estimate the lifetime of a battery, divide...