There are a number of ways you can power Arduino projects but if you want to create something which is mobile or is not located near a computer or power point then you have no choice but to use batteries.
Powering Arduino projects using batteries is relatively simple however there are a number of things which should be considered like your budget, required run time, voltage requirements and size/weight restrictions.
What voltage to power the Arduino?
According to the documentation the Arduino can be supplied with 7-12V although 7V is ideal. If run with less than 7V you may experience problems powering outputs and any more than 12V and the regulator can overheat and damage your board.
Once you have decided what voltage you want to use the easiest way to power the Arduino is using a battery holder with a 2.1mm centre-positive plug which can be connected to the DC input on the Arduino.
Can I use a 9 volt battery?
You can power the Arduino using a 9V battery but you shouldn't. The voltage regulator on the Arduino is inefficient resulting in large losses. This combined with the fact that 9V batteries have smaller capacities than AA batteries means that you will not be able to power your Arduino as long as with other options.
I have devices requiring different voltages?
If there are other devices in your project which have different voltage requirements then the simplest solution is to have multiple battery packs. If you decide to use multiple power sources make sure you wire all grounds together or things won't work properly. For example if you have a project where you need to power the Arduino and servos you could have a 7V battery pack connected to the Arduino and a 5V battery pack connected to the Servos (don't forget to wire the grounds together).
Rechargeable versus disposable batteries
There are differences in capacity, terminal voltage and price which should be considered when deciding between disposable and rechargeable.
If you don't already have rechargeable batteries lying around then it is expensive to get started. Rechargeable batteries typically cost around 7 times more than their equivalent non rechargeable equivalent and require charging equipment. These chargers can range in price from quite cheap (mine was bundled with batteries) to expensive (over $100).
The capacity of rechargeable batteries is typically lower than disposables. If you want to get close to the capacity of a disposable alkaline battery you will need to buy the more expensive rechargeable batteries rated around 2300mAh.
An important thing to note if you choose to use rechargeable is that they have a lower terminal voltage of 1.2V versus 1.5V for disposable. This will need to be taken into consideration when choosing a battery holder for the project. For example to power an Arduino using 7V you would require 6 rechargeable batteries compared with 5 if disposables are used.
Hopefully you are now equipped with the knowledge to make an informed decision when deciding how to power your Arduino project using batteries.