The first step to powering your Raspberry Pi with a portable battery pack is to decide how much current you need to be able to supply to your setup.
The Raspberry Pi draws about 500mA, and adding additional USB devices require more current. If you are adding more USB devices, ensure that they use less than 140mA and are compatible. The compatibility list can be found at http://elinux.org/RPi_VerifiedPeripherals.
If you require a device that draws more than 140mA, then you will need a powered USB hub. A tutorial on creating a portable USB hub will be posted soon
Next, you must decide how long you want your Raspberry Pi to run on battery power. The amount of energy in a battery pack is measured in milliampere-hours. For example, a 1000mAh battery pack can supply 1000mA for 1 hour, or 500mA for 2 hours, or 2000mA for half an hour. You must also ensure that the battery pack you choose can supply your maximum current. If your setup will draw 1700mA (1.7A), but your battery pack can only supply 1.5A, you may run into problems. For most situations setups, however, less than 1A will be drawn, and most battery packs will be able to supply this. Most battery packs are in the range of 1000mAh to 10000mAh. The downside to a larger battery pack is cost and weight, and the downside to a smaller battery pack is less operation time.
Now that you have chosen the approximate size of your battery, you have two options, depending on the following questions:
- Do you have any experience wiring circuits, and if not, are you up to the challenge? (It is not too difficult!!)
- Are you supplying power to any other devices (especially motors, servos, or any other high current devices) or are you powering only your Raspberry Pi and any attached USB devices?
If you are not comfortable wiring a circuit manually, or you are looking for a quick and easy setup with minimal hands on preperation, a portable phone/device charger may be right for you! An example of one is: http://www.amazon.com/PowerGen-External-Blackberry-Sensation-Thunderbolt/dp/B005VBNYDS/
These are battery packs that already have Micro USB adapters perfect for powering the Raspberry Pi; just plug it in!
If you want a challenge, or are looking to provide extra power, you may need another type of battery: lithium polymer (commonly known as Li-Po). These batteries are similar to those found in laptops and cellular phones, and are used in hobby aircraft and RC cars. The benefit of using Li-Po is they can supply much more current for additional devices, but the downside is you must do all of the wiring manually and you must buy an external charger!
There are many sizes of Li-Po batteries, ranging from ~150mAh all the way up the 10,000+ mAh. Again, choose the best size for your needs. In addition to the battery, you will need a charger. I suggest what is known as a "balancing" charger, since they require little additional knowledge. You must also get a charger that matches the number of "cells" in your battery pack. A cell is a small battery; almost all Li-Po batteries are made up of multiple cells. Since most chargers have a range of cells they can handle, just make sure your battery falls in that range! (There are many good tutorials online for choosing, charging, and using Li-Po batteries... almost all of them on RC/hobby sites)
You will want to make sure your battery is a slightly higher voltage than you need. If all you are powering is a Raspberry Pi and USB devices, you will need to supply 5 volts, so a 7.4 volt battery would be fine. If you need to supply 9V to another device, however, you must take that into consideration.
Next, you must get something called a UBEC, or Universal Battery Elimination Circuit. It contains something known as a switching regulator, which reduces the voltage of the battery to your desired 5V without much loss of energy (they are very efficient). Make sure you buy a UBEC that supplies 5 volts, as many other voltages of UBECs exist. Next, you will simply connect the output leads of the battery to the inputs of the UBEC. The output of the UBEC will be a steady 5 volts, perfect for the Raspberry Pi, but how do you connect it!?
This will require a USB cable with the male Micro USB plug on one end. To use this, you must cut off then end opposite the Micro USB plug, and inside, you will find four wires. The only ones that interest you are the red and black wires (perhaps red and blue) as these supply the power! Simply connect the red wire to the red output of the UBEC, and connect the black or blue wire to the black or blue output of the UBEC. Done! Simply connect the USB cable to the power connector on the Raspberry Pi and admire your work.