You might do some projects that need your raspberry to be portable (means for a certain range of time, it can be brought to anywhere) for example remote control project, drone, etc. Any type of raspberry pi has its own power supply specification especially for current amount. Meanwhile up to now all raspberry pi type needs 5Volt. Raspberry pi 3 needs at least 1A but it wont function normally (I have tried with 1A so it wont connect through ethernet card and wifi and so on) and typically consume 2.5A to power all pins (50mA), camera(250mA), hdmi display (50mA), ethernet card, and wifi port.
Step 1: Parts
1. Turnigy Battery 3s 11.1v 2200mAh (You may use 9V single battery)
2. Raspberry Pi 3B
3. BEC 5v 3A
4. Jumper wire
5. Mini Breadboard
Step 2: Connect Them
UBEC 3A 5V or 6V would be fit to convert a single Lipoly battery 3S 11.1V to 5V. Besides, UBEC as switch regulator keeps the power more efficient instead of linear one (with 85% efficiency percentage). Even it's a little bit expensive, it avoids damaging your raspberry as it keeps voltage amount very still and I recommend you very strongly to use BEC. Furthermore, please ensure the output voltage range with multimeter digital and it must be in range 5V - 5.3V or it can fry your RasPi. Then, Plug the output BEC into GPIO Pin 4 and 6 (Board Mode). Wait for a while until your raspberry pi blinks green signal and lastly you might check to login to RasPi through router.
And I will let the rest to you, how can you package the portable Pi from the battery :D