There are a couple of pre-built solutions for powering the Raspberry Pi on a portable basis. Some are actually ‘out there’, some are… a bit delayed, we’ll say. Instructables user DylanD581 has written a great tutorial which uses an Adafruit HAT prototyping board and a bunch of other components to create a portable power source which uses a LiPo battery. It gives you the following possibilities in terms of powering your Pi:

• Power the Raspberry Pi with a LiPo Battery Only
• Power the Raspberry Pi with a Micro USB Cable Only
• Power the Raspberry Pi with an External Power Source Only
• Charge the LiPo Battery and Power the Raspberry Pi with a Micro USB Cable Only
• Charge the LiPo Battery and Power the Raspberry Pi with an External Power Source Only

It’s really quite versatile and well worth the time it takes to solder it up. Take a look at the tutorial here.