I've recently obtained a Pi 3 Model B, with the official 7 inch touchscreen case, as well as the allied electronics portable case and I am looking at options to provide power to it. Currently, from looking at the documentation on the official site as well as this blog, the recommended option is to use a 5.25V/2.4A Micro USB AC Power Supply. I have one, however, I would like a more portable means of providing power to it. So far, the best solution I have found is using a portable phone charger.
So I guess my question is, what are my best options in terms of long-term battery life? Also, is there a way to monitor the battery life on the PI once its connected to the battery pack? Ideally, I would like to use this as a computer (note taking, drawing, surfing the net) and perhaps even connect a second monitor to work with my 7 inch screen.