Since you didn't complain about poor signal strength, I'll focus solely on the issue of running out of power. When I had a phone with replaceable batteries, I carried two extra batteries that were always fully charged. Even with opportunistic charging habits, I would regularly swap out to a fresh battery in the early/mid afternoon. For the few times a month I went through both extra batteries, I had a 20k mAhr external battery pack I would fall back on. With this setup, I never ran out of power. I have the RAVPower battery pack and have been immensely satisfied with my purchase. I've also have heard very good things on the Anker line. I used this setup for over two years successfully.
For the past 3 months, I'm using a current gen smartphone without an interchangeable battery (or SD slot! life sucks). I've been relying on a mophie battery case and the battery pack to get me through the day. Honestly, I haven't had to use the battery pack as the energy drain during the day is significantly less and the mophie has been sufficient to keep me charged up.