Either that or just make sure your phone is charged overnight so it’s at 100% for the start of the day.
My average work day is 8-9 hrs, during which my nearly two year old iPhone 6s gets used for email, network file browsing, wireless testing, job logging with photo evidence and at a push for radio streaming or local music playback during auditing (if it’s school holidays). Even with a mix match of that, I can get back home with around 30-35% of my battery remaining.
Now if using GPS for a large amount of the day, yep a PowerCore 10000 can be a lifesaver for me