I have used portable battery banks in the past and found them to be heavy, clunky and found it is more parts to lose. Using an iPhone, I learned that the Apple produced piggyback silicone cases with battery and pass through charging as a good way to go.
They are relatively heavy but add double to triple the time between needed charges. They are expensive, but combine a protective silicone case with the contoured battery. WIth the Apple piggyback battery on my old i6s iPhone, I could go a minimum of three full days on Camino, using GPS constantly, before I had to recharge.
Also, and just FYI, earlier today, I read that Apple has just released these cases for the iPhone X, XS, and XR models. I now have an XR, so this is of interest. I presently get 2 full and long days between charging. According to the specifications, the new case should double my time to 3 - 4 days. I am thinking on it...
IMHO, a battery case that doubles as a protective case is the way to go. Several makers produce these for Android phones, and for Apple products. But, as far as I know, only Apple makes the contoured silicone case with the pass-through charging capability for iPhones.
Hope this helps.