Remote supplemental battery for iPhone 8

I mount my iPhone 8 on the handlebar of my bicycle to run the Polar Beat app that displays heart rate, distance, speed and more using GPS and bluetooth connection to a chest heart monitor, plus a music app (usually Pandora). Polar Beat is an energy hog. I clipped a 1-meter lightening cable to the bike connecting the iPhone to a free 2200mAh 7.92W supplemental battery placed in my under-seat bag. Even with both batteries at 100% charge when I start out, the internal and supplemental batteries are depleted too soon. Furthermore, the iPhone will become hot when connected to this free battery, so I stopped using that battery.

Many supplemental batteries ("power banks") are now on the market, and many of those boast compatibility with iPhones, but my experience with my hot iPhone 8 suggests it is not a simple match-up. I wonder if there are specifications to be aware of to take advantage of extra battery capacity while avoiding damage to the iPhone and its internal battery.

• What specifications should I look for when buying a larger capacity (but compact) battery to connect to my iPhone 8 via lightening-USB cable that will work properly with my iPhone 8?
• Are there iPhone settings to be aware of when connecting a supplemental battery?