My phone's battery is absolutely fine, although it isn't expensive in the slightest (weird of you to assume it is...) but regularly hits below 30% battery towards the end of the day even without playing games. I could use a battery bank, but I'd still need that external controller which is yet another thing to carry and is awkwardly shaped and wouldn't fit anywhere near as well in a pocket.
Again, weird to think I'm "worried" about anything. I'd just rather reserve my phone's battery for when I need to use it, instead of wasting substantial amounts of it playing games.