Have you actually had the need for a powerbank?So, it turns out that my Anker Powercore was the culprit, as it's not compliant with the USB Type C charging standards, using the cable supplied with the phone would stop the charging at 84 percent. Going to sell it and buy the Powercore+
My phone gets me through the day with no problems at all. Haven't had to carry my powerbank around since I got this phone.
Sent from my SM-G955F