For example, a 15,000 mAh powerbank can only fully charge an iPhone X (2716 mAh) 3 times before the powerbank needs to be recharged. My iPhone X would have only taken 8,148 mAh after its 3rd full recharge from this powerbank.
This power bank is considerably new and has only been fully charged 3 times since I got it.
Is it possible the battery inside the power bank still needs to be conditioned by discharging and recharging it a few times?
Read more: https://iphoneus.org/faq/why-is-the-actual-mah-of-a-powerbank-less-than-what-is.html