Please explain how Anker calculates the capacity of its chargers. For example, PowerCore 13000 product description says it will charge an iPhone 8 “almost five” times. Published current time of the charger is 13 amp-hours. At 5 volts, that’s 234,000 Joules or 65 Watt-hours. I’ve read that the iPhone 8 battery capacity is 26,870 Joules or slightly less than 7 Watt-hours. So why wouldn’t the PowerCore 13000 charge an iPhone 8 almost nine times?
Please explain how Anker calculates number of phone charges in a charger
Short answer, losses during power conversion and transmission to phone. Say the powerbank is fast charging your phone at 12 volts the internal circuitry of the power bank first converts the voltage from the internal cells to 12 volts where losses occur mostly in the form of heat which you can feel when you pick up your power bank halfway through a charge and it is slightly warm. Next the power bank transmits the power to your phone where additional losses occur.
This process is not 100% efficient, nothing is.
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Hey, Dude. Thanks. I knew there was entropy, but I assumed it was much less than that. Is it really that much? Your profile doesn’t say anything about your background in physics.
To add to TheDude some of that battery pack is also powering its own circuitry (overvoltage, signal conditioning, monitors, etc). A typical AC to DC converter of mid range is around 75% efficient due to various factors. Battery Packs will follow similar scenarios.
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After the 5th time anything else is a bonus 
I don’t know the mathematical physics behind it lol
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I have no background in physics but I do know somebody that will be better able to answer your question.
@professor care to share your knowledge?
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Hey, Dude. That link answers my question. Thanks. I was converting the current to energy using standard USB 5 volts. But according to your link, which sounds credible, the power banks cannot really sustain 5 volts for their full energy capacity. The real energy capacity in Watt-hours or Joules is the rated amp-hours times 3.7 Volts, not 5 Volts. Reducing that by about 10 percent entropy the number of charges published by Anker comes out approximately what I would expect. Contradiction eliminated. I also infer from the link you provided that as the power bank is depleted it charges devices more slowly.
18650 Voltage Curve
As you can see the voltage stays fairly steady throughout the discharge cycle.
It’s feasible that the battery would charge slower if the cells were depleted beyond 25% charge, temperature is also a factor.
The good professor never showed 