I have a power bank which is rated to store 20000mAh of charge and 72Wh of energy. It is recommended by the manufacturer to charge it with a Quick Charge charger which provides 1.5 Amps with 12 Volts. The company claims that using this charger, the battery can be fully charged within 7 hours.
Now, here's what I don't understand. The charger would provide 1.5 x 12 = 18 Watts of continuous power, which means that would provide it with 18Wh of energy after an hour of charging at the same rate. Being charged completely in 7 hours is understandable since the charger would reduce the wattage as the battery fills up. What I don't understand is if the charger is providing the battery with 1.5 Amps, shouldn't it take at least 20/1.5 = 7 hours 20 minutes to provide it with 20Ah of charge, which certainly disagrees with the claim of the company? Where is my reasoning wrong?