Why is the PowerBank not charging as specified? It's lying (Description)

You will often find The Power Bank 30000mAh but why it can charge the 3000mAh phone only 4 or 5 time Or will say that can only be charged 50-60% of it specified.
I find the answer of my method And found The Power Bank battery is 3.7V, When the Power Bank Charging the phone it use 5V, the current will change form 3.7V to 5V, then it will reduce the current in same time
i calculate the 30000mAh from 3.7Vto 5V is 22200mAh, but in Actual use 18956mAh.
My Calculation Method is (mAh) / 5 * 3.7
And in the most cheap Power Bank from China use 3.3V Battery or less than 3.3V , the current will less than 3.7V Battery
#PowerBank #battery #charging