A quick rule of thumb is to assume that the real world capacity of your powerbank is 2/3 of the theoretical capacity mentioned on the package. So for a good quality 10000 mAH powerbank, it will be about 6600 mAH or about 4 charges for the iPhone 8 and for a good quality 26800 mah power bank it will be about 18000 mAH, or about 10 charges for the iPhone 8.
Understanding Power Bank Capacity
The reason for this mismatch is that mAH is not a good unit to measure battery capacity when the voltages of the battery and charged devices are different. Li-ion batteries used in power banks output 3.7 volt (nominal) but your phones’ batteries get charged at 5 volt.
mWH or watt-hours is the ideal way to measure a battery’s stored energy as it is voltage-independent and takes into account the total energy of the battery.
So a power bank with 10000 mAH capacity actually has 10000 mAH capacity at 3.7 volt. Total energy in such a battery in mWH will be 10000 mah x 3.7 volt = 37000 mWH.
When the output is at 5 volt, the Mah capacity of this battery will be lower. The capacity of the battery while charging at 5 volt output will be 37000 mWH divided by 5V or 7400 mAH.
In addition to this, energy is also lost during the conversion process from 3.7 to 5 volt, due to the resistance of the connecting cable, and during the charging process. You may have noticed that the phones often get warm and mildly hot during charging. This is simply electrical energy getting wasted as heat during the charging process and is unavoidable in any charging process.
Many other factors, such as ambient temperature and the exact chemistry of the powerbank and smartphone batteries also determine the real charging performance of the powerbank. Assuming a 10% loss (or 90% efficiency), this leads to an effective capacity of 7400 x .90 = 6660 mAH . This is approximately 2/3 of the on-paper or advertised 10000 mAH capacity of the powerbank.
As the actual charging performance may vary from device to device, you can roughly assume the 2/3 capacity number to calculate how many times your smartphone will get charged by your powerbank.
For example, a quality 26800 mAH powerbank such as Anker PowerCore+ 26800 will have a real world capacity of 26800/.66 = About 18,000 mAH. Hence, it will charge an Apple iPhone 8 (1820 mAH battery) about 18000 / 1820 = about 10 times! It will charge the Samsung S9 about 18000 / 3400 or about 5+ times.
Similarly, a good 20,000 mAH power bank such as Aukey 20100 Lightning Input + USB C PD or Anker PowerCore 20100 Lite USB-C Input will charge iPhone 8 about seven times and Samsung S10 about 4 times. A quality 10000 mAH powerbank such as EasyAcc Brilliant 10000 will charge iPhone 8 nearly four times.
Here are some tips to maximize the power capacity of your power banks.
Maximising Power Bank Capacity
To extract maximum power from your powerbank follow the following tips –
Use Fast Charging or QuickCharge ports only when required
Most modern smartphones ship with fast charging technologies like QuickCharge or Fast Charge over USB-C. Such fast charging will charge your phone at extremely fast speeds. For example, Apple iPhone XS/XS Max, XR, X/8/8+ can get charged from zero to 50% in 30 minutes with USB-C PD fast charging powerbanks such as the Aukey 10000 Lightning Input PD powerbank. Quickcharge 3.0 phones such as LG G8, G7 or V40, V30 can get charged from zero to 80% in 35 minutes using QuickCharge 3.0.
Fast charging is great when you are in hurry. However, fast charging uses high voltages (9 volt or even 12 volt) and this further reduces the usable battery capacity of the powerbank and drains power bank battery much faster.
Using standard 2.0 amp or even 1 amp ports on the powerbank will make your powerbank give as much as 20% to 30% more charging capacity.
Use Good Quality Powerbanks
Quality powerbanks have higher efficiency of up to 95% due to optimized circuits.
Use 1 Amp or 2 Amp Regular USB Charging ports if not in a hurry
Charging via non-fast-charging 1 Amp or 2 Amp/2.1 Amp regular USB ports will ensure greater efficiency than charging at fast-charging high current Quickcharge USB ports as more power is wasted at higher voltages and currents.
