What is the real output capacity of a Power bank?

Do you have any idea of how many charges are 10000mAh? and what about 20000mAh?

Many users want to know how many times they can charge their smartphone with a power bank and they make the mistake of dividing the capacity of the power bank itself by the capacity of the device, namely:

Number of charges = Power Bank Capacity (mAh) / Device Capacity (mAh)

For example, if we have a 10400 mAh power bank and a smartphone with a 2600 mAh battery. How many times can we recharge our phone with this power bank?

Well, if we take the above formula as a basis:

Number of charges = 10400 mAh / 2600 mAh = 4


This result would not be correct, as we are taking into consideration that the capacity indicated by the manufacturer is that of the power bank itself when, in fact, it represents the capacity of the internal battery stored in the power bank.

And this is the main problem many users find when buying a power bank without taking into account the real capacity they will have at their disposal:

customer reviews amazon power bank

You should know that when you connect a smartphone to the power bank’s USB port, it is charged at a voltage of 5V, but the power bank battery works at a voltage of 3.6-3.7V.

This means that the power bank’s internal circuit must boost this voltage to the one the USB port needs, which will decrease the power bank capacity available to charge our devices.

Which are the components of a Power Bank?

First of all, you should get familiar with the 2 basic elements that make up a Power bank:

  1. battery, which is formed by one or several cells connected in parallel.
  2. An electronic circuitBattery Management System (BMS): this one manages the charge and discharge process.

The real capacity of a power bank will depend on the quality of the components used in its manufacture and how efficient is the circuit that performs the voltage conversion.


A power bank may have a battery formed by one or more cells, either lithium ion (type 18650) or high-density lithium polymer.

battery 18650 li-ion vs lipo

However, cutting-edge power banks use Lithium polymer cells, resulting in a more compact and lighter product.

Each cell used has a nominal voltage of 3.6-3.7V, with a capacity ranging from 1500 mAh to 3600 mAh.

If our power bank battery consists of several cells connected in parallel, the battery voltage would be the same as the nominal voltage of the cells and the total capacity would be the sum of the capacity for each of the cells.

For example, if we build a power bank with type 18650 cells with 3350 mAh at 3.7V, the power bank’s total capacity and battery voltage would be:

  • If it is one-cell: 3350 mAh at 3.7V
  • If it is two-cell: 6700 mAh at 3.7V (2 x 3350 mAh)
  • If it is three-cell: 10050 mAh at 3.7V (3 x 3350 mAh)
  • If it is six-cell: 20100 mAh at 3.7V (6 x 3350 mAh)

Electronic circuit

The power bank’s USB port that is used to recharge our devices operates at a 5V voltage but remember: the power bank’s battery has a voltage of 3.7V.

The power bank internal circuit is responsible for boosting the battery voltage from 3.7V battery until reaching the 5V needed by the USB port.

In addition, this circuit performs other functions such as protection against input and output overvoltage, short circuit, overcurrent, temperature resistance… and it is also responsible for monitoring the cell load, controlling the LED indicators or showing charging status on a LED display…

All these functions have an energy cost that has an impact on the remaining capacity of the power bank to charge the devices.

This “energy cost” will depend on the efficiency of the circuit itself. Some power banks will consume more energy and others less, depending on the quality of the components used in their manufacture.

For this reason, we don’t recommend buying cheap or unknown-brand power banks which, likely use low quality cells and circuits that aren’t very efficient and with dubious safety.

In general, the external batteries in the market have an internal circuit with an energy efficiency of 85% but some brands such as Xiaomi or Anker have an efficiency of over 90% in their models.

However, it should be clear that this efficiency will never reach 100%. It is physically impossible to transfer all the energy from one device to another without losing some energy in the process.

How to calculate power bank real capacity?

Once we are familiar with the main components of a power bank and that it will have better or worse performance depending on its quality, let’s review a procedure to calculate the real capacity of a power bank:

Energy from the Power Bank

First of all, let’s clarify what’s the difference between capacity and energy:

  • Ampere-hour (Ah) is a measure of electric charge that represents a battery’s capacity. Normally, it is measured in milliamperes-hour (mAh).
  • Watt-hour (Wh) is a measure of electrical energy. It’s calculated as Wh = Ah x V.

Continuing the above example, if we use a power bank with a capacity of 10400 mAh, this means that it is formed by 4 lithium ion cells with 2600 mAh each, connected in parallel to a 3,7V voltage.

Therefore, the total energy that this specific power bank can supply would be:

Total energy = 10400 mAh x 3.7V = 38480 mWh

Voltage conversion

As we mentioned above, the power banks have the USB port at a 5V voltage. Then the internal circuit must boost the voltage from 3.7V to 5V.

After this conversion, the electric charge available at the output is reduced:

38480 mWh / 5V= 7696 mAh

Circuit energy efficiency

On the other hand, we must keep in mind that the power bank’s internal circuit consumes energy during the voltage conversion process.

Assuming we use a good quality power bank and its internal circuit has a conversion efficiency of 90%, the available capacity would be then:

Actual capacity = 7696 mAh x 0.90 = 6926 mAh

In other words, this 10400 mAh power bank has an output delivery of 6926 mAh, the actual capacity we will have to recharge our smartphone.

Number of charges

Therefore, the number of times we could recharge our 2600 mAh smartphone with this power bank would be:

Number of charges = 6926 mAh / 2600 mAh = 2.66 times

Keep in mind that all this procedure is carried out under certain ideal conditions or, in other words: it is assumed that a new power bank is used and the battery is in good condition.

Moreover, we should not use our phone during the charging process as it consumes more energy: open applications in the background, active Bluetooth or Wifi, screen brightness… all these decrease the external battery capacity for future recharges.