What is the real output capacity of a Power bank?

Do you know how many times you can recharge your cell phone with a 10000mAh or a 20000mAh power bank?

A very common mistake among those of us who want to know the number of charges we can get on our devices with a power bank is to divide the capacities of both devices, i.e.:

Number of Charges = Power Bank Capacity (mAh) / Smartphone Capacity (mAh)

For example, if you have a smartphone with a 2500mAh battery capacity and you want to buy a 10000mAh power bank, how many charges could you get?

If we apply the above formula we get 4 full charges:

Number of Charges = 10000 mAh / 2500 mAh = 4 charges

THAT'S A BIG MISTAKE!!

We are sorry to tell you that this calculation would be incorrect because the 10000mAh of the power bank refers to the capacity of its internal battery.

A result closer to reality would be to use the actual capacity available at the output USB port of the power bank:

Number of Charges = Actual Power Bank Capacity / Smartphone Capacity

If we assume that the actual capacity is 6000 mAh we would have enough juice for 2 full charges of our smartphone:

Number of Charges = 6000 mAh / 2400 mAh = 2,5 charges

Although this second formula is still technically incorrect (capacities divide at different voltages) we wanted to make you see in a simple way that the number of charges is much lower than expected despite the fact that the power bank is advertised with a capacity of 10000 mAh.

 

In the following article we are going to explain in depth how to calculate the real capacity and the number of charges of a power bank in any device and for different charging voltages.

Although the content of this article may be a bit technical and boring to read, we have tried to make it as easy to understand as possible for anyone without previous experience.

After reading this article you will become a power bank expert!

 

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. A rechargeable battery of a certain capacity (mAh) and nominal voltage (V).
2. An electronic circuit that controls the charging and discharging process of the rechargeable battery, as well as performing other important functions such as, for example, protection against over-voltage, over-discharge, temperature control, etc.

As we will see below, the actual output capacity of a power bank will depend on the quality of such components.

 

Battery capacity and voltage rating

The battery of a power bank consists of lithium-ion (Li-Ion) or lithium polymer (LiPo) cells.

Usually, they use cells with a nominal voltage of 3.7 volts (V) and a capacity ranging from 1500 to 5000 milliampere-hours (mAh). However, cells with other voltages are also available on the market, e.g., 3.6V, 3.8V or 3.85V.

 

Additionally, the battery may be made of of a single cell or several cells connected together:

►One cell

If the power bank battery is made of a single cell, its capacity and rated voltage will be the same as that of the cell itself.

►multiple cells

If the power bank battery is made up of multiple cells, its capacity and voltage rating will depend on the individual capacity and voltage of each of the cells as well as the configuration used (series or parallel circuits) to connect the cells together.

Generally, a power bank battery uses cells with the same capacity and voltage nominal connected in parallel.

With a parallel connection, the voltage of the power bank battery is the same as the voltage of the cells and its capacity is the sum of the individual capacity of each of the connected cells.