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.

And this is the main problem that many Amazon users encounter when buying a power bank:

In the following article, we are going to explain in depth how to calculate the actual 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!

👇OTHER GUIDES YOU SHOULD READ👇

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 a single cell or several cells connected together:

So, from now on, whenever you see the capacity of a power bank advertised, remember that it refers to the capacity of its internal battery!

USB Output port voltage

We have already seen that a power bank is composed of an internal battery of a certain capacity (mAh) and nominal voltage (V) which is usually 3.7 volts (V).

However, when charging a device with a power bank we should know that the USB output port of any power bank works at a standard voltage of 5V.

Even this voltage can be higher (9V, 12V or 20V) if both the connected device and the power bank support fast charging protocols such as Quick Charge (QC) or Power Delivery (PD).

This difference between the battery voltage and the power bank output voltage is the reason why the capacity of a power bank at its USB output port is different from the capacity indicated on its internal battery.

For example, a 10000mAh power bank would have a capacity of 7400mAh at its USB output port at a charging voltage of 5V.

To sum it up

In this section we learned that:

• A power bank is made up of an internal battery (consisting of one or more Li-Ion or LiPo cells) and an electronic circuit.
• The capacity advertised on a power bank indicates the capacity of its internal battery and is different from the capacity available at the output port.

How to calculate the actual output capacity of a Power bank?

Once we are familiar with the main components of a power bank, let’s learn a step-by-step method to calculate the actual output capacity of a power bank for any charging voltage (or output port voltage).

Additionally, to better understand the theoretical concepts, each section will have a practical example using the real data of a power bank of 10000 mAh capacity, the Ugreen (model PB178):

The specific data we are interested in knowing about this model are the capacity (mAh) of the device and the voltage (V) of its battery:

• Battery Capacity: 10000mAh 3.85V

Later, we will also explain what the Rated Capacity data means, which some manufacturers are starting to include in the specifications of their power banks.

Stored energy

As we already know, a power bank is an electronic device that stores energy in an internal battery to later transfer it to the battery of other devices, or even power some of them.

This energy is measured in watt-hours (Wh) and is calculated by multiplying the capacity (mAh) by the nominal voltage (V) of its internal battery:

Voltage conversion

During the process of charging a device, the electronic circuit of a power bank raises the nominal voltage of the battery (e.g., 3.7V) to the voltage of the USB output port (5V standard voltage).

If we assume that the voltage conversion (from 3.7V to 5V) was an ideal process, i.e., without energy losses, all the energy stored in the power bank battery should be completely transferred to the USB output port.

Therefore, we can define the following equivalence:

Power Bank Stored Energy = Power Bank Output Energy

Let’s deconstruct this equation in terms of capacity and voltage:

Battery Capacity (mAh) x Nominal Battery Voltage (V) = Output Capacity (mAh) x Charging Voltage (V)

From the above equation, we know all the variables except the capacity at the USB output port.

If we solve this variable, we get a first approximation to know the real capacity at the output port of a power bank according to the voltage at which the device is charged (5V being the standard voltage):

Output Capacity (mAh) = [ Battery Capacity (mAh) x Nominal Battery Voltage (V) ] / Charging Voltage (V)

Energy efficiency

So far, we have calculated the actual capacity of a power bank at its output port considering that it is able to supply 100% of the energy stored in its battery.

However, we are sorry to tell you that the formula defined in the previous section is never going to be fulfilled in real life:

Power Bank Stored Energy ≠ Power Bank Output Energy WRONG!

The energy at the output port of a power bank is ALWAYS going to be less than its stored energy and the amount supplied will depend on the quality of its 2 components:

We can define the discharge energy efficiency of a power bank as the ratio between the energy supplied at its USB output port and the energy stored in its battery:

It should be clear that the energy efficiency of a power bank will never be 100%:

Even if we buy a new power bank and its battery is in perfect condition, there will always be an energy loss due to the voltage conversion process performed by the electronic circuit of the power bank in order to charge a device.

This energy loss will be higher or lower depending on the quality of the electronic circuit.

Usable Energy and Real Capacity

Once we know the concept of energy efficiency in a power bank, we are ready to calculate the energy and capacity it will have at its USB output port.

With the Energy Efficiency formula as our starting point, we take the Stored Energy variable to the other side of the equation and, in this way, we get the energy available at the output port of the power bank:

Power Bank Energy Efficiency = Output Energy / Stored Energy

The energy calculated with this formula is the usable energy of the power bank, that is, the energy that we will have available to use to recharge our devices.

If we analyze this formula, the only element we don’t know is the energy efficiency of the power bank.

This data must be established by us and, you may wonder, what value should I use?

Based on our experience, after analyzing a large number of models, we recommend using an efficiency of 85% (0.85):

There are power banks that have an efficiency higher than 90% as there are also those that have it below 80%, but if our power bank is of a good enough quality, its discharge efficiency will be around 85% *.

*Note that this efficiency may be slightly reduced when working with fast charging protocols.

We have already seen how to calculate the usable energy of a power bank, if we want to know what is its real capacity at the output port for a given charging voltage, we simply have to express the above equation in terms of capacity and voltage:

Output Energy = Efficiency x Stored Energy

Output Capacity (mAh) x Charge Voltage (V) = Efficiency x Battery Capacity (mAh) x Nominal Battery Voltage (V)

Next, we solve our variable which would be the actual capacity at the output port and finally we get the general formula:

This formula is valid for any charging voltage if, for example, we want to calculate the real capacity at the output port of a power bank for a charging voltage of 5V we would apply the following formula:

In case you indicate the power bank capacity in energy terms (watt-hours Wh) you can use the following formula:

To sum it up

In this section we have learned:

1. That the energy at the output port of a power bank is known as usable energy and is ALWAYS going to be less than the energy stored in its battery.
2. That the usable energy of a power bank will depend on the quality of its components (battery condition and efficiency of the electronic circuit).
3. That the energy efficiency of a power bank is the ratio between the energy supplied at its output port and the energy stored in its battery.
4. How to calculate the usable energy (theoretically) and the real capacity at the output port of a power bank knowing its specifications and assuming an energy efficiency of 85% when its battery is completely discharged.

Formulas

• Stored Energy (Wh) = [Battery Capacity (mAh) x Battery Voltage (V)] / 1000
• Energy Efficiency (%) = [Output Energy (Wh) / Stored Energy (Wh)] x 100
• Output (or Usable) Energy (Wh) = Stored Energy (Wh) x 0.85
• Actual Output Capacity (mAh) = [Output Energy (Wh) / Charging Voltage (V)] x 1000

Use Case

We applied the formulas with the data of the power bank used as an example in this section assuming it has an energy efficiency of 85%:

Ugreen Power Bank Data:

• Internal Battery Capacity of 10000mAh and nominal voltage of 3.85V.
• Discharge Energy Efficiency of 85%.

Results:

1. Stored Energy (Wh) = (10000mAh x 3.85V) / 1000 = 38.5 Wh
2. Usable Energy (Wh) = 38.5Wh x 0.85 = 32.725 Wh
3. Actual Output Capacity at 5V (mAh) = (32.725 Wh / 5V) x 1000 = 6545 mAh

How to measure the actual output capacity of a Power bank?

So far, we have seen some formulas that allow us to know what would be the usable energy and the real capacity of any power bank before buying it.

These formulas are based on the assumption that a generic power bank has an energy efficiency of at least 85% when fully discharged.

However, as we will see below, energy efficiency may vary from one power bank to another.

Comparison of real vs. theoretical data

If we have already purchased a power bank and we want to know its real capacity, usable energy, and energy efficiency, it would be necessary to discharge it completely (from 100% to 0%) by connecting to an output USB port and to an electronic load at a constant voltage (V) and current (A) and use a multimeter to measure the total energy provided.

Let’s discharge our Ugreen power bank with a 10W electronic load (5V/2A) and compare the data obtained by the multimeter with the results of the formulas presented in the previous section:

We see that the real and theoretical data are quite similar; even for this model, we obtain better results than predicted by the formulas.

In short:

For the theoretical calculation of the usable energy and real capacity of a power bank we can use another value of energy efficiency, for example, 80% or 90%, however, we consider that 85% is a valid average value for pretty much every power bank.

More real-world examples: The PowerBank20 project 📋

In order to recommend the best power banks on the market, one of the tests we perform in PowerBank20 is the analysis of the energy efficiency of the power bank when it is completely discharged.

In the following table you can see some of the data we recorded in this test for models analyzed on our website of well-known brands in the market:

If you are curious about all the data for each power bank analyzed you can visit this section.

Conclusions

Before buying a power bank:

1. The usable energy and actual output capacity can be estimated by knowing only their specifications and establishing a theoretical energy efficiency of 85%.
2. Once we know the usable energy of the power bank we can estimate the number of charges it could perform on a cell phone.

How to calculate the number of charges you can get from a Power bank 📱

To calculate the number of charges that a power bank can deliver to a device we need to know:

Once we know this data, we simply apply the following formula to obtain the number of charges:

Remember, this is a general formula, and it works for any device that can be recharged with a power bank (cell phone, tablet, smartwatch…).

On the other hand, its result, logically, is not 100% accurate but it helps us to have an idea of how many approximate charges can the power bank that we are thinking of buying to charge our devices provide.

Example: How many charges are 10000 mAh?

Here, we will estimate the number of charges that we would have on our Bq Aquaris X2 Pro cell phone, which has a 3100mAh battery, assuming that we want to buy the 10000 mAh power bank from Ugreen.

Subsequently, we will compare the result with the actual data obtained from the measurements once we have purchased the power bank.

The formula for estimating the number of charges on the Bq Aquaris X2 Pro cell phone using the Ugreen power bank is as follows:

Number of charges = Power Bank Usable Energy / Recharging energy of a device

The following table shows the specifications of both devices:

Specs	Ugreen PB178 Power Bank	Bq Aquaris X2 Pro Smartphone
Battery Capacity	10000 mAh	3100 mAh
Battery Voltage	3.85V	3.85V
Energy Stored	38500 mWh = 38.5 Wh	11935 mWh ~ 12 Wh

Let’s calculate the usable energy of the Ugreen power bank assuming it is capable of supplying 85% (0.85) of its stored energy (38.5 Wh):

Power Bank Usable Energy = 10000 mAh x 3.85 V x 0.85 = 32725 mWh ~ 33 Wh

Next, we calculate how much energy our cell phone would need to recharge its battery assuming that the recharging process (cable, voltage conversion…) has an efficiency of 85% (0.85):

Recharging energy of a device = (3100 mAh x 3,85 V) / 0,85 = 14041 mWh ~ 14 Wh

We can see that the battery of the Bq Aquaris X2 Pro needs to receive approximately 2000 mWh extra energy (14000 – 12000 mWh) to fully recharge its capacity of 12 Wh.

Finally, we calculate the estimated number of charges:

Number of charges = 33 Wh / 14 Wh = 2.36 charges

Therefore, we know that if we buy the Ugreen power bank of 10000 mAh capacity we would have 2 full charges (from 0% to 100%) on our Bq Aquaris X2 Pro mobile and, in addition, we would still have energy for a third partial charge (from 0% to 36%).

However, for our more curious readers, let’s check the actual number of charges we would have on the mobile after purchasing the Ugreen power bank.

Data validation

The following tables show the results obtained from the tests of recharging the mobile and discharging the power bank both for standard charging (the usual charge for any device with a USB charging port) and for Quick Charge 3.0 fast charging and Power Delivery (this mobile is compatible with both protocols).

We also verified that the average efficiency obtained in both tests differs from the 85% established in the theoretical formulas.

Bq Aquaris X2 Pro Smartphone

1. We note that this cell phone has an average recharge efficiency of 80% with a required power supply of approximately 15Wh.
2. The charging cable is taken into consideration for the measurement of the recharging energy of the device.

Ugreen PB178 Power Bank

1. We observed that this model is capable of supplying more energy (90%) than what we established for the theoretical calculation (85%).
2. To measure the energy that a power bank can supply through its output USB port we perform a complete discharge by connecting an electronic load at constant power. Taking into account the charging power of a cell phone we performed our tests with 10 and 14W for standard and fast charging respectively.

With these data we calculate the actual number of charges by applying the above formula: