Why is there a difference between rated and real battery capacity?

Why is there a difference between rated and real battery capacity?

It is undeniable that power banks have made our lives easier when it comes to extending the battery life of their portable devices, especially smartphones and tablets. However, a very common scenario is when users proceed to charge their cell phones only to find out that the battery of their power banks has drained “ahead of time”.

Most people have logically deduced that there is a difference between the power bank capacity and the actual power transferred to their cell phones. They have also noticed that this capacity is not sufficient to charge their cell phones the number of times they have initially calculated, leaving them disappointed.

But is there any explanation behind this? It turns out that a power bank cannot pass the energy stored in it without experiencing losses.

 Misleading advertising on power bank capacity?

The biggest misconception about power banks is that they can provide the same charge as the one listed in the technical specifications. A common assumption is that, for example, a 10,000 mAh will be able to charge the iPhone 7 battery 3.3 times. However, buyers ignore an important fact: Power losses.

In reality, power banks have been designed to supply less mAh than the so-called capacity. Manufacturers and merchants are to blame as they usually commercialize the “full capacity” as the actual value to calculate the number of charges users can get for their smartphones or tablets. This is done with the sole purpose of boasting about having a higher power capacity and boosting sales.

The result is that users feel baffled by this disparity, even believing that they have acquired a power bank of a lesser capacity or one that is malfunctioning.

Thus, how can we, as buyers, choose a power bank that fits our needs without neglecting power losses? The truth is that it requires a few tips to truly master the art of understanding the real capacity of power banks.

How do Power Losses occur?

Power banks are composed of batteries, as well as electronic circuitry to manage the power flow delivered to devices. As in the case of any lithium battery, power banks store energy rated at a nominal voltage of 3.7 V. Sadly, the standard USB output is 5 V, and thus the voltage is elevated through a converter circuit placed in between, incurring in initial power losses.

In addition, the batteries of electronic devices are lithium made and operate at 3.7 V, which means that another power conversion takes place, generating even more losses.

Besides, the USB cable also causes power losses by Joule heating, which is due to the internal resistance. This is expressed as an efficiency rating, and it is generally between 80% and 90%. The efficiency rating varies from one power bank to another, although many manufacturers opt not to disclose it to customers.

How to calculate the actual capacity?

Many have adopted the 2/3 of the advertised capacity as a rule of thumb as an indicator of the real power capacity.  But, where does this principle comes from? As we have seen, both voltage conversion and efficiency rating are factors to take into account.

Then, the actual power capacity can be obtained using the following simple formula: 

Actual capacity = 3.7 V x Advertised capacity x efficiency (in decimal) / 5 V 

The 10,000 mAH Xiaomi Mi Power Bank PRO is one of the best-reviewed power banks in the market because it has an efficiency rating up to 93%, which is actually stated in the technical specifications under the term “conversion rate”:

If taken as an example, then the actual capacity calculation is as follows: 

Actual capacity = 3.7 V x 10,000 mAh x 0.93 / 5 V = 6,882 mAh 

This result indicates that only 68.82% of the advertised capacity can be supplied to devices.

However, it is interesting to notice that if the power bank had an efficiency factor of 0.9, the results would be 0.666; in other words, two-thirds. Therefore, the assumption that 2/3 of the capacity on-paper is the actual capacity is not far from being accurate.

How many times can a Power Bank charge a device?

The exact number of times differs according to the power bank and the battery of the device itself. Nevertheless, there are other factors such as the ambient temperature, USB chord length, and the rate of discharge, which is either 1 Amp or 2 Amp depending on the USB port.

Anyhow, the charge cycles of a power bank are obtained by dividing the actual capacity by your smartphone’s capacity:

Number of charges = actual capacity/device battery capacity 

For instance, the battery of an iPhone X has a capacity of 2,716 mAh. Then it can be recharged two and half times using a 10,000 mAH Xiaomi Mi Power Bank PRO. 

Number of charges = 6,882 mAh / 2,716 mAh = 2,53 charges. 

Overall, the higher the capacity of the power bank, the higher the number of charges is. 

How to test the actual capacity of a Power Bank?

It is impossible to know the exact internal capacity without disassembling the power bank, but regardless, it is possible to measure the USB output.

For carrying out this procedure, a USB cable is connected to a full-charged power bank, while the other one is cut, insulating the four colored wires. A 5 Ohm resistor is then connected to the black and red terminal (1 and 4). A current of 1 Amp or 1,000 mA will circulate through it as 5 V is the standard USB output.

The voltage is monitored with a voltmeter for a determined number of hours according to the power bank capacity. If the power bank battery lasts for the same number of hours as listed in the capacity, then it is the actual capacity.  In reality, this capacity is less due to power losses.

For example, for a power bank of 12,000 mAh, a constant current load of 1 Ampere per hour will be drawn for 12 hours. However, the voltage should drop to between 3 V and 4 V at an earlier time, approximately two-thirds of the capacity (8 hours), which indicates the power bank real capacity.

As for the efficiency rating, it can be obtained using the formula to calculate the actual capacity: 

Efficiency (in decimal) = Actual capacity x 5V / 3.7 V x Advertised capacity 

Provided that the battery supplied power at 5V for 8 hours, the efficiency rating will be: 

Efficiency (in decimal) = 8,000 mAh x 5V / 3.7 V x 12,000 mAh = 0.90 


Overall, for choosing a power bank that adjusts to individual needs, it is important to consider a bigger capacity than the one the device has; but this is not the sole indicator. In fact, users should be aware of efficiency rating and power conversion to avoid deception. Nevertheless, anyone can estimate the actual capacity only by considering 66.6% of the advertised value.