What does “Rated Capacity” mean in Powerbanks?
You bought a 10000 mAh power bank (mobile charger), and your phone is a product with a 5000 mAh battery. Logically, most people do not know the difference between the power bank capacity and the actual power delivered to their mobile phone. They are disappointed when they realize that this capacity is insufficient to charge their mobile phone the number of times they initially calculated.
A 10000mAh charger can fully charge a 5000mAh phone battery twice. However, this is not the case. Let’s see why:
Understanding the actual capacity of a power bank
Most power banks are created using Li-ion batteries with an average voltage of 3.7V. This is the voltage manufacturers use to calculate the theoretical capacity of power banks. So when you see a power bank with a capacity of 10000mAh, it is based on a 3.7V value.
However, they do not provide 3.7V when using power banks to charge other devices. Instead, they must provide 5V, which is the required USB standard. If you look at any power bank’s output ports, you’ll most likely see one of these values next to it: 5V/1A, 5V/2A, 5V/2.4A, etc. Always 5V and not 3.7V.
So when the local 3.7V is converted to the supplied 5V, the capacity also decreases. 5V capacity can be calculated by the formula:
formula
5V capacity = (3.7V * Declared Capacity) / 5V
For example, if we were to buy a 10000 mAh power bank, its actual capacity would be:
5V capacity = (3.7V * 10000mAh) / 5V = 7400mAh
As you can see, the supplied capacity is 26% smaller than the theoretical, advertised capacity.
But that’s not all! The actual capacity of the power bank is even smaller!! This is because of another factor that must be considered: power losses.
Understanding power losses
As mentioned, power banks have a native 3.7V, but they must provide 5V. The voltage is thus increased through a converter circuit between the power bank and the charged device, causing initial power losses.
Additionally, batteries for electronic devices are made of lithium and operate at 3.7V, which means another power conversion occurs, creating even more losses.
Finally, the USB cable also causes power losses due to its internal resistance.
All these factors are summarized and expressed as an efficiency rating, usually from 80% to 90%. The efficiency rating varies from one power bank to another, but many manufacturers choose not to disclose this to customers.
How to calculate actual capacity?
As we can see, both voltage conversion and efficiency values are factors to consider. Then, the actual power capacity can be obtained using the following simple formula:
formula
Actual capacity = 3.7 V x capacity x efficiency (in decimal) / 5 V
For example, the 10000mAh Xiaomi Mi Power Bank PRO is one of the best-reviewed power banks on the market, as it has an efficiency rating of up to 93%, which is mentioned under the term “conversion rate” in the specifications.
Then, the actual capacity calculation is as follows:
Actual capacity = 3.7V x 10000mAh x 0.93 / 5V = 6882mAh
This result shows that only 68.82% of the advertised capacity can be delivered to devices.
However, it is interesting to note that if the efficiency factor of the power bank were 0.9, the results would be 0.666; in other words, two-thirds.
Therefore, if you want to know the actual capacity of a power bank but do not know its efficiency coefficient, then you can estimate that 2/3 of the theoretical capacity is its existing capacity. However, actual capacity will vary from manufacturer to manufacturer.
How do we test the actual capacity of a power bank?
Knowing the exact internal capacity without disassembling the power bank is impossible, but it is possible to measure the USB output regardless.
One USB cable is connected to a fully charged power bank to perform this procedure, while the other is cut to insulate the four coloured wires. A 5Ohm resistor is connected to the black and red terminals (1 and 4). Since it is a 5V standard USB output, a current of 1Amp or 1000mA will pass through it.
Voltage is monitored with a voltmeter for the hours determined according to the power bank capacity. If the power bank battery lasts the same number of hours as stated on the capacity, this is the actual capacity. In reality, this capacity is less due to power losses.
For example, for a 12000mAh power bank, a constant current load of 1 Ampere per hour will be drawn for 12 hours. However, the voltage should drop to between 3V and 4V sooner, around two-thirds of capacity (8 hours), indicating the actual capacity of the power bank.
As for the efficiency rating, it can be obtained using the following formula to calculate the actual capacity:
formula
Efficiency (in decimal) = Actual capacity x 5V / 3.7V x Advertised capacity
Provided that the battery provides power at 5V for 8 hours, the efficiency rating will be:
Efficiency (in decimal) = 8000mAh x 5V / 3.7V x 12000mAh = 0.90
For more methods of testing the capacity of a power bank, check out this dedicated article.
Conclusion
In general, it is essential to consider a larger capacity than the device has to choose a power bank tailored to personal needs, But this is not the only indicator. Users should know the efficiency rating and power conversion to avoid cheating. However, anyone can estimate the actual capacity by considering only 66.6% of the advertised value.
