A: Before we proceed with the explanation, we need to lay down some information about the basic parameters of the components and workings of a power bank. For purpose of illustration, we will use a power bank with an energy capacity rating of 10,000mAh. Warning: This topic may cause nose bleed as the discussion may get a little bit technical. Reader should exercise discretion in reading this topic. Please review the topic in No. 3 above with regards to the concept and computation of “Power”. :D
Of all the power banks we encountered while doing the above experiments, and based on our observation, the published energy capacity of each power bank refers to the capacity of its internal battery. To explain further this concept of energy capacity of power banks, first, we need to clear out some things. The published energy capacity rating for example of 10,000mAh refers to the capacity of the internal battery that is housed inside the power bank. All power banks have internal rechargeable Li-ion battery that has a working voltage of 3.7V. In reference to No. 3 above, if we are to compute for the “Power” (which is expressed in “Watt”) of this power bank’s internal battery,
Take note that when we are charging up our power bank, the input voltage is 5.0V and we are charging its internal battery that has a voltage of 3.7V. But when we are using the power bank to charge our devices, the output voltage of the power bank through its USB output port is now at 5.0V (The standard voltage at any USB port is always rated at 5.0V). And this time it is using 5.0V output voltage to charge your device’s internal battery which has a voltage of 3.7V. When the power bank is used to charge a device, the electronic circuitry inside the power bank will have to step up the output voltage of its internal battery which is at 3.7V to its output voltage of 5.0V! This is where the discrepancy in the energy capacity of power banks stems out from. Thus, the loss in its energy capacity. Now, take care of your nose for it may bleed. Based on the “Law of Conservation of Energy” which states, “Energy can neither be created nor destroyed. It can only be converted into other forms of energy”. From Number 3 above, “Power” is simply the product of multiplying the energy (in Ampere) by voltage (in Volt). For the purpose of simplicity, Power is also a form of energy that is expressed differently. By applying the law of conservation of energy, the total power of the Power Bank shall remain the same throughout whether it is receiving charge or giving out charge. Given the example above, the total energy content of the power bank (10,000mAh) expressed in “Power” is at 37Wh. Now, this is how we would different the capacity of the power bank when it is being charged and when it is used to charge other devices.
As you can see, the above sample power bank has a battery capacity rating of 10,000mAh when it is being charged. This is also the rating capacity claimed by its manufacturer. But when this power bank is used to charge your device its “theoretical” capacity immediately drops to just 7,400mAh due to voltage output difference. It has to start at 3.7V (the actual voltage of its internal battery) and then steps up its output voltage to 5.0V. This explains the lost in its effective capacity. If we do the computation, this is equivalent to 74% of its original capacity (7,400/10,000 x 100% = 74%). This would translate to a energy lost of 26% (100% - 74% = 26%). But the observable net % is at about 60% for true branded power banks and not 74%. The additional lost in energy is due to inefficiencies in the whole power bank circuitry system. Efficiency in its simplest term is a measure of how “efficient” a system works. The perfect system has 100% efficiency – meaning if we input 100 units into the system, we should get an output of 100 units also. In the above case of the power bank, in its simplest term and not to further complicate things, our input value for the energy we supply to the internal battery is at 10,000mAh, then the theoretical output energy is at 7,400mAh. This 7,400mAh is the “ideal” output capacity due to voltage step up conversion loss (3.7V stepped up to 5.0V) and has nothing to do with efficiency. We then go to the 60% observed actual output capacity which is at 6,000mAh. From the ideal 7,400mAh, we only get 6,000mAh. This is where the issue of efficiency kicks in. Ideally we should expect 7,400mAh, but we only get 6,000mAh. If we do the simple computation, this is how it would looks like:
Take note, some manufacturer will call this 81.08% as % Conversion or Conversion Rate
The above has an efficiency of 81% by taking into consideration the voltage step up conversion loss. This is the more correct (fairer way) to compute its efficiency.
But for simplicity of computation and without taking into consideration the voltage step up conversion loss, we simply use this practical formula to measure the actual output capacity or “quality” of the power banks available in the current market:
Now, you know why we arrive at this 60% efficiency guide for true branded power banks? When we simply divide the actual capacity output by the claimed capacity of the power bank then multiply by 100, we get the % Effective Capacity.
B. As to the second question: Why don’t they just publish the actual rated output capacity to avoid the confusion?
Of all the branded power banks we evaluated, only Sony published their rated output capacity. The rest only publish their internal battery capacity. The only reason I can guess is that manufacturers want to simplify things. If they publish both, then they may have to explain the difference. As you can see from our explanation with regards to this matter, if you are the manufacturer, will you entertain the idea of explaining this energy capacity matter? I guarantee you, it won’t be a short explanation. :D