Power Banks Frequently Asked Questions

The concept of the word “bank” in power bank has the same meaning as compared to the “bank” in commercial banks that we know. We normally deposit “something” in Banks with the purpose of using that “something” sometime in the future by “withdrawing” it out. Just like Commercial Banks, we deposit our cash, then we withdraw from it in the future if the need arises. The same concept extends to Blood Banks, where we deposit human blood. The human blood is screened and processed, then preserved by the organization/institutions like hospitals, or the International Red Cross. Whenever a patient needs blood transfusion, he or she may request or “withdraw” the blood that he or she needs. With power banks, electrical energy is stored or “deposited” to the internal rechargeable batteries that are housed inside the power bank’s casing . When your cellphone’s battery becomes “low bat”, you can use your power bank’s stored energy (or “deposited power”) by withdrawing it out and use it to charge your cellphone. This is very similar to the concept of transacting with your Commercial Banks, except that power banks don’t involve interest earnings!
A: “mAh” means milli-Ampere hour. It is a quantitative (quantity or amount) expression of “energy” in the form of Electric Current. Milli-Ampere (mA) is a unit that is used to measure electric current or the amount of electrical energy passing through the circuitry or the amount of energy that is contained in a battery. The higher the mAh, the higher is its energy content. Higher mAh would mean the battery can operate your device for a longer period of time. The hour (h) is the hour (60 minutes) that we use on a daily basis to keep track of our time. When the two units (mA and h) are combined into one expression, as in mAh, this would mean that the total energy content in this device can sustain delivering energy of a certain milli-Ampere in the given period of time expressed in hours. For example, if your power bank has a rating of 9000 mAh, this would mean the device can deliver 9000mA of electrical energy and can sustain the delivery of this quantity of energy for 1 hour. To illustrate further in another example, let’s say, the power bank is connected to an equipment (light bulb) that draws 1000mA of electric current, how long can this power bank delivery the needed electric current of 1000mA. The answer is 9 hours. 1000mA x 9 hours = 9000mAh.
Note: 1 Ampere = 1000 milli-Ampere (mA), just like 1 Liter of Coke = 1000 milli-Liter (mL) of Coke.
A: Good question. In order not to get too technical, I will try to explain this in as simple as possible. With basic electricity and applying its principle to the example above, the unit Ampere or milli-Ampere measures the amount of energy that is contained in the battery. The hour is simply the amount of time this battery can sustain delivering the Ampere that will be consumed by the equipment using this battery. To arrive at the unit Wh, I have to explain first what Wh means. W= Watt, h= hour. 1 W = 1000 mW, just like 1 A = 1000 mA, 1 L= 1000 mL. Watt is the unit we use to measure the “Power”. Just remember, for simplicity and for explanation purposes, we use Ampere (or milli-Ampere) to measure “Electrical Energy” and Watt (or milli-Watt) to measure “Electrical Power”. So how is Electrical Energy related to Electrical Power? Be patient, here it goes. We simply multiply the Electrical Energy (expressed in Ampere or A) by the voltage rating (expressed in Volt or V) of the battery to arrive at its Electrical Power. For the given example, the energy content is 2600 mAh and the voltage is at 3.8 V (V= Volts). But before we do the multiplication, we must first use the correct unit. We cannot simply multiply the mAh with V. We must first convert the mAh to Ah then multiply it to V so that both will have the same uniform unit. Before we proceed to getting the power, we have to convert the mAh to Ah first, here’s how it should look:

Then we simply multiply this 2.6 Ah by 3.8 V, which is = 9.88 Wh

Now you know that a battery with a capacity rating of 2600mAh with a voltage of 3.8V will have the same capacity with another battery that has a capacity of 9.88 Wh that has a same voltage rating.

In order not to complicate things, all you need to remember is this. To get the power, simply multiply the energy expressed in Ampere by the voltage expressed in Volt. If one of the component has a “milli” prefix attached to its unit, first convert to remove the “milli” then do the computation.

To make sure you have understood the above discussion, let’s illustrate one more example. If the battery of your smart phone has a rating of 4.44 Wh and has of voltage of 3.7 V, what will be its energy capacity in mAh unit?

But since we are accustomed to mAh units, we will just simply convert the above Ah unit to mAh as shown below:

In simple terms, to convert from Ampere to mill-Ampere, we simply multiply the Ampere by 1000. To convert milli-Ampere to Ampere, simply divide the milli-Ampere by 1000. The same applies to Watt and milli-Watt.
A: Based on the explanation discussed in No. 3 above, and by using simple mathematical operations, we can easily compute for the “ideal” number of charges we can get from our power banks. I said “ideal” and this is discussed in the next FQA. To compute for this, we have to get two parameters. The rated capacity of the power bank and the rated capacity of the battery of the phone we want to charge. Then simply divide the capacity of the power bank by that of the phone’s battery. For example, if the power bank has a rating of 20400mAh and that of the phone’s battery has 2000mAh, then the total number of charges will be:

A: If this was the case, then the power bank that you had bought was “one of those” low cost power banks that over claimed their rated capacity which is a common practice among low quality generic or imitation brands. In order to provide our customers with answer to question like this, Windsor Computer Center has conducted several experiments to evaluate the performance of several selected power banks that are available in the current market. Here is the guide that we recommend to our customers. The best brands (not imitations!) like Sony, Samsung, Sanyo, APC, Huntkey, Bavin, msm.hk, Oppo i-like, etc., will give you approximately 60% of power based on their published rated capacity. The next in line of “more acceptable” brands will provide around 40-50%, while the rest of the generic / imitation brands will only provide 20-30% of their published rated capacity. To illustrate, let’s say we have a true rated branded one (i.e. Sony), with a capacity of 10,000mAh and will use this to charge your phone with a battery capacity of 2,000mAh. Let’s see how many cycles can it provide.

If we are going to use the above low cost power bank you bought with a claimed capacity of 20,400mAh and assuming it has the best performance of 60%, then this is what we will get in terms of effective capacity and charging cycle with a 2,000mAh phone:

But based on your observation, it only lasted for 3 cycles. On this basis, and applying the formula above and working backwards,

In this case, the power bank that you have bought provided only about 29% of actual output instead of the 60% (the highest one can get). In other words, this 20,400 mAh power bank can only delivery 50% (29.41% as compared to 60%) of what it claims. For true branded power banks, they can provide 60% and for this 20,400mAh, it only provided 29.41%!
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

A: With regards to your question, we have these two recommendations. First, rely on the recommendation of your trusted vendor since the evaluation of power banks is quite technical. Second, if you are a technical person, we would be glad to assist and guide you so that you can do the technical evaluation yourself.
A: Yes and no. Generally speaking, higher selling price normally will give you higher quality products. But nowadays, some unscrupulous vendors will sell low quality imitation product disguished as high quality products for a higher margin. Or a few honest vendor selling such low quality products without knowing they are such. They themselves are also victims. The best way is really to rely on the reputation and technical know-how of your vendor.
A: Yes. We have power bank that can charge laptops. It has a battery capacity rating of 33,600mAh. It can charge simultaneously one laptop of almost any brand from Apple to HP, Acer, Dell, Lenovo, Samsung, etc., and one smart phone or tablet. It comes with a LCD display window indicating the output voltage and the % charge status of the battery reserve of the power bank.
A: Yes. The concept is basically the same. The internal rechargeable battery of this power bank has an operating voltage of 12V. This matches perfectly with its output voltage of 12V which is the operation voltage of all automobile starter motors. In addition to this, the current uptake of the car’s starter motor is very high. Such is taken into account in its design so that it can produce an output current of high intensity in the range of about 100-300Ampere! Yes as high as 100-300Ampere! But since the time needed for each engine start is just a second or two, it won’t drain out the battery of the power bank that easily. So, a fully charged power bank for car should be able to at least jump start your gasoline car about ten (10) times and about half for diesel engines.
A: Currently, we carry several branded power banks like Sony, APC, Bavin, MSMHK, Junction one, Alibaba, Oppo i-like. We also carry other lower cost generic brands. We will recommend the power bank that suits your requirement based on your application and budget. If your budget would permit, we strongly suggest you go for the popular brands like Sony, APC, Bavin, etc..
A: Conversion Rate is a technical term used to describe the battery quality of Power Banks. Higher Conversion Rate means higher quality. Higher Conversion Rate means less energy is lost when the energy from the Internal Battery of the power bank is transferred to the external device that is being charged. This topic is discussed in full details in Q&A #6A together with % Effective Capacity.
A: Please find below the table of results for our comparative evaluation of several power banks that are available in the market. The power banks that are shaded in yellow are the true-rated ones which are true to their claims. Our guide for true-rated power banks are those that have a % effective capacity of 60% and above. For the above average ones, our guideline is 40% - 58% (shaded in green) while the rest (average) of the power banks are 19% - 39% (no shade), which constitute most of the population of the lower cost market. The average low cost power banks tend to overstate their capacity rating to make them more attractive to buyers. This is precisely the reason why we conduct experiments so that we can validate or dispute their claim.