What’s power bank?
Power bank serve as an ‘extra battery’ or external charger for your phone or other electronic devices.

Where is power bank made from?
Majority of the power banks in the market are actually made in China.This post is my attempt to shed some light on the never ending questions related to choosing the right power bank for your gadgets. You may consider this as a poor man’s guide in determining the power conversion efficiency of a particular power bank. If you can share some interesting facts, post your comments below so that I can update this post for the benefit of every power bank users.

To begin with, the first thing that a power bank user should know is the power requirement or the input rating of his/her gadget. Normally, you can get this information from the charger that comes with the electronic device. Input rating has two parts – voltage (V) and current (A). Typically, smartphones and tablet has an input voltage of 5V while input current (A) ranges from 0.5 A to 2A. The next step is to determine your gadget’s battery capacity which is often expressed in mAh that reads milliampere-hour. You can see this information printed on your device’s battery or the manual that comes with your device. The input rating of your mobile device is very important because this is critical in choosing the power bank with a corresponding output rating that matches the input voltage and current. Choosing a power bank with an output rating higher than the input rating specified on your smartphone can damage the internal battery.

Summary:First thing to know is the battery capacity of your gadget and the input rating. You will use these information in selecting the right power bank for you. Also, please avoid charging your device or power bank with charger whose output rating is higher than the input rating required.
Once you have the battery capacity information and the input rating of your tablet or smart phone, you can then choose which power bank will suffice your needs. This is done by asking for information like capacity and power conversion efficiency. Similar to the battery found in most of our gadgets, the capacity of power banks are expressed in mAh. Power conversion efficiency is derived from rated capacity which is always lower than the power bank’s battery capacity. The reason behind this is that there is an energy loss during the power transfer from the power bank to your smartphone or tablet. To explain further, batteries inside a power bank has a nominal voltage of 3.7 V or 4.2 V when fully charged. To charge a 5V device, the 3.7 V or 4.2 V should be converted to 5V hence, attributing to energy loss.
Note: The battery voltage inside the power bank drops from 4.2 V to 3.7 V during charging so it is better to get the average efficiency using these values. Please check my power bank review to give you an idea.
Estimating power conversion efficiency can be computed by multiplying its capacity by 3.7 (or 4.2) then divide by 5. This is done to get the rated capacity at 5 volts. The result that you will get should be greater than the battery capacity of your smartphone or tablet. But take note that, here, what you see is not what you get. In most cases the output will be less and you will only know the performance of your power bank after using it for two to three times. But always remember that efficiency should not be less than 80%. I think that power bank should be at least 85% efficient. So in this case asking for product warranty is very important. Some of the power banks I knew comes with a minimum of 3 months up to 6 months warranty. While there are power banks with 1 year warranty period, the cost is much higher.
Summary:Multiply the capacity (mAh) of a given power bank by 3.7 then divide by 5 to get a rough estimate of its rated capacity. Ex. Mobile Power 4000mAh power bank has a (4000 * 3.7)/5 rated capacity which is equal to 2960mAh. If you’re planning to use this on your smartphone with 3000mAh battery, then you’re good to go. It will at least give you 98% efficiency. Always remember though that it is normal for a power bank to produce less than the rated capacity because of several factors. But keep in mind that the conversion efficiency should be not lower than 80% and that most power bank operates at 85%.
The best thing that you can do is to try charging your mobile device with 2% charge left with your fully charged power bank and see how much charge the power bank can produce. After depleting the power bank see how much charge is left on your mobile device. Then calculate that percentage in mAh to get a rough estimate of the transferred power.
Example:
A given power bank has 2500mAh capacity.
Rated capacity @3.7 = (2500*3.7)/5 = 1850mAh;
Rated capacity @4.2 = (2500*4.2)/5 = 2100mAh;
After using the power bank, 2% battery charge becomes 52% on your mobile device with a battery capacity of 4000mAh.
52% – 2% = 50%; 50% of 4000mAh = 2000mAh; @3.7 efficiency is >100%; @4.2 efficiency is 95%

I think that there’s no such thing as 100% efficiency unless you’re using a power bank whose output capacity is greater than the battery capacity of the mobile device used during the test. In this case, we can assume that the nominal voltage is not 3.7 volts but higher or the power bank is really performing well. This is just an example so real test results should further confirm its performance. If you can’t find any review of a certain power bank, you could at least ask for a money back guarantee or product warranty. I hope you find this post helpful on your quest for looking the right power bank for you.

Have a nice day!


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