Saturday, December 19, 2015

FAQ-4 How do power banks work?

Good evening everyone, as a continuation of this frequently asked question post I will be writing about something pretty new, interesting and helpful too. I wrote about how to make this device but I think it is better to write another separate one just giving the idea of how it works, so here it is how power banks or portable mobile/laptop chargers work.


Before jumping into how it does what it does, let's see what it does. Power bank is essentially a small portable device that stores energy and provides you with that while on the go. Usually most small power banks provide 5V on an USB port so it is very convenient to charge any mobile device although fast charging might not be available on most of them but it will get the job done. Most of them provides about 1A of current some can give 2.1A of current, depending on the model. There are however some slightly larger ones that has the ability to power small laptops too, they usually can store more energy so that they can provide more energy. That's pretty much what they do.

Picture taken from Internet

To understand things a bit better, let's take a look at a block diagram of how these devices work, after that we will see some of the major and minor components of it. 


Major components:

Moving on to the main question, how does it work? To my knowledge all of these devices use Lithium Ion/Lithium Polymer battery as their power source, so in the inside they have these type of batteries, remember this is the same type of battery that is inside of your phone, tablet, laptop or most of the portable devices that we use these days. These cells are so popular and widespread due to their high weight to power ratio, Lithium batteries can provide about 300W per kg where the other ever popular lead acid can only do 180W per kg. So more power, less weight that's what is needed on a portable device.

So we have got the battery for such devices, lithium ion or polymer, now we need to make sure it can provide proper voltage and current to the device. Lithium cells has a nominal voltage of 3.7V and can go all the up to 4.2V while fully charged, but the USB ports that we use has 5V with strict regulation. So we need to change this cell voltage to 5V. How can it be done? Usually it is done by a boost converter that provides a steady 5V on its output regardless of the battery voltage being at 3.7V or 4.2V or anything in between them. If more than one cell is used, what I have seen is manufacturer uses them in parallel, so they can use the same circuit just have better service time. Although the one that I made, I used two 18650 cells in series and use a buck converter to lower the voltage to 5V. Either way output voltage is 5V, we got it on USB so we can charge any device. Another important thing about this boost converter is, it usually limits the output current too, depending on the circuit it can provide different amount of current. In some cases, where it is suppose to charge a laptop however, the output voltage can be 12V or 19V depending on the model or depending on the device that it's manufacturer is aiming at. So we have a power source inside and a circuit that can provide our desired voltage and current, everything is fine so far but we need to charge this cell right?

So next part, charging the battery that is inside of them. Charging a lithium ion is a bit tricky. You have to make sure to put proper voltage and current in and change the charging profile when necessary and you have to control the voltage otherwise it can blow up! Lithium ion takes a steady/constant current up until they reach their 60-70% capacity then it charges at a constant voltage, steadily declining the input current to them. So you need a charging circuit that have such ability. Also it is safe to use a charging current that is half of it's amp-hour rating. For smaller capacity power banks, usually an USB input is provided for charging it up, as smaller capacity devices have smaller cells it is easy to top them up with a smaller current from any USB port. Larger capacity ones however include their own power brick or AC adapter with them to charge it from AC main line.

Now we have a working power bank that can take in power from USB or an AC adapter(similar one that is found with any phone, tablet or laptop, but might have a different output voltage) and charge a battery inside it and can power your device using that stored energy. Done? Not really, there are some more bits and pieces of stuffs in their. Let's talk about them.

Minor components:

Usually these devices, or at-least the good ones have indicator that shows the battery percentage, so you can see how much power it has left or should you charge it or not. So we need a circuit that can sense the battery capacity and show us by maybe a multi-color LED or maybe a bar graph or maybe with a LCD. This circuit will also tell us when it is fully charged so we can unplug it. Some of these devices have some built in meters for measuring voltage and current, usually on the output section and they use LCD or LED display to show that. Some more advanced ones can even show how much time left for it to power the load that is connected to it. Without these circuits, the power bank will still work but they make stuffs a lot easier and more suitable for using.

There are some more circuit to ensure the safety of these devices, like temperature sensor or current sensor. What it does is while charging or supplying a load it keeps on checking the battery or the circuit temperature to ensure that both of them are within their standard operating range, if something goes sideways it automatically turns itself off. They also measure output current too to see if the output current is within it's limit, if not, it will automatically turn off. Another protection circuit is there to protect the battery from going below a certain voltage level, some battery has this protection built in but if it doesn't have that circuit it might be added to the design to ensure good health of the battery.

This is pretty much how power banks work, but all the circuit blocks that I have mentioned here might not be available in many designs but reputed manufacturer will have them for sure. There are huge amount of power banks available with many different capacities and it is pretty overwhelming in most cases to select the perfect one. Yet simple advice go for the reputed manufacturer or the one with better warranty policy. Some power banks might come with torch light too, get them if you find it necessary.

Some power banks might have wireless or induction charging that uses different circuitry to make magnetic field to transfer power, This post is already kind of long so won't get into details. 

More questions:

Q1: Capacity that is printed on these devices, is that true or false?

A1: Usually they lie about the capacity, you will see a lot of very small and thing 20k-30k mAHr rated power banks that are very cheap, open them up you will be greeted with maybe 4k-6k mAHr rated cell inside. With those cells there is no possible way for them to provide that much power. Cost is another thing, lithium ion is not cheap, so no one can just shove huge amount of power in those power banks without increasing the cost.

Q2: How can I know if my power bank has proper rating or not?

A2: If it is from a reputed brand, probably it is telling you the truth, if not it might not be true. You can always open up the power bank to see and test the actual battery or just charge it from 0 to 100% by using a known source, if a 20k mAHr power bank fully charge within 3 hours via a 500mA USB port, you know it is fake.

Q3: Charging phone or tablet using these power banks, how safe it is?

A3: It is safe, as long as it is providing proper voltage and adequate current with least amount of noise, it will be fine. There is a lithium ion charging circuit inside of the phone too, the same or similar one that is inside of the power bank, and it will take care of that 5V power and usually reputed manufacturer's power banks are safe to use.

Q:4 If I connect the output of it to the input of it will it keep on charging itself for infinite amount of time?

A4: No, these circuits make heat, so they waste power and for that the lithium battery will be depleted after some time.

Q5: Which one is best for me?

A5: Ask yourself, what kind of device you are planning to charge, how many times are you planning to charge, what form factor suit you best. After getting all those answers you will know what to buy. If you want to charge a tablet with a 4000mAHr battery, there is no point buying a 3000mAHr power bank, because you won't even be able to charge once, but hey if form factor is the issue, go ahead and buy, you will still be able to get some.

I think that answers most questions that I found on the internet and in real world if you have any more question feel free to ask.


1. Boost Converter.
2. Lithium Ion Battery Charging.
3. Battery Capacity.
4. How chargers work.
5. Making a portable charger or power bank.
6. My other posts.
7. Inductive Charging.

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