Moving on to the main question, How does a power bank 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.
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 does a power bank 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.