A portable power bank has become an indispensable accessory for all of us. As long as battery technology does not improve drastically, our smartphones and other electronic gadgets will continue running out of power at times of need. And we will become more and more dependant on these power banks. But have you ever wondered how these power banks work?
A power bank or a portable charger works by first taking power from an external source like a wall power outlet. It then stores this power in the form of electrical energy in a rechargeable battery. Then it uses this stored energy to charge or power up the internal battery of other electronic devices like a smartphone or a tablet, thereby discharging its own battery.
Sounds simple, isn’t it? But you will be surprised to know that to achieve this charging, storing, and discharging cycle, a number of sophisticated electronic components are used.
And yes, all these components are nicely tucked inside that rectangular brick-like structure of the power bank.
Main Components of a Power Bank
Now that you know how a power bank works, you must be aware that the most important component of a power bank is the rechargeable battery pack inside it. And since it needs to charge itself as well as other electronic equipment, there has to be additional electrical circuitry to achieve this.
The main components of a power bank include a rechargeable battery pack, input and output ports, charging circuit, discharging circuit, battery protection and monitoring circuit, controller, switch, and indicators. Sophisticated power banks may also include components like temperature and current sensors for additional safety measures.
Here is a block diagram that shows how the different components are arranged in a power bank to make it work. Each component has a specific job to do and all of them working together as a unit is what makes the power bank function.
Let us go through these components one by one to understand how a power banks works.
Rechargeable Battery of a Power Bank
A power bank is basically a rechargeable battery with accompanying electronics to charge and discharge it. Hence the battery pack inside it is its most important component.
Nowadays, all rechargeable batteries used in power banks are Lithium based. It can be either Lithium-Ion or Lithium-Polymer.
The main reason for choosing Lithium based batteries are:
- High energy density: Large amount of electrical energy can be stored in a relatively small battery
- Low weight-to-capacity ratio: The weight of the battery is relatively less compared to other similar battery technologies, having the same capacity
- Low self-discharge: The rate of self-discharge is lower than other competing battery technologies
The above three characteristics of Lithium-based batteries are what helps in making high capacity power banks portable enough to be carried in your pocket.
In case you are curious, batteries used inside mobile phones, tablets, and most other electronic gadgets are also Lithium based.
But which is better? Lithium-Ion or Lithium-Polymer? It turns out, both have their own advantages.
Advantages of using Lithium-Ion batteries
- Has more charge-discharge cycles compared to Lithium-Polymer. This means the battery will last longer before it starts to lose its efficiency.
- The manufacturing process is less expensive compared to Lithium-Polymer. This helps in keeping the price-performance ratio of the power bank low.
Advantages of using Lithium-Polymer batteries
- Has higher energy density compared to Lithium-Ion. This enables to pack a large amount of energy in a relatively small form-factor for the power bank.
- Less chance of explosion, compared to Lithium-Ion. This helps in increasing the safety factor of the power bank.
The better the quality of the battery used, the more efficient the power bank will be. A good quality power bank from a reputed manufacturer will hold almost 80% of its full charge even after 3 months. Whereas, a low quality power bank will discharge itself within a month. In fact, this charge retention capability is an important criteria to determine the quality of a power bank.
Most power banks will have one or more rechargeable batteries connected in serial or parallel. The configuration will depend on the desired energy output for the power bank.
18650 Lithium-ion cell is a popular battery model used in many power banks. It has a nominal voltage of 3.7 volts and a capacity of 2500 mAh. So to construct a power bank of capacity 1000mAh, four 18650 Lithium-ion cells can be connected in parallel.
Most high quality Lithium batteries can withstand more than 1000 charge/discharge cycles, and still be able to hold 80% of its initial capacity. This is the main reason why these are used in all portable power banks nowadays.
The Battery Protection and Monitoring Circuit
Lithium-based batteries (especially Li-ion batteries) are tricky to charge. If the voltage goes beyond a certain threshold (this is typically 4.2V for Li-ion), the battery temperature starts to increase.
This can lead to the battery catching fire, or even explode, causing serious damage. Ever wondered why there are certain restrictions on carrying power banks in flights? This is the reason.
As such, the battery pack in a power bank is connected to a Battery Protection Circuit.
The duty of the circuit is to continuously monitor the input and output voltage and current of the battery. The battery protection circuit can also have a temperature sensor for added protection.
Main functions of the Battery Protection Circuit in a Power Bank
- Disable charging when the battery voltage level goes above a certain threshold (overcharge protection)
- Disable discharging when battery voltage level goes below a certain threshold (over-discharge protection)
- Disable discharging on detection of short circuit (short circuit protection)
- Disable charging/discharging if the battery or circuit temperature goes above or below their standard operating range (temperature protection)
This circuit can also have additional features, like monitoring the remaining charge in the battery. Usually, it is connected to LED indicators that can display the remaining charge in the form of bars or dots on the body of the power bank.
Some high-end power banks have dedicated battery monitoring circuits with built in meters that can display voltage and current levels, and also remaining charge in the form of percentage. Some even display the amount of time left to completely charge and/or discharge the battery.
The Charging Circuit of a Power Bank
The charging circuit in a power bank is responsible for charging the rechargeable battery pack. But why do we need a dedicated circuit to charge a rechargeable battery?
This is because Lithium-based batteries cannot be charged by a regular power supply. If you fail to charge the battery following certain rules, its operation can be impaired. In worst cases, the battery may catch fire and even explode. So then, how do you charge a Lithium-based rechargeable battery?
There is a basic algorithm for charging a Lithium-based battery. In the first phase, the battery has to be charged at a constant current until the voltage reaches a maximum value (usually 4.2V). Then in the next phase, the voltage is held constant, and charging current is gradually lowered to about 10% of the initial charge rate, called the termination current. The maximum voltage and the termination current vary slightly depending on the model and the manufacturer.
The first phase charges almost 80% of the battery fairly quickly. The second phase which charges the remaining 20% is a fairly slow process. This is the only way of safely charging any Lithium-based battery. The charging circuit inside the power bank implements this charging algorithm.
As the charging happens through a USB input port, the input voltage is usually 5V. The charging circuit consists of a buck converter module that converts the input voltage to either a constant current or constant voltage, as per the charging algorithm of the Lithium-based battery.
The Discharging Circuit of a Power Bank
As the name suggests, the discharging circuit handles the discharging of the rechargeable battery inside the power bank thus generating an output power that is used to charge other electronic devices.
The nominal voltage of a Lithium based battery is 3.7V. However, electronic devices like cellphones and tablets need a source that can deliver 5V. Just like the input port, the output port in a power bank is usually USB which has a standard voltage of 5V. So how do you think this can be achieved by the power bank?
To reach an output of 5V, the voltage level has to be stepped up. The discharging circuit consists of a boost converter module that does exactly that.
The discharging circuit can also limit the current depending on the usage and hence can deliver higher output voltage too. For example, if the power bank is meant to charge a laptop, then its discharging circuit can be designed to output 12V or 19V, as per the laptop’s charging requirement.
Controller And Switch of a Power Bank
The switch in a power bank is attached to a controller which actually performs some of the user controlled functions of the power bank. These include:
- Start charging an external electronic device by turning on the boost converter in the discharging circuit
- Automatically turn off the boost converter when it detects no current is being drawn by the external device
- Display the battery level during charging and discharging
Charging and Discharging Ports
Most power banks use USB ports for input and output. Usually, Micro-USB port is used for input and USB Type-A port is used for output.
Nowadays, USB Type-C is quickly becoming the standard as it can be used both for input and output. Another advantage of USB Type-C is that it supports the USB PD (Power Delivery) specification for high speed charging and discharging.
Conceptually, a power bank works like a rechargeable battery that can charge itself and then can be used to charge other devices. But even the most basic power bank has some complex circuitry inside it. From the above explanation, you can see that apart from its most important component – the rechargeable battery, a power bank has a lot of other electronic components that work in tandem to make it function as it should.
With the advancement in technology, the power bank will simply become more and more efficient and portable. However, the basic working mechanism will remain more or less the same in the near future, until of course, some new invention in battery technology significantly changes how we use our electronic devices.