Lesson 43: All About Power Banks and Its Specs
Welcome back to MIUI Device Classroom! This is Rahul from the MIUI Device Team. In the last class, we learnt All About Wi-Fi by @CLAYTON_WWC. Hope you guys found it informative and useful. Today we will be talking about power banks. Let us begin with some basic info.
A small portable device which comprises a special battery in a special case and a unique circuit for controlling the power flow is known as power bank. In simple words a device which acts as a bank, stores the electrical energy and provides you the stored power on the go.
There is a recent shift in its popularity because the battery life of our devices easily get outstripped. It's the advancement in technology that makes it more power deserving and hence battery life reduces.
The same thing applies for charging via a power bank. It is recommended not to use the phone while charging.
They are capable of charging almost any USB charged device including laptops. It's just the matter of their capacity.
Till date the highest capacity of a power bank is up to 26800 mAh. (Well that's a power house) So till here we learnt about power bank and its various types. Now we will move on to its components. Here it's divided into two parts.
What are the types of Power bank?
Here I have considered 4 types of power bank, but the list goes on with the advancement of technology. So let's categorize them.
Since we want power banks to be light and portable, majority of the power banks make use of Lithium Ion/Lithium Polymer cells as power source. Do note that the same tech is used in battery of your mobile, laptop, music players etc. All around we can say the most of the portable devices that we use these days consist these battery types.
So why are they popular?
Well here a simple weight to power ratio is followed. As per the studies lithium battery can provide 300W per Kg as compared to the less popular lead acid which can proivde only 180W per Kg. So more power & less weight that's what is preferred for a portable device.
Voltage and Current
We need to make sure proper voltage and current supply is provided to device. The nominal voltage of a lithium cell is 3.7V and can go up to 4.2V when fully charged, but the USB ports used has 5V with strict regulation. Therefore we need to change the 3.7V-4.2V to 5V.
Now here comes another question,
How can it be done?
Well there is a Boost Converter. It can provide a steady 5V on its output regardless of the battery voltage being in between 3.7V to 4.2V. Parallel Boost converter is used if more than one cell is used under the Power bank. Its major role is to limit the output current and depending upon the circuit it can also provide different amount of current.
Now we have powersource inside and a circuit. Everything is fine but we need to charge the cell, right? So here the next part comes.
Charging the cell
Proper current and voltage is required to make sure the battery inside the power bank takes a steady or constant current. Here also Boost converter plays an important role. The converters are usually bidirectional convectors. It helps to reduce the Voltage from 5V to 4.2 while charging and do the vice versa while discharging.
Charge indicator and Small circuits
Usually power banks or at least the good ones have an indicator which performs various functions, showing the battery percentage, time left, battery signal etc.This indicator works with a circuit which could sense the battery capacity. The advanced power banks may carry an LCD/LED display which could feature the time left for charge, power load, time etc. These are not necessities for a power bank. A Power bank can work without them, but including them can be quite convenient in providing more information for users.
Ans: Well mAh refers to milliAm pere-hour. It is an unit of battery capacity. It helps in distinguishing the capacity of the battery by describing the amount of power stored at a time.
- How safe it is when we charge phones/tablets with a Power bank?
Ans: It is safe to charge as long as the power bank is providing proper voltage and current to the device.There is a lithium Ion circuit too inside the phone which is almost similar of that of a power bank and it will take care of voltage and reputed Power banks are usually safe to use.
- What is the life span of a Power bank?
Ans: Generally a properly maintained power bank can retain up to 80-90% of its original capacity at 400-500 charge and discharge cycle (charge then discharge = 1 cycle, regardless of whether you charge/ discharge it partially or fully).
- Why can't a 4100 mAh Power bank fully charge a 4000 mAh phone battery?
Ans: All the power banks resemble their capacity in mAh but on most power banks there is an another number quoted too which is Wh(Watt per hour).
Take an example: A power bank has 3350 mAh (that's 3.5 Ah) and has a capacity of 12.06 Wh (and Watt=Amps*Volts), So the battery is 3.5 -3.6 approx. Now generally smartphones and battery of power banks run at 3.7 volts, and as I said before USB ports works at 5 volts, to go from 3.7V to 5V some conversion is required that's Boost Converter. Since Watt capacity is fixed, then going from 3.7V to 5V will decrease the number of amperes that the power bank can supply. That actually means that a 12.06 Wh battery (3350 mAh at 3.5V) can deliver 2412 mAh at 5V. But when energy reaches to phone it is converted again, from 5V down to probably 4.2V or 4.4V. So 12.06 Wh battery (3350 mAh at 3.5V) can deliver 2871 mAh at 4.2V. Each of these conversions to step up the voltage and then to drop it down again causes power loss, anywhere from between 8% to 25% (depending on the design and quality of the converter).
Ans: Each cell has its own capacity which is affected by size, in parallel the capacity is increased and thus it can store more power.
- Why some power banks have more cells than others?
That's all for today's class. Hope you guys enjoyed learning! So let's take break for now and wait for the next class by the Device Team.
Credits:@soumya_2002 for suggesting me the topic.
Thanks to @marcus_keong, @BharatG and @candicesu and all my team for their help & suggestions.
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