## Power Bank Circuit using 3.7V Li-ion Cells

The article presents a handful of assorted power bank circuits which can be built by any individual for their personal emergency cellphone charging functionality. The idea was requested by Mr. Irfan

The Request

Dear sir i want to make a power bank using 3.7v rechargeable cells. Can u help me to make a circuit for power bank. I want the output 5v 2amp... thanks in advance.

Irfan

The Design

### What is a Power Bank

Power bank modules have gained significant popularity today due to their portability and ability to charge any cell phone while traveling and during emergency requirements.

A power bank is basically a battery bank box which is initially fully charged by the user at home, and then carried by the user during the course of a journey. And when the user finds his cellphone battery reaching low, he connects the power bank to his cellphone for a quick emergency top-up charging of the attached cellphone.

### How Does a Power Bank Works

I have already discussed one such emergency charger pack circuit in this blog, which used chargeable Ni-Cd cells for the intended function. Since we had 1.2V Ni-Cd cells employed in the design we could configure it to the exactly required 4.8V by incorporating 4 of these cells in series, making the design extremely compact and suitable for optimally charging all types of conventional cell phones.

However in the present request the power bank needs to be built using 3.7V Li-ion cells whose voltage parameter becomes quite unsuitable for charging a cellphone which also uses an identical battery parameter.

The problem lies in the fact that when two identical batteries or cells are connected across each other, these devices begin exchanging their power such that finally an equilibrium condition is achieved wherein both the cells or the batteries are able to attain equal amounts of charge or the power levels.

Therefore, in our case suppose if the power bank utilizing a 3.7V cell is charged fully to about 4.2V and applied to a cellphone with a drained cell level at say 3.3V, then both the counterparts would try to exchange power and reach a level equal to (3.3 + 4.2) / 2 = 3.75V.

But 3.75V cannot be considered the full charge level for the cell phone which is actually required to be charged at 4.2V for an optimal response.

### Making a 3.7V Power Bank Circuit

In order to correct the above situation and make sure that the 3.7V power bank is able to provide the required 4.2V at a consistent rate until the cellphone is completely charged at this level, a step up circuit becomes imperative.

A simple boost charger circuit could be used for this purpose as explained in the following paragraph:

The inductor L is made by using 5 turns of 22SWG super enameled copper wire over any suitable toroidal ferrite core.

We can see two variable resistors (presets) included in the design, these are required to be optimally set for acquiring the most effective and efficient performance outcome from the boost charger circuit.

The voltage across "C" is the output which is used for charging the external device, and in our case the voltage here must be fixed at around 5V.

The entire setting procedure for the above boost power bank circuit can be learned from the following article:  Calculating Inductor Value in SMPS

The circuit also includes a built-in 3.7V Li-ion charger circuit made up of a TIP122 emitter follower stage, a 5V zener diode and a small 6V/100mA incandescent bulb.

As long as the 3.7v cell charges through the transistor emitter lead, the series bulb remains illuminated and as the cell nears the full charge level, the illumination on the bulb becomes dimmer until finally it shuts off indicating a full charged 3.7V cell.

The 5V zener diode ensures that the emitter voltage of the TIP122 never exceeds the 5V range, which is further pulled down to around 4.3V by the associated series 1N4007 diode.

Another 1N4007 diodes can be seen connected with the base of the BC557 transistor, this configuration makes sure that the BC557 remains shut off disabling the 555 IC stage and boost converter stage during the charging phase of the cell.

The charging input is preferably obtained from any standard SMPS cell phone charger unit.

So the above takes care of a power bank using a 3.7V cell, the AH level depends on the load specs, or the cellphone required to be charged, for smart phones the AH level should be preferably  above 5000mAH.

Although the above design is efficient and recommended for an optimal response, the idea may not be easy for a newcomer to build and optimize. Therefore for users who might be OK with a slightly low tech design but much easier DIY alternative than the boost converter concept might be interested in the following configurations:

The three simple power bank circuit designs shown below utilizes minimum number of components and can be built by any new hobbyist within seconds

Although the designs look very straightforward, it demands the use of two 3.7V cells in series for the proposed power bank operations.

The first circuit above makes use of a emitter follower transistor configuration for charging the intended cellphone device, the 1K perset is initially adjusted to enable a precise 4.3V across the emitter of the transistor.

The second design above uses a 7805 voltage regulator circuit for implementing the power bank charging function

The last diagram here depicts a charger design using an LM317 current limiter. This idea looks much impressive than the above two since it takes care of the voltage control and the current control together ensuring a prefect charging of the cellphone.

In all the three above power bank circuits, the charging of the two 3.7V cells can be done with the same TIP122 network which is discussed for the first boost charger design. The 5V zener should be changed to a 9V zener diode and the charging input obtained from any standard 12V/1amp SMPS adapter.

#### Swagatam Majumdar

Hi Friends, Welcome to my site, a place where you will discover a massive collection of electronic circuit ideas, mostly requested by the dedicated readers and exclusively designed by me for their customized application needs. I have posted more than 1100 circuit designs in this site, if you have a personalized circuit requirement you may feel free to request it through the comment box, if it seems feasible to me then surely you may find it published here with your credentials attached in the post, thanks and please keep reading>

1. Thanks sir.. there is some confusion for me.
1- Is it control the discharge levels of Li-ion cells..
2- is it control the over charging of Li-ion cells...
3- is it delivers the 2amp for smart phone sharge
Once again thanks for the circuit... B-)

1. Irfan, the cell will not discharge unless connected to a cell phone

the cell will not overcharge but when the indicator lamp shuts off you should remove the input supply...

2amp may be obtained if the cell is rated at 3000mAH

2. Also tell where charging pin or female usb connector is attached...?
and which point is used to connect the cells...
And how much cell we connect in this circuit...

1. Irfan, Everything's shown in the diagram, I am sorry if you are not able to understand the diagram then it would mean that you are very new in the electronics field, and this circuit can be extremely difficult for a new comer...so I think you should rather try the other 3 circuits instead of the first....

3. Thanks sir for this circuit
What is the output amp in 1st circuit
I want 1 amp output from 1st circuit

Thank you very much

1. Mayank, you can calculate and change the 0.6 ohm resistor for fixing the desired current capacity of the circuit...accordingly the cell mAH will also need to be upgraded to the intended level

2. the formula for calculating the resistor is

R = 0.6 / max current output (amps)

3. sir this circuit can give cAPAcity of 10000 mah ?

4. Sir the circuit above is so interesting.

Is there any idea that can modefie or improve the above circuit like for example adding of a small solar panel to charge up the battery 3.7v.

1. Thanks Angelous, yes definitely you can use a suitably rated solar panel and use it to charge the 3.7V cells.

2. sir why did you used bulb? it is possible to change that into the red LED?

3. Bulb will pass sufficient current for charging the battery and also indicate the situation, LED will not pass sufficient current and will not serve purpose, it will remain lit forever, not allowing the battery to get charged.

4. LED will require an external circuit for the indications... which can make the circuit more lengthy...

5. i have been waiting for so long for this kind of circuit
i will definately build it sooner n give updates thanks for this BOSS SWAGATAM

1. you are most welcome Adesina!!

6. good post sir, what is the max current of the last current control circuit, can we build boost converter using attiny micro controller and a 7 segment display interface , i want to increase efficiency because i used 6*1.2v 2700mah which doesent charge my 2000mah android a single time the 7805 getting heated up and a lot of power is wasted there, i bridged 2 7805 ics together but there is no improvement the charging current is very low 150ma i connected directly 4 cells without any regulator and the phone got charged with with 1500ma current but due to voltage fluctuations the charge controller in the mobile disconnecting the power source. any help is appreciated thank you sir.

1. Thanks Seelamsetti,

just use 4 of the 1.2V cells in series and connect the output directly to your cellphone, no need of using a 7805 IC, as shown below:

yes MCU can be used for making a power controller circuit

2. ...if your cellphone is disconnecting the power due to fluctuations then you can add another pack of 4 cells in parallel or try bigger 1.2V cells

7. Hi sir,
Can you please help me to make a 3.7v mobile battery level indicator, I want to do this by using ATtiny85, for this I have use concept of voltage divider, so I have taken Vin as 4v, R1 as 130 ohm and R2 as 910 ohm for 3.5v Vout.
Thanks.

1. Hi Pawan, sorry, I do not know how to do it with a ATtiny85....

8. Hi Swagatham
For Ni-Cd cells, the charging current is calculated by deviding 'Battery capacity Ah/10'.

How can I calculate the safe charging current and charging timeof Li-Ion and Li-Po batteries....?

1. Hi Anil,

as a rule of thumb for Li-ion and Lipo batts you can use a charging current as high as their AH level...so for example if the AH of the battery is 2AH then the charging current could be 2amps, but the temperature of the batt could be crucial and will need to be monitored manually or by some automatic method

9. Sir this powerbank is 3.7V to 5V/2A ? Right? So how about if the battery is at 4.2V What will the output .. ?

and also i want to rem0ve the bulb ...and replace it with led and resistor c0nnected to supply batt. Or output ? How can i done this?

1. Romeo, you can adjust the PWM pot to get a precise 4.2V

the bulb is a safe and an easy option to verify the charging condition...LEd indicator will not work in that position and might require a complex circuit using an opamp

10. Sir curcuit is good but a bit complicated
And also please tell me which type of bulb used in curcuit
A tourch bulb ?

1. Prince, you can use a 3V torch bulb rated at 200mA or above.

11. And also please tell me how much volt requierd for charging a 18650 cell

1. it's same as described above...4.2V

12. I would like to know what are the theories used in creating a power bank?

Thank you.

13. respected swagatam

my powerbank charging only 10 scecond then auto cut off why?
if i can make another one with its 2 battery (3.7v*2ps)--if that batteries are sutable for ur circuite used LM317 ic --is this ic will heat-

1. Hi AZ, check the voltage with a voltmeter while the cell is charging, it could be reaching the full charge value quickly...this may be because your charging current is high,,,,check this first

14. sir how i can check a 7805 ic

1. you can do it by actually supplying an input voltage across the specified pins and checking exactly 5V across its output pins

15. Sir! you have mentioned the no of turns and the gauge of wire used for inductor. what is the value of inductance?
or it can be made just by giving five turns of 22SWG on ferrite core?

16. How much baat including in perelal and improve the nex by nex Mobil charj

17. Can i replace 3.7v battery to 6v lead acid? It is compatible??

1. yes you can do that

18. Well done sir, I tried to charge my power bank with combination of three cells from laptop battery, I don't know their capacity. So my power bank leds that used to be off, now is always on blinking as if it's charging whether is not connected to a charger. Please do you know what might have happen to my charger and how-to rectify the problem?

1. Arhyel, what is the input specification of your power bank?...this specification will help us to know how much input is required to be fed safely to the unit...if it was more than then your PB could be damaged

19. Thanks a lot sir, nice design. I can't find the charging unit and battery connector on the 3rd simply diagram.

1. thanks Ochin, the charging of the 3.37V cells can be done through any standard adapter externally.

in the last paragraph I have mentioned the word "LM317 charger" with reference to the cellphone which is supposed to be charged using the shown emergency power packs.

the battery of the power pack needs to be charged from an external charger which could be your cell phone charger unit

20. sir can u pls tell me the inductor value thanks in advance

21. Sir IAM USING LGABB41865 BATTERYS
OF 5 BATTERYS SINGLE BATTERY HAVE
2600mah TOTAL 5 BATTERYS mah of 13000 mah because please give circuit for that power bank charging and out of 1amp and 5volts please help me sir

1. give all the specifications correctly

22. Sir there is a large power loss in case of second circuit using ic 7805. Is there any way to minimize the loss?

1. you can try a buck converter circuit instead

23. Hello Sir I'm now in 5th from ECE branch i wish to be design a Power bank. Sir... Of 10000mAh capacity Sir... Help me Sir

1. Hello, please provide all the technical details of the power bank, as per your specifications.

24. THANK YOU SO MUCH SIR
FOR YOUR USEFUL CIRCUITS AND EXPLAIN .

25. Sir, what's the rating for the transistor that's suitable to stop the power bank from turning of while a phone is charging (the voltage of the 5 power bank battery is 3.7v each )

1. Chinwike, I could not understand your question...which transistor are you referring to??

26. sir... is this circuit using cmos version of 555. from the sheet normal ne555 need minimal voltage 4.5 volt... or did you ever succeed with normal ne555

1. 7(555) indicates a CMOS version, please refer to the diagram and see the number on the IC.

27. Hi. My power bank circuit is complaint. Its battery is 3.7v but the output is required 5v1A. .plz help me..plz given a simple booster circuit diagram.am not a professional. .

1. you can use any IC 555 based boost converter circuit for this....I have already posted this in my website...you can search it using the search box at top right.