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Li-Ion Battery Charger Circuit Using IC 555

Li-Ion Battery Charger Circuit Using IC 555

The following post explains a straightforward yet precise automatic Li-Ion battery charger circuit using the ubiquitous IC 555.

Charging Li-ion Battery Can be Critical

A Li-ion battery as we all know needs to be charged under controlled conditions, if it's charged with ordinary means could lead to damage or even explosion of the battery.

Basically Li-ion batteries don't like over charging their cells. Once the cells reach the upper threshold, the charging voltage should be cut off.

The following Li-Ion battery charger circuit very efficiently follows the above conditions such that the connected battery is never allowed to exceed its over charge limit.

When the IC 555 is used as a comparator, its pin#2 and pin#6 become effective sensing inputs for detecting the lower and the upper voltage threshold limits depending upon the setting of the relevant presets.

Pin#2 monitors the low voltage threshold level, and triggers the output to a high logic in case the level drops below the set limit.

Conversely, pin#6 monitors the upper voltage threshold and reverts the output to low on detecting a voltage level higher than the set high detection limit.

Basically the upper cut off and lower switch ON actions must be set with the help of the relevant presets satisfying the standard specs of the IC as well as the connected battery.

The preset concerning pin#2 must be set such that the lower limit corresponds to 1/3rd of the Vcc, and similarly preset associated with pin#6 must be set such that the upper cut off limit corresponds to 2/3rd of Vcc, as per the standard rules of the IC 555.

How it Works

The entire functioning of the proposed Li-Ion charger circuit using IC 555 takes place as explained in the following discussion:

Let's Assume a fully discharged li-ion battery (at around 3.4V) is connected at the output of the below shown circuit.

Assuming the lower threshold to be set somewhere above the 3.4V level, pin#2 immediately senses the low voltage situation and pulls the output high at pin#3.

The high at pin#3 activates the transistor which switches ON the input power to the connected battery.

The battery now gradually begins charging.

As soon as the battery reaches full charge (@4.2V), assuming the upper cut off threshold at pin#6 to be set at around 4.2v, the level is sensed at pin#6 which immediately reverts the output to low.

The low output instantly switches off the transistor which means the charging input is now inhibited or cut off to the battery.

The inclusion of a transistor stage provides the facility of charging higher current Li-Ion cells also.

The transformer must be selected with voltage not exceeding 6V, and current rating 1/5th of battery AH rating.

Circuit Diagram

Li-Ion Battery Charger Circuit Using IC 555



If you feel that the above design is much complex you could try the following design which looks much simpler:


Li-Ion Battery Charger Circuit Using IC 555

How to Set up the Circuit

Connect a fully charged battery across the shown points and adjust the preset such that the relay just deactivates from N/C to N/O position....do this without connecting any charging DC input to the circuit.

Once this is done you can assume the circuit to be set and usable for an automatic battery supply cut off when fully charged.

During actual charging, make sure the charging input current is always lower than the battery AH rating, meaning if suppose the battery AH is 900mAH, the input should not be more than 500mA.

The battery should be removed as soon as the relay switches OFF to prevent self discharging of the battery via the 1K preset.

IC1 = IC555

All resistors are 1/4 watt CFR

IC 555 Pinout


IC 555 pinout


About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

84 thoughts on “Li-Ion Battery Charger Circuit Using IC 555”

  1. Howdy, Friend! Interested to Learn Circuit Designing? Let's Start Discussing below!
    • Hi Vishwanath, you can easily procure a 6V crank flashlight and use its output for the charging purpose if possible I’ll try to update the design soon…

  2. Hi… This is Suma here… I am trying to make Li-Ion Battery Charger Circuit Using IC 555 and I found your circuit relevant.. But after connecting I am getting many doubts.. Please clear my doubts…
    In the following circuit, I am not getting how much voltage is supplied to the 3.7V Battery while charging? The output from the bridge rectifier output will be how much?
    For 78L05 regulator to give 5V output minimum 7V we have to give input to it. The same voltage is given to the battery positive terminal. 7V is given to battery for charging?

    • Hi, yes in the first circuit 7V would reach the Li-ion cell but due to the 2 ohm resistor the current would be sufficiently reduced, which will mean the voltage dropping to the required safe level for the battery, and as it begins charging and reaches the 4.2V level, the IC can be expected to cut off the supply and stop any further charging.

      if you are not comfortable with the 7V, you can modify the design using zener and resistor as done in the last diagram.

    • you can use a 4.7V zener and a 5V input, alternatively the 7V can be reduced by adding 4 series 1N4007 diodes for the battery

  3. Hi sir,
    Sorry i've one more question :
    1. If i change the supply voltage should i change the zener diode too ?
    2. i've drop voltage from 5v to 4.3v with plcaed 1N4007 diode after relay, is it correct ?
    Tks in advance

    • Firkan, No change would be required, the zener value can remain as indicated in the diagram.

      yes a 1N4007 will drop around 0.7 V….. but a diode a is not actually necessary

  4. Hi
    Li-ion battery have some charging state initially it charges through constant current then after it will start charging through constant voltage of 4.2V. In this circuit you cut the charging when battery reaches to 4.2V (during constant voltage stage) then how it will charge the battery??

    • Hi, the above charger is presented in its basic form, which may not be 100% efficient but sufficient to charge the cell to at above 90% level.

      cutting of at the full charge level was important for ensuring a risk free charging…. however for enabling a trickle charging at 4.2V the relay's N/C could be configured appropriately.

  5. Hello sir
    My laptop charging circuit is damaged and its battery interfacing- circuit also . I have no idea or schematic to repair it ( 6 cell battery protection circuit ) plz give me link if any . And
    Which circuit I need to charge this 6 cell li ion laptop battery of 11.1 volt , 4400 mah for charging it externally

  6. I Want to built auto cutoff charger for lithium ion battery 11.3v 3Ah with led indication can u help me out sir please

  7. sir,I want to say that my power bank is 10000mah but problem is that when we connect the led to the pcb circuit and battery connection after that pcb board become heat and sometimes led will glow continously.so please share me your suggestion how to check or improve this problem.

  8. Thanks a lot for the clarification. Maybe you need to indicate in the post, the CMOS 7555 requirement, since the NE 555 needs minimum voltage of 4.5V.

  9. Hi, thanks for your posts. But looking at the first circuit, I wonder how the 555 gets powered initially for the sensing to work, since the 7805 ground is not connected until the transistor is on. May be I am missing something please can you clarify?

    • Hi, it will get the power from the battery side and use 3.7V for the 555 IC for the detection…the IC should be preferably IC 7(555) so that it can work with 3.7V supplies also

  10. Hi Swagatam thanks for the circuits. I need a circuit charger for my lithium one cell battery 100ma which have dropped it's voltage. A guy told me I have to charge it with 1/10 of it's amp capacity which means 10ma. Can I modify the 555circuit?

    • Thanks Aborg, Lithium cells can be charged at their full AH rate, meaning you can charge your cell at 100mA rate also, however at 10mA it will be much cooler…. but might take around 10 to 14 hours to charge fully.

      you can try the second 555 circuit for your application. use a 10 to 50mA current input for the recommended charging procedures.

    • Dear Movies-point, first apply the lower threshold to the circuit from the battery side, then adjust the pin2 preset to just cause the pin3 to become high….then without removing the supply increase the voltage to the higher threshold level and adjust the pin7 preset until the output just becomes low

  11. Dear Swagatam majumdar. I visit your site a many time your work is awesome. I keep your site ay my bookmarks. . Now i want to build a circuit which could charge a lithium cell of laptop battry from main 220v ac whithout transformer. I have a lot of lithium cell of laptop battries in good condition. I want to fit them in a handy tourch . Could you please help me to build a simple and cheap circuit which donot over charge and a full charge indicator. Please help me . I will be thank full ….. Imran Arain from pakistan

  12. Plese help me. I have 4.2v battery 2300MA. And i meke this circuit. But not working.. And i meke it expert regulator.. Is this problem? , powersupply input voltage is 4.96 and output 4.96 (no lord ) and put battery now output voltage is 4.12.. But not charging… Ic is damaged or i didn't put regulator? What's the problem?

  13. Dear Swagatham
    I have 2 Li-Ion batteries (3.7v, 2500mah) removed from my Samsung mobile, when it became unusable in mobile.
    I removed the tiny circuit board from the battery. Now the battery I am using for testing my hobby circuits. The battery can store charge for long time.
    I need an automatic charger circuit with fullcharge cut-off and led indication. My idea is to charge both batteries in series (7.4v).
    The battery do not discharge through the charging circuit, when it is fully charged.
    You have published many Li-ion charger circuits. Which one is suitable for me.

    • thanks Melisa! the NO contact is anyway not required, so a single N/C should do the job, you can try with it.

      just connect the N/C and the pole of the relay as shown in the diagram.

  14. a 3.7v liion battery should give charge at 4.2v, but this circuit construction i always get more than4.2v and this is not good for liion battery that what i have learned. with input voltage change output voltage also change, how to fix that problem?and how to make output voltage exact 4.2v..thanks

    • Bhanu, I would suggest you try an opamp based circuit from this blog, the above circuit will be again difficult for you to set, it can be used for 12V battery

  15. Hi,
    Can I connect the solar panel instead of 220V mains input?
    while my solar is 19v, can it be connected to D2-D5 section?

  16. Dear, Swagatam

    Sir, Can i replace BD139 transistor with
    ant NPN transistor? i have C1904 here… and what is the use of two 10k VR resistor there? to adjust the voltage ??? thank yoy.

    • Dear Darel,
      you can use any NPN which can handle in excess of 1amp current, so please confirm the same through their respective datasheets.

  17. hello sir i am following your circuit design from many days.here you have used ne555 ic as a comparator. in your another post LM741 opamp was used to charge battery. but in the both circuits i have noticed the transistors used(here bd139, or tip122) emiter and collector terminal both connected to negative to the power source. how transistor worked then? please explain.

    • helo skl,

      the transistor acts like a switch in series with the negative line of the circuit.

      It toggles the positive voltage that's flowing from the circuit into the negative of the power supply and thus helps to switch it ON/OFF in response to the IC output signals.

  18. Dear Swagatam

    what the max ampere level of this circuit and what changes will i have to make if i want to charge 4000mah li-ion battery

  19. Dear Sir,

    Can a 12v 1 amp transformer suitable to charge 3.7 V 1000 MaH Battery or suggest a suitable transformer

    Thanks in Advance

    • Dear अमृतमन्थन,

      You will have to first drop it to 4V by using a 7805 IC and a 1N4007 diode in series, then it would become suitable for charging the cell, but be cautioned, you will have stop the charging as soon as the cell voltage reaches 3.9V

    • Sir
      Can U set a LED indicator in this circuit which indicates that charging is on and another LED indicator for charging is full

    • Connect two LEDs: one across positive and pin#3, second across Pin#3 and ground respectively…..each LED must have individual 1K resistor in series…

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