Home » Home Electrical Circuits » Li-ion Emergency Light Circuit
Li-ion Emergency Light Circuit

Li-ion Emergency Light Circuit

The post presents a simple Li-ion emergency light circuit with over charge and low battery cut off features. The Circuit was requested by Mr. Saeed Abu and Y0f4N.

Technical Requirement

Bro thanks for ur reply. Actually im Pharmacist(M.Pharm) & Electronics is my hobby. So i go through ur mentioned link & i dont understand ur suggestion to modify that diagram also about ur mentioned cut off transistor. So would u mind to send me the complete circuit diagram 

My requirement is:(1) Circuit is Operated by Nokia standard cell phone charger

(2) Battery nokia 3.7 volt

(3) Auto ac to dc changeover system when ac fail

(4) Battery overcharge protection system(auto battery full charge cutoff) with LED indicator.I have tried many times to develop such type of circuit but i failed. So Please bro help me urgently. Please design it simple.

The Design

The proposed Li-ion emergency light circuit with over charge and low battery cut off features functioning may be understood with the the following points:

Transistor T6 is basically configured to automatically sense and shut off the LED during the presence of mains AC and vice versa. Here a mobile charger is used for powering the T6 circuit.

As long as the mains input is available, the 1 watt LED stays switched OFF due to the presence of a positive potential at the base of T6,  T6 starts conducting the moment AC mains fails illuminating the connected LED with the help of the attached Li-ion battery power.

T1 and T2 form the low battery detector stage and does the same when the Li-ion battery voltage falls below a certain predetermined level set by P1.

When this happens, T1 just stops conducting forcing T2, T3 to switch ON hard.
T3 passes the battery voltage to the base of T6 choking its conduction thereby shutting off the LED and inhibiting any further loss of voltage under the situation.

T4 and T5 are configured for the opposite function, that is for detecting the full charge of the li-ion battery.

P2 is appropriately set such that T4 conducts fully at this battery voltage.

With T4 fully ON, the base of T5 is unable to acquire the required negative biasing via  R6 and thus is prevented from supplying the charging voltage to the battery, which in turn protects the battery from getting over charged and damaged in due course of time.

The red/green LEDs indicate the relevant states of the battery and the cut off conditions.

The 10 ohm with the negative of the battery may be eliminated, it's not worth with so many existing protections.

For getting a better response from the over-charge cut-off stage, the above circuit could be modified further with an additional transistor stage T5, as shown below:

Referring to the following circuit, we are able to the see a few crucial additions and removals:

IC 7805 has been added, diode at T6 collector is removed, and D1 position changed. These changes ensure that an exact 4.3V is able to develop across the emitter of T6 and ground, irrespective of the input voltage level.

D5 has been removed in order to provide a better illumination for the LED at the collector of T2.

All high value resistors have now been reduced to 1K for an increased current biasing for the BJTs.

As suggested by one of the avid readers of this blog Mr. Syed, the above diagram needed some corrections.

The finalized diagram of the Li-ion emergency light circuit with over charge and low battery cut off featurescan be seen below:


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!

185 thoughts on “Li-ion Emergency Light Circuit”

  1. hi swagatam
    i have a question, in 3.7v li-ion battery the charging voltage is 4v how about lead acid battery rated at 4v what will be the charging voltage ?

    • Hi Abba, you can use any type battery as per your preference, Li-ion batt can be charged quickly that’s the only advantage of it

  2. I set cut off preset I adjust 4.2v voltage unplug adapter then used the battery voltage is decrease 4.1v connected circuit then not charging I check circuit out voltage 4.1v or 4.2v then adjust cut off preset voltage increase 4.3v or 4.4v then battery charging then over charging again I set cutoff voltage 4.2v again and again I set cutoff voltage I want to parent one time set the cutoff voltage sir please update any mobile charger supportable single ic used low cost circuit diagram please sir

  3. charging cut off led glowing in unplug adapter Glowing led then charging led glowing in unplug adapter Glowing led this problem please sir solution

  4. p1 & p2 is variable resistor in 1st diagram??? I will use 3.7v 1a (Sunca match box size battery) battery to this circuit. so what fixed resister I have to use in place of p1 & p2??

  5. dear i need temperature controlled AC Fan Dimmer circuit and Birthday song tone Calling Bell circuit diagram.do u have this pleas share it.

  6. Ok sir. ill check on breadboard first. Another thing, how do i change the output voltage to 1.5v instead 3.7v? as i want to run a 1.5v (150mA approx) dc motor instead LED.How may i do that?

    • for changing the output to 1.5V you can either change the battery to 1.5V or use 3 nos 1n4007 diodes in series with the output positive

    • sir, if I use 1.5v battery, then what's need to be changed? and on 3.7v battery, you told to use 3 diode in series. Why not a simple 10 ohm resistance? will you kindly explain?

    • add 3 more diodes in series after 7805 while charging the 1.5V batt

      a 10 ohm will affect and reduce current to the load, using 3 diodes will keep current intact, but reduce volts to the preferred limit.

  7. Dear sir, what will be the input voltage? as there is a 7805, i need more then 5v for that, right? but as schematic, it says to use 5v cell phone charger.what to do?
    Please help!

    • Dear white dragon, a cellphone charger will normally have a slightly higher voltage than 5V, so it's OK to use a cell phone charger as the input, the 7805 is only for ensuring that the li-ion cell does not get an over voltage due to a wrong high voltage input

    • Oh i see. i didnt know that. 🙂 thanks for the prompt reply. I am designing a pcb of this schematic. Another thing i don't understand. On 3rd schematic, from the collector of T6 toward emitter of T7, it is shown to use 2 pcs of diode, but shows only one. do i need to add 2 diode parallel or serial to there?

    • since D2 is introduced, therefore only one diode at the emitter of T7 is sufficient, no need of two diodes here.

      before designing a PCB I would recommend to test it out over a general purpose board for a trial and verification.

    • bro i haven't tested it but it looks perfect because most of the issues have been corrected in the last design and it should work as proposed if it's done exactly as shown.

  8. The reason why i am skipping the series config. is that if an LED fails in a branch all the LEDs associated with that LED in that particular branch will be turned off affecting the overall brightness considerably

    • Today all LED lamps depend and use series LED configuration, some street lights even use 50 LEDs in series. Good quality LEDs will never fail no matter how these are configured, as long as everything's done as per the specs.

  9. Sir,
    I need a simple LED light circuit to fit inside a car….
    I am using 10 no.s of white LEDs… The LEDs are connected in parallel for maximum efficiency… So 10 parallel connections are there… Power for the bank is taken from the Car battery voltage… I need to drop a maximum of 600 mA from the source ( 60 mA for each LED ) for maximum brightness….
    By calculations i have found the following ;

    Resistance needed for a single LED to draw a current of 60 mA is:
    ( 12 – 3.1 ) / 0.06 = 148.33 ohms
    Power rating of that resistance :
    ( 12 – 3.1 ) * 0.06 = 0.534 W
    150 ohm 1W resistance can be safely used…
    So 10 no.s of such resistances have to be used… The main drawback of this config. is, more of the board space is getting utilized.

    Now i am gonna chose for a simplest design which uses only one main resistance instead of placing separate limiting resistances for each LEDs…

    The total current required = 600 mA = 0.6 A
    Voltage = 12V – 3.1V = 8.9 V
    So R = 8.9 / 0.6 = 14.833 ohm
    Power rating of the resistance = 8.9 * 0.6 = 5.34 W

    Are my calculations correct sir ??
    I need your valuable comments on these

    • Arun, series connection gives more efficient results for LEDs, make two led strings with 3 leds in series on each string, and connect them in parallel.

      the series resistor on each string would be then:

      12 – 9.9/.06 = 35 ohms, 1/4 watt

  10. Sir,
    Sorry for disturbing you much…
    All the problems facing by me are due to my carelessness while making out the circuit on BB…
    I got the result of the above circuit in a fruitful way so that i can proceed further…

    Hope you will understand my lack of time in concentrating much on the electronics due to a lot of works related to my profession to be completed …

    Also expecting your valuable help in future projects that i am planning to do..

    Thank you sir

  11. Sir,
    On checking today,
    the diagram i have sent to you via email, when assembled on the bread board, acts as the same in the VERO board..

    The Battery full indicator illuminates even on power failure… also the following points were noted

    – Battery full voltage is 4.7 V

    the Base of T7 gets voltage about 6.4 V when a 7.0 V charging source ( to charge the 4 V 1000 Ah battery taken from the mosquito bat ) source is used, thereby the LED bank connected between its collector and ground is not active…
    but when source is disconnected ( power failure ) the base voltage is getting reduced only to 3.8 V not to zero, even-though the LED bank begins to illuminate

    The battery full indicator LED continues illuminating even after power failure

    Correct adjustment of Higher voltage cut of can't be achieved

    Even if the higher voltage cut off is achieved, the charging voltage available for the battery doesn't becomes zero..

    I am expecting better solutions for the mentioned problems in the above points

    • Arun, others have also tried the above circuit they are quite satisfied with it, I am not sure why you are facing so many issues.

      Anyway here are the answers:

      "Battery full" LED will stay illuminated because it's connected with the battery line, so it's fine, you can eliminate this LEd entirely and just keep the red and green LEd for the indications.

      1) This case is also normal, D2 is dropping the 7V to 6.4V, however despite my of repeated suggestions you have not used a 7805 IC that means you will get all the wrong and dangerous results while the battery gets fully charged.

      On power failure T6 gets activated via R7 that's why you see 3.8V at its base, it's due to battery voltage minus the 3.8V which could be 0.6V, which is exactly what a BJT needs to conduct.

      2) i have answered this above.

      3) This is because you have not used 7805 IC and did not take care about making the emitter of T6 exactly equal to the full charge level of the battery.

      4) the answer is same as above

  12. bro ur given last circuit is overcome all the drawback of over charge cut off feature which i have faced during build it?

    • Thanks for the feedback bro, that's great… a small additional refinement could make it more perfect, the emitter of T6 needs to be shifted to the anode of D2 from its existing position, this will make it more reliable in my opinion.

    • …sorry the above recommendation is not correct, rather another diode needs to be added between the emitter of T6 and the base of T7 so that the battery voltage does not reach the emitter of T6 during backup.

  13. Hi Pro! I have done this circuit and it works quite well with the charger plugged in and cut, even with a low battery saver mode also works well. I changed T7 with TIP127 to control parallel 10Led 1w, however when I measure the voltage at the base of the LED light is getting 2.8V, this can happen due to too many connections I Led?. This is my 2nd T6 is used to charge controller TIP127, however after the last TIP127 voltage has been reduced from 5.1V down to 3.96v, so the pressure drop due to TIP127 this?. I tried removing T6 and check the charging mode and charging cut still works well. So according to Pro then need to add back to T6?. I checked voltage charger when the battery is not connected 4.4. I have combined this circuit with controller enabling / disabling of IR and used more direct mode without batteries. Looking forward to the comments of Pro to help achieve the best performance circuits. Thank Pro!

    • Thanks Truong for improving the design, I appreciate your efforts.

      I hope your suggestions will help the interested viewers to learn and modify the circuit as per their personal applications.

    • remove the battery and check the voltage across its collector and ground, it should be 4.2V and the current should be minimum 500mA, if these are satisfied then your batt will get correctly charged.

  14. But still i am facing a low charging current problem… The 1000 mAh battery doesn't get the minimum chaging current of 100 mA, only 56 mA maximum is available…

    WHAT 2 DO SIR ????

  15. Finally i got it Sir. I have included the LDR dark sensor portion to the last diagram…. I have sent the altered diagram to your hitman inbox. Plz check it sir.

    Now..can u suggest a suitable and efficient method to adjust the brightness of LEDs in that circuit ?

  16. Sir,
    Can you provide a dark sensor which can be used with the last schematics such that the emergency light should only be turned on during night time..

    The dark sensor given in the below schematics is not working since the collector voltage of the BC557 in the DARK SENSOR section doesn't become zero when the LDR faces darkness ( i have confirmed it by testing it many times )..


    Plz help me to solve this by obtaining zero volt at the base of T7 when darkness is detected.

    • Arun, you should first learn how PNP devices work. The shown circuit will definitely work
      In the given link, the collector of BC557 should become equal to the battery voltage in order to shut off T1.

      The LDR should get sufficient light to turn off T1, or may be you can try using another BC557 to make a Darlington configuration for increasing sensitivity.

    • …OK sorry, I slightly misunderstood you question.

      At complete darkness any good LDR resistance will reach in Megaohms.

      With 100k across its base/positive, a resistance in Megaohms across its base/ground will have no effect on the BC557 so how can it keep conducting??

      Check the base voltage of BC557 at this position if it doesn't get equal to the battery voltage would indicate a faulty BC557.

  17. Hellow sir in the circuit about which i was discussing the problem stated in the last comment, the problem persists even after many trial and errors
    Here i am again giving the direct link to the schematics


    The LEDs associated with the high voltage cut off section ( either charging LED or full charging indicator LED ) continues to blink even after power failure ( or retains its state as they were before the power failure occurs ).

    On close examining the circuit i have reached to some conclusions… I am giving those here…. Plz give your valuable opinions about these..

    Since the diode at the collector of T6 has been removed, the battery voltage during power failure becomes available at this terminal causing an emitter voltage of T6 ( about 2.6 V to 3.0 V for battery full voltage ) .. That emitter voltage is being utilized by the Over voltage protection circuit along with the battery voltage ( by the 47 K pot )… This may be the real reason for those LEDs to illuminate even after power failure….
    I could found this by testing the LEAD terminals of T6 and the problem was solved by placind the diode D3 as before.

    Some voltage will also be present as in the first case by shorting the base of the LED driving transistor T7 directly to the voltage rail ( in which the voltage from the mobile charger arrives during charging phase )… So i have used a diode in place of the actual connection with cathode pointed to the base of T7 and anode to the above voltage rail..

    The above said two modifications solved the issue…

    Now the LEDs ( which are causing the problem ) do not illuminate during power failure..

    Check your hitman inbox sir… I have send the altered diagram to it…

    Hoping your reply soon

    • Hello Arun,

      Thanks for the detailed assessment!

      What you have suggested in point 1) is strange because in a PNP device voltage will "flow" from emitter to base/collector, never in the opposite direction, therefore the diode at the collector shouldn't make much of difference.

      However your 2) point makes sense wherein I am assuming that you have shifted the emitter of T6 to the anode of D2 from the existing cathode of D2, right?

      The above correction would be preventing a leakage voltage from T3 collector to reach the emitter of T6 and perhaps this could have been the real issue.

      Try removing the T6 collector diode with your 2) suggestion in place, and see if it reverts to the issue or sustains the present good condition.

    • your point 2) modification would be also preventing T7 emitter/base battery voltage from reaching T6 emitter, correcting the discussed issue

  18. Hai sir,
    In the circuit given in the below link, the Charging LED ( green ) illuminates even during power failure….


    I have placed a 4.7V zener in series with the Green LED with proper position, which solved the issue.
    Green LED turned off when power failure occured.
    But a new problem arised that, the charging current of the battery becomes very less compared to the previous case..


    • Hi Arun, a zener diode is not required, and there's no way the green LED can illuminate with the input supply switched OFF, because if the emitter of T6 has no voltage from where will the green LED receive power? The battery voltage cannot pass through collector of T6 to the green LED, right? so then from where could the green LED be getting the power if there's no input supply at the emitter of T6?

      You can try disconnecting different links that may be related to the green LED to find the leakage path for troubleshooting the issue.

  19. Sir i am using two unknown rechargeable batteries taken from two broken MOSQUITO BATS….
    On searching the internet i found that they might be 4V 1000 mAH batteries.
    One of the battery shows abotu 4.3V and the other one shows 4.1 V..

    (1). So what do you think about the MAXIMUM CHARGING VOLTAGE and MINIMUM
    CHARGING CURRENT of these batteries ?

    (2). Will the criterion 1/10Ah holds the same for this case so that the parallel config. of these
    batteries require 200 mA current as minimum charging current ?

    (3). What config. will be most preferable here ? A Parallel config. or a Series one ?

    (4). What z about the maximum charging voltage ?

    My requirement is to make the present circuit able to provide suitable charging levels ( voltage and current ) to the battery bank and the brightness of the LEDs ( 10 no.s ) should be high enough to make it useful at night….

    On inspecting the working of the present circuit, i came to know that, the charging current supplied by the collector terminal of the T6 is only about 15 to 30 mA which is too low ..
    Also the discharging current is also less so that more than one LEDs can't be used instead of the single one…..

    Hope you will suggest better remedies to solve all these issues….

    • Arun,

      the full charge voltage for most batteries and in your case also would be 16 to 18% of the actual battery voltage rating, so for 4V it would be around 4.8V. Similarly the lower may be calculated as 12% less than the actual battery rating.

      The charging current in the above design can be increased by changing the 2N2907 with TIP127, this will significantly improve the current specs of the design.

    • neither series is good nor parallel, ideally you should charge them separately, but parallel config is better than series according to me.

      1/10th rate will be good if the the batts are Ni-CD type.

  20. Sir,
    How to use this circuit to operate on set of 10 white LEDs ?

    Can you plz check your hitman inbox Sir…
    I have sent a mail to it.

  21. Sir,
    i have tune the P1 which is working perfectly.i have tested it.but the problem is that the 1 watt led doesnot off. emitter and the collector of the bc557 is connected with each other as mentioned in all three diagram.
    i have just replace the connection of emittter to the positive of the battery.and it is working perfectly means it cutoff the 1 watt led when the red led glows.
    sir i have a question about p2.When i tune it in that position when the green led is comming to be off and the green is in starting to glow but it does not works it remain in this condition even i connect a full charged batt or a weaker once or the charger.
    When i connect full charged batt and adjust the p2 when the blue led is glowing (just when it start glowing)and then replace the batt with weaker batt ,the blue led is still glowing even when i connect the charger.Then how to set P2??????

    • Syed, yes you are correct, T3 emitter should be connected to the battery positive for the required LED glow cut-off, I misunderstood your previous comment, I thought you were referring to T6.

      As for the P2 high voltage cut-off setting, when you connect a new battery you will need to switch off the whole circuit, then connect the new battery and then switch ON again for initiating the actions afresh….if you don't switch OFF the circuit the transistors will be latched on to the previous conditions due to their high hysteresis characteristics.

    • Sir,
      The circuit is working with its all features.
      I have make two changes.
      1) i have connect the emitter of bc557 directly to the battery.
      2)i have replace the connection of R5(1k) to the positive of battery.
      That all ,the circuit is working..

    • Thank you Syed for the updates, that's interesting.

      the connection of BC557 emitter to batt positive is valid and looks perfect but R5 should work even in the shown position.

      But anyway, if it's working for you let's take it in that way.

      I'll update the modified diagram soon…

  22. sir i think that the emitter of bc557 should be connected to the positive of the battery to sense through the for cut off. Please describe the function of T4 and T5 so i can accurately adjust the P2.

    • emitter of T6 is connected with the battery via its own collector. as soon as a discharged battery is connected, the emitter potential will drop and come down to the battery voltage level.

      P2 is set so thst T4 just begins conducting at around 4.1V, when this happens T5 conduction gets interrupted since its base begins getting grounded via T4 collector/emitter.

      With T5 not conducting, T6 is unable to receive the negative potential via T5 collector/emitter, and therefore T6 now begins getting choked and soon stops conducting….the battery thus stops receiving the charging voltage at around 4.2V

    • OK now i got your point, I misunderstood T3 with T6, you were referring to T3 and I mistook it to be T6….you are right T3 emitter must be connected with the battery positive for the cut off, I'll make the corrections soon.


  23. and also how to set the lower cut off limit and upper limit accurately… On tweaking the pots associated with each section, only a gradual change in detection of voltage has been noted, not an abrupt change which means that the higher cut off during charging and the lower cut off during discharging occurs gradually ?

    • yes you will a gradual change not a sudden change because these are crude BJT stages not opamp stages.

      an opamp circuit such as a LM358 circuit can be used for a snappy response….

      a relay can also be used for the same but 3V to 5V relays are not easily available and will make the circuit costlier..

  24. Sir in this circuit when i am using a 5.0 V battery ( taken from a broken mosquito bat ), only 3.58 V to 4.0 V is available at the collector of T6 ( positive terminal to The LED lamp )…
    What to do sir ?

    what are the minimum cut off voltage and maximum cut off limit of such a battery taken from the Mosquito bat ? No signs of voltage or current written on the surface of the battery

    • Arun, in the last circuit, I have removed the diode at the collector of T6 did you remove it in your circuit?

      the minimum cut off could be within 3.7 and 3.5V and the upper cut off from 4 to 4,2V for a 3.7V Li-on battery

  25. Sir, i will change my circuit according to new design(3rd Circuit).
    Do you have made this circuit.This one is a best emergency light with all features.Every one want to made this. Please verify that circuit practically.

    • No I did not check it so far, but as you can see everything's very basic and very tight, there's no space for any fault now, especially after adding the 7805 now you don't have to worry about the input voltage level and the voltage corrections at the emitter of T6.

      Do it exactly as per the last diagram, it will surely produce the mentioned results, but the cut-offs may not be as sharp as an IC circuit would do, because this circuit is crude compared to an IC circuit.

  26. Sir. I want to ask about the adjustment of P1 and P1 .
    When i connect a weak battery and adjust the P1 the red led(low batt) does not glow at any point.i have the reduced the value of the resistor connected with Low battery led.but it was not glowing and when i connect the charger it glows..
    if the charger has output of 9 or 12v volts, will it effect the components?
    For P2 when i connect full charge battery and charger to adjust, and after adjusting it in that position when the Blue led(full batt) glows i change the battery with weaker once but the led is still glowing.. (O_o) .i have check all the connection these are perfect..
    Please describe the adjustment of P1 and P2 .

    • Syed, remove the diode D5, it's dropping 0.6V and making it difficult for the LED to glow at 3V.

      the voltage across emitter/ground of T6 (in the last diagram) from the adapter should be exactly equal to the full charge specification. this must be first confirmed without connecting any battery. for example if you are using a 4V battery, amke sure the emitter of T6 shows 4.4V initially without any battery connected…9V or 12V is too high

      The amp of the adapter should be 1/2 of the connected battery AH. for 1.2AH, the input amp should not be more than 500mA

      also make all the 33k resistors to 10K for better response

      All these things should be confirmed otherwise the circuit will keep producing wrong results.

  27. Ok sir,
    but i am in trouble with P1, But i have adjusted P2 ..
    i have seen the above the comment for adjusting this but when i attach a weak battery of about 3.0 volts and adjust the P1 the Red Led does not glow properly but it glow.and my Main Led(600mA) does not Off..

    • You can change the series 10k resistor to 1K and see the response, it should brighten up the LED…

      sorry, i could not understand your last question…." but it glow.and my Main Led(600mA) does not Off".

    • …OK got it now, you mean to say at low voltage, the emergency 1 watt LED lamp does not shut off fully

      what did you use for R4?

      use 1k resistor for R4 and check the response

    • Sir how to set the P1 and P2.
      i have set according to above comments but not successfully set it.Please explain it using multimeter..

    • Syed the setting procedures are very easy, without connecting any battery feed a 3.6V directly at the emitter of T6 through a variable power supply, now adjust P1 such that the red LED just begins to light up.

      Next, increase the above voltage to around 4.2V directly at the emitter of T6.

      Now adjust P2 until the base LED of T6 just shuts off.

      your setting up procedure is complete.

      disconnect the variable power supply and connect a phone charger to the input of 7805, connect the desired battery for getting the intended automatic cut off actions as per the above settings.

  28. Hello sir i have made this circuit..
    i want to ask that the circuit will perform well on this configuration.
    i have used tip127 so can i use led upto 5a??but i am using led of 0.6a with 4v 1.2a battery..Battery is not Li-ion battery.

    • Hello Syed, yes it will work, but with a 600mA LEd your battery will become flat quickly, you will need a minimum 4AH battery for getting around 3 to 4 hour backup

  29. Bro i can not get ur point properly.What will be the base voltage of T5/T6 when battery get its full charge?Pls describe it properly so that i can examine this properly.

    • Bro, base of T5 should become zero or near zero at 4.2V, this will force the base of T6 to become 4,2V or near 4.2V….all these should be measured with respect to ground or negative of the supply, while P2 is being adjusted.

      You can do it in the following manner:

      Without connecting any battery first make sure the voltage at T6 emitter and ground is 4.2V.

      At this position (without any battery connected) slowly adjust P2 until just the green LED shuts off or becomes too dim.

      Your circuit is set now, after this you may connect the battery for getting the required over charge cut-off at 4.2V

      make all the LEDs to red for better response.

    • ,,,,bro, I have made one small change in the above circuits to ensure a better response.

      the upper circuit is now supplied from the emitter of T6, and not from the battery….see the red line it joins with the positive line of the upper circuit.

      also due to the presence of D3 please make sure that the charging voltage from the source at emitter of T6 should be 4.2 + 0.6 = 4.8V this will produce a correct charging voltage of 4.2V for the Li-ion battery after passing the drop through D3.

  30. bro u above 2 circuit the battery get charged max. 3.90v.Also with PCM the full charge auto cut off is occurred but no full charged indication by LED. Without PCM no full charge auto cut off is occurred also no full charged indication by LED. However, ur other feature is ok. Bro i think u can add 6v dc relay to fix the problem.Is it possible?

    • sorry bro with this circuit a relay cannot be used, we'll have to go for an opamp circuit for operating a relay….
      I am 100% sure about the above circuit that it will work just needs to be tweaked by observing the base voltage T5/T6 with a multimeter while adjusting P2.
      unless its not done with a multimeter, the results cannot be confirmed

      If the LEDs are not glowing properly we can change their positions, no issues, but the important thing is to make sure that the base of T6 becomes equal to its emitter voltage when the battery voltage reaches 4.2V

      And for this to happen the input voltage at the emitter of T6 should be precisely 4.2V, if it's more T6 will never shut off.

    • the SCR circuit which you are presently building also does not have an emergency feature, my suggestion was with reference to the SCR circuit not the above.

      which circuit are you referring to?

  31. Dear Bro thanks for trusting on my experiment at last.Though i am Pharmacist electronics is my heart. There is no difference between human body and circuit.So i feel it so much.Actually,The PCM makes the Li-Ion battery superior from others.However i am still working on ur given circuit for better outcome.I will give u feedback ASAP.

    • You are welcome bro, yes surely we are all just like machines too.

      By the way with a PCM connected you can simply use a 7805 IC for supplying the Li-ion cell, it'll do the job perfectly, no other additional circuits would be required….for getting an LED indication, you may use a few of the above circuit stages

    • Sir your 1 watt led emargency ac operated circuit . I can under stand BT136 pinout in data sheet there show M1 M2 G . In our city 100uf/225v not aveliable can i use 100uf/160v. And 2uf/400v is it 205/400v ?

    • Ravindra, don't make the circuit using SCR or triac it could be a lot difficult for you, instead you can try the following:


  32. Dear Bro we call that circuit is PCM=Protection Circuit Module.However u can go through these link:

    Which will encourage u to do that experiment.

    Also ur comment ''any battery will stop taking charge after the voltage reaches its maximum charging capacity'' is totally wrong because i have charged a battery with PCM up to 4.25v then it stop to take charge(In that situation the voltage was around 6.5 to 7.5v at the battery with connecting the charger, i have checked it by Digital meter)then i take the same battery removing the PCM it is now charging upto 4.4v and continue to charged and i stop it for explosion risk.

    Now explain me how can differ the full charge voltage between The Battery with PCM and the same battery without PCM???

    • Dear bro, right at the beginning I had asked you provide a link regarding this and you said that no link was required because you had tested it practically. however I wanted to believe it only through reliable source and not your experiments, since now you have provided it now it's very clear what you meant to say.

      Now it makes sense, and it also proves that how sensitive Li-ion batteries are and without a strict charging circuit it could explode or start leaking.

      By the way all Li-ion batteries may not have this built-in protection that's why an external protection is required.

      I did not want to say that ''any battery will stop taking charge after the voltage reaches its maximum charging capacity'' but since you were not agreeing and proving your experiment to be correct without any reliable article by saying that your cell was not taking above 4,25V so I had no choice but to say this. I know that a battery may keep charging to dangerous levels and that's why protection circuit is imperative.

      Still I did not understand your last question.

      My above circuit can be set to any desired cut-off level, you can set it to cut off before the PCM level or the same level….but the PCM will have no connection with the above circuit, they are independent and will not read and react to each others conditions.

    • …..the above circuit will cut-off at the same level every time with PCM or without PCM it won't make any difference. If it's set to cut off at 4.2V then it will cut-off at this voltage even without the PCM connected.

      It's better to test it without a PCM fitted.

      The crucial thing is the input voltage at the emitter of T6, which should be 4.3V so that it can cut off at this voltage.

  33. Dear bro r u saying me that by doing that experiment or not? If not then do it then challenge me that was my wrong experiment.In addition, i have made many times for my many project full charge indication by zener diode using cell phone battery's self over charged protection system feature. So please do that experiment as per my direction.

    • Dear Bro, any battery will stop taking charge after the voltage reaches its maximum charging capacity but that doesn't mean you can keep the charging switched ON…because that would force the chemical reactions to continue inside the cell and start damaging it slowly.

      I'll advise that you should first research online how to charge Li-ion batteries before getting so much confident.

  34. Dear bro i have done many experiment on Cell phone battery last 3 to 4 years and still doing.Also i have discussed about it so many mobile technician in my locality.In that way i am running many project by cell phone battery but no problem is raised.So before comments on my experiment u have to do that experiment practically.I hope u will do that and give me feedback ASAP.Also u asked me why do i need an overcharge protection in the above circuit? Because i need a fully automatic feature like Inverter.That built in circuit of battery only covered over charge feature but not covered low cut off and ac to dc change over feature.However i will check that circuit again also ur two new link.i will wait for ur that experiment which i have told u bro.

    • sorry bro, no battery has such a feature, especially with li-ion batteries we need to follow strict procedures while charging so that they don't get ruined soon and to prolong their working capacity. therefore Li-ion batts are always charged using sophisticated chargers.

  35. Dear bro no need to show any article / image to prove this u can easily prove it by yourself.At first:
    1)U have to take a Nokia Battery and a Standard Charger.
    2)Now u simply charge that battery with that charger directly(no use any circuit, just join the anion and cation of the charger directly to the battery anion and cation section).
    3)Wait for the voltage reached at 4.24v of the battery and then connect digital meter on the battery to see the voltage in charging condition.
    4)U will see when voltage reached 4.25v of that battery then it stop to take the charge automatically and the voltage suddenly increased approx. 6.5v to 7.5v which mean that the battery did not take any charge.
    5) Now u remove the top built in circuit of that battery carefully.U will find Anion and cation section and connect to the charger accordingly.Then u have to do same experiment according to my above way.
    6)At last u will share the result of this experiment ASAP.

    • Dear bro, that's a wrong impression you have got, if it works in that way then why do you need an overcharge protection in the above circuit?

      If you continue doing in that way, your battery could bulge after some days and start leaking or even explode.

      The following is the last and final adjustment that I am presenting for the last circuit.

      Without connecting any battery first make sure the voltage at T6 emitter and ground is 4.2V.

      At this position (without any battery connected) slowly adjust P2 until just the green LED shuts off or becomes too dim.

      Your circuit is set now, after this you may connect the battery for getting the required over charge cut-off at 4.2V

      If you find all these difficult there are much sophisticated circuits as given under, you can try them out:



  36. Bro i mean that the over charged cut off feature of above circuits will work if i connect a battery without it's self over charged protection circuit(circuit which remain on the top of mobile battery) or 4.2v dry cell battery? Please confirm it first then i will share the other problem oh these circuit.

    • Bro, do you mean the batteries have its own built-in overcharge protection? I have never heard of this before, can you show me an article or image which explains this??

  37. Bro i go through ur two updated circuit diagram but no positive feedback.At first u have to clear me that ur given circuit is capable for true over charge cut off feature because i have seen that it is only depend on battery's self over charge protection circuit which i have told before many times.If this circuit not then what about this Circuit(https://homemade-circuits.com/2013/12/usb-automatic-li-ion-battery-charger.html). Can i use this for my same requested purpose?

    • Bro, i did not understand what you are saying.

      are you using the above circuit with another auto-cut off circuit??

      If it is so then you should remove it and use it only with a simple 5V adapter.

      I suggested you to measure the voltage at the base of T4 while adjusting P2, but you did not do so.

      The above circuit will surely work if it's done as per the rules, not by trial and error.

      anyway you can try the other one which you have asked, but if you could not make this one you wouldn't be able to make that even, because it has a opamp which will require even more knowledge and precise adjustments.

  38. My battery can not charge above 3.96v. i use mobile battery charger is it correct.? 2N2907 eqvalant or can i use 2N2222 ? I know 2N2907 matal pinout but i want to plastic transistor pinout

    • 2N2222 is NPN, not PNP…. moreover it's also a metal can transistor.

      you may use use 8550, or BEL188, or BD140

    • the lead which is close to the small body projection is emitter, center lead is base, the last one is the collector


  39. Sir, please add LDR circuit for LED on when night if day light mains power goan LED OFF only night time LED on

  40. Dear bro, i have tried again ur revised circuit but problem is still exist, these r:

    1)Ur developed Over charge cut off system is not worked only work in case of battery with self over charge protection circuit which i have told u before.
    2)During low cut off and Over charge cut off stage the charging Green LED is glowing gently (when i disconnect the charger.)
    3)During over charge cut off stage the full charge indicator Yellow LED is glowing gently.If i adjust the P2 to increase the brightness of Yellow LED then Green LED glow gently and vice versa.

    So i am hoping a nice solution of it from u.

    • Sorry Bro, that looks impossible…

      If the blue LEd in the last diagram illuminates it means T4 is conducting.

      If T4 is conducting it would ground the base of T5, stopping it from conducting.

      When T5 is stopped from conducting how will the green LEd glow???

      Because as long as T5 is conducting the green LED is getting its ground through the collector/emitter of T5, once T5 stops conducting this passage is blocked and the green LED also has to stop glowing, resulting in shutting off the 2N2907 transistor completely.

      Check the voltage at the base of T5 when the BLUE LED is glowing. It should be zero and the green LED must stop glowing.

      Please do the above, we'll troubleshoot it right here…..

  41. Thanks bro I'll wait for ur corrected diagram also i need true over charge cut off circuit which only worked by ur developed circuit not by the battery's self over charge protection circuit.Pls bro update ur circuit ASAP.

  42. Pls bro correct the diagram ASAP.I am waiting for this.Also i need true over charge cut off system i mean battery will be over charge cut off by ur circuit not his(battery) self over charge circuit.

  43. Dear bro i have tried again ur new circuit as per ur direction but no positive result.The problem is here:

    1) The charging green LED is glowing during charging but no so much glow.
    2)The yellow LED is not glow during battery voltage is 4.25 also i can not glow that by adjusting the P2. At 4.25v the voltage remain in yellow LED is approx. 0.65v.
    3)Also ur developed auto high voltage cutoff circuit (battery over charge protection) system is not working.Only working Battery's self overcharge protection circuit when voltage reaches at 4.25v( I have tested it again by connecting a battery without it's self overcharge protection circuit, it getting charged 4.40v then i disconnect the charger).
    4)But ur other system low battery cutoff and ac to dc change over system is ok.

    Bro what will be the solution of this.I think u have to develop this circuit practically to find out the solution of this.

    • Dear bro, the second circuit is perfect in every way.

      again I'll say if you do without understanding, you will keep getting problems.

      if T4 and the yellow led are not conducting it means either one of them is faulty or the connections are wrong in your circuit.

      make R5 = 1k (I had suggested this before also)… and check.

      as soon as T4 conducts, T5 will switch OFF and the green LED will also switch OFF

      you can try making the series resistor of green LED to 1k for increasing its current and brightness.

      practical making won't be required because I am sure this circuit is OK.

    • …bro, i think got the culprit.

      the yellow led should not be placed at the emitter of T4. connect the emitter of T4 directly with ground

      put it at the base of T4….and check.

      the emitter position is not allowing T5 to stop conducting completely.

      I'll correct the diagram soon.

  44. Dear bro, I have tried again as per ur latest direction but i am again failure.here the problems are:

    1)The voltages at the base of T5 cannot make zero from 0.6v by setting up P2.
    2) The green LED keep glow gently at 4.25v of the battery.
    3)Also ur developed auto high voltage cutoff circuit (battery over charge protection) system is not working.Only working Battery's self overcharge protection circuit when voltage reaches at 4.25v( I have tested it again by connecting a battery without it's self overcharge protection circuit).
    4)But ur other system low battery cutoff and ac to dc change over system is ok.

    So bro what can i do now to over come above problem.Please do something.I think u should practically make this circuit then u can easily find out the problem.

    • Dear Bro, I have modified the diagram with an additional stage, please do it as per the new diagram and check the response.

  45. Dear Sir,

    I request you to kindly suggest me a simple circuit to cut off power supply to a 4V-4Ah lead acid battery on full charge (over charging protection) from a 5V mobile charger.

  46. Bro i have tried again as per ur modification guideline but result is not positive.The green LED is not glowing during charging the battery so there is no question of shutoff the green LED When the voltage reaches 4.25V.Also ur developed auto high voltage cutoff circuit (battery over charge protection) system is not working.Only working Battery's self overcharge protection circuit when voltage reaches at 4.25v( I have tested it again by connecting a battery without it's self overcharge protection circuit) .So Bro i think u have to redesign the circuit freshly and upload it again.I am waiting for ur great feedback.

    • Bro, electronics is not easy, you won't get results even from the easiest of the circuits until you do it by understanding it.

      The above circuit is perfect in all respects according to me, however if you can pinpoint the faults in the design then surely we can debate on that.

      Alternatively you can try testing the design by checking the voltages at the base of T5 while setting P2. The base voltage should almost become zero at 4.25V.

      confirm this by disconnecting the emitter of T4, this should revert the T5 into conducting mode again. once confirmed connect it back in position.

    • …check it by putting red prod of the meter on the positive line and black on the base.

      when conducting, it should show near 0.6V, keep adjusting P2 until it drops to zero at 4.25V.

  47. Bro thanks ur reply.However today i have developed ur circuit as per ur given model and i successfully done auto ac to dc changeover system,battery low voltage cutoff system but i can not success in auto high voltage cutoff(battery over charge protection) system.The problem is here:

    1)The green LED is not glow during battery voltage is 4.25 when charger is connected but when charger is disconnected from circuit then green LED is glow but so dim.

    2)Then i replace R6 into 1k then green LED is glow normally.

    3) I tune P2 very accurately at battery voltage 4.25 but auto high voltage cutoff(battery over charge protection) system is not worked.

    4) Only working Battery's self overcharge protection circuit when voltage raised at 4.25v but not ur given over charge protection circuit that's why the green LED is not glowing.

    5) I have tested it by connecting a battery without it's self overcharge protection circuit with ur circuit.Ur circuit is continued to give charge that battery and battery voltage is raised up to around 4.75 then i stop to give charge my battery.

    So bro it seems to me that in auto high voltage cutoff(battery over charge protection) system there is something wrong.Pls refine it.This circuit is badly needed for me.I have tried to refine it but i failed.

    • Bro, remove the green LED from the shown position, and connect the collector of T4 to directly to positive.

      Now connect the green LED in series with the base of T5, anode to base, cathode to T4 emitter.

      When the voltage reaches 4.25V, this LED will shutoff indicating that T5 has been switched OFF and the battery is receiving no charging voltage.

      ….also reduce R5 to 1K

      please try this and let meknow the results.

  48. bro is R7=1k is needed and why? Also i can not collect TIP 127 in my locality.So am i use BD 140 or any other NPN Transistor?

    • Bro R7 is for enabling the transistor to conduct when mains is absent, without it the LED will not glow.
      BD140 will do, it's not an NPN, rather a PNP

  49. Bro thanks ur reply.Where should i use TIP127 for 5/6 cells(Parallel connection) in above circuit?Also any change is required in Charge section in above circuit for 5/6 cells(Parallel connection) ?

  50. Bro thanks for ur developing my requesting circuit and also ur so talent on electronics.If i use 5/6 cell in parallel then this circuit will work properly?What is the alternative transistor of 2N 2907?

    • my pleasure bro, you can use a TIP127 for 5/6 cells.

      but please note that the circuit is not yet tested by me, although i believe it would work

  51. Hello sir,
    Just I wana to make a circuit wich work in such a way that when ac supply is off then dc supply automatically on but when ac supply is on then dc supply automatically off.pls reply me sir ….

  52. Swagatam,

    What modification should I do in this circuit to support 4 rechargeable pencil cells of 1.2 volts each, at 1000mah. thanks this will be tried out soon, what can i use as the equivalent of BC 547 available.


  53. Hello Swagatam,
    I just want add some detail for your article at first circuit since use variable resistor. For expert will be no issue to adjusting P1 and P2. As your blog mention, we are here for learning an discussion.
    To setup P1 as use for protection low batt
    – Remove / unplug the input power supply then connect your low battery (can use 3.2 V or 3.0 V) for me prefer 3.2 V.
    – Adjust P1 until red LED was glow and main LED was off
    To Setup P2 as use for protection overcharge
    – Plugin the input with power supply and put batt with full charge. let say about 3.7 V
    – Adjust P2 until green LED are glow
    hope this can help
    Thanks & Rgds,

  54. Hello Swagatam,
    thank you for your fast design.this design more cheap and easy to find for componen at local store.I just modify for LED driver with replacing T6 with BD 139 and adjust R7 with 10 K for 3 Watt. In this design better you remove D1,D2 as T5 already protected the battery. A
    P1 is key to prevent battery to drop to deep and P2 to protect battery from overload.
    Just suggestion for protection battery , when we set 3.2 volt as cutt off battery and full charge about 4.1 Volt, then we can modify P1 and P2 as fixed resistor. as I see R1,P1 and R5,P2 are voltage devider.may be it's not easy to find match resistor
    Thanks & Rgds,

    • Helo Yofan,

      You are welcome!
      yes, after confirming the exact values, P1/P2 may be appropriately replaced with fixed resistors..it won't be much difficult

Leave a Comment

Do NOT follow this link or you will be banned from the site!