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Simplest, Safest Li-Ion Battery Charger Circuit

Simplest, Safest Li-Ion Battery Charger Circuit

Li-Ion cells are probably the most complex when it comes to charging them, because these cells are quite vulnerable to overcharging, and tend to get hot at unfavorable conditions. The following post explains a simple yet a safe way of charging a Li-ion cell which can be easily constructed at home.



Advantage of Li-Ion Battery

The main advantage with Li-Ion cells is their ability to accept charge at a quick, and an efficient rate. However Li-Ion cells have the bad reputation of being too sensitive to unfavorable inputs such as high voltage, high current, and most importantly over charging conditions.

When charged under any of the above conditions, the cell may get too warm, and if the conditions persist, may result in leaking of the cell fluid or even an explosion, ultimately damaging the cell permanently.

Under any unfavorable charging conditions the first thing that happens to the cell is rise in its temperature, and  in the proposed circuit concept we utilize this characteristic of the device for implementing the required safety operations, where the cell is never allowed to reach high temperatures  keeping the parameters well under the required specs of the cell.

In this blog we have come across many battery charger circuits using the IC LM317 and LM338 which are the most versatile, and the most suitable devices for the discussed operations.

Using LM317 as the Controller IC

Here too we employ the IC LM317, however this device is used only to generate the required regulated voltage, and current for the connected Li-Ion cell.

The actual sensing function is done by the couple of NPN transistors which are positioned such that they come in physical contact with the cell under charge.

Looking at the given circuit diagram, when power is applied to the set up, the IC 317 restricts, and generates an output equal to 3.9V to the connected Li-ion battery.

The 640 ohm resistor makes sure this voltage never exceeds the above limit.

Two NPN transistors can be seen connected in a standard Darlington mode to the ADJ pin of the IC.

We know that if the ADJ pin of the IC 317 is grounded, the situation completely shuts off the output voltage from it.

It means if the transistors conduct would cause a short circuit of the ADJ pin to ground causing the output to the battery shut off.

With the above feature in hand, here the Darlingtom pair does a couple of interesting safety functions.

The 0.8 resistor connected across its base and ground restricts the max current to around 500 mA, if the current tends to exceed this limit, the voltage across the 0.8 ohm resistor becomes sufficient to activate the transistors which "chokes" up the output of the IC, and inhibits any further rise in the current. This in turn helps keep the battery from getting undesired amounts of current.

Using Temperature Detection as the Parameter

However the main safety function that's conducted by the transistors is detecting the rise in temperature of the Li-Ion battery.

Transistors like all semiconductor devices tend to conduct current more proportionately with increase in the ambient or their body temperatures.

As discussed, these transistor must be positioned in close physical contact with the battery.

Now suppose in case the cell temperature begins  rising, the transistors would respond to this and start conducting, the conduction would instantly cause the ADJ pin of the IC to be subjected more to the ground potential, resulting in decrease in the output voltage.

With a decrease in the charging voltage  the temperature rise of the connected Li-Ion battery would also decrease. The result being a controlled charging of the cell, making sure the cell never goes into a run away situations, and maintains a safe charging profile.

Safest Li-Ion Battery Charger Circuit

The above circuit works with temperature compensation principle, however it does not incorporate an automatic over charge cut off feature, and therefore the maximum charging voltage is being fixed at 3.9V.

At 3.9V we cannot assume the battery to be fully charged.

To counter the above drawback, an automatic  cut off facility becomes more favorable than the above concept.

I have discussed many opamp automatic charger circuits in this blog, any one of them can be applied for the proposed design, but since we are interested to keep the design cheap and easy, an alternative idea which is shown below can be tried.

Employing an SCR for the Cut-Off

Here, an SCR is used across the ADJ and ground of the IC. The gate is rigged with the output such that when the potential reaches at about 4.2V, the SCR fires and latches ON, cutting of power to the battery permanently.

The threshold may be adjusted in the following manner:

Initially keep the 1K preset adjusted to ground level (extreme right), apply a 4.3V external voltage source at the output terminals.
Now slowly adjust the preset until the SCR just fires (LED illuminated).

This sets the circuit for the auto shut off action.

Easy and Safest Li-Ion Battery Charger Circuit

How to Set-Up the Above Circuit

Initially keep the central slider arm of the preset touching the ground rail of the circuit.

Now, without connecting the battery switch ON power, check the output voltage which would naturally show the full charge level as set by the 700 ohm resistor.

Next, very skilfully and gently adjust the preset until the SCR just fires shutting off the output voltage to zero.

That's it, now you can assume the circuit to be all set.

Connect a discharged battery, switch ON power and check the response, presumably the SCR will not fire until the set threshold is reached, and cut off as soon as the battery reaches the set full charge threshold

SHARING IS CARING!


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!



52 thoughts on “Simplest, Safest Li-Ion Battery Charger Circuit”


  1. Howdy, Friend! Interested to Learn Circuit Designing? Let's Start Discussing below!
  2. Hi Swag,
    In the 2nd circuit with the SCR, 700 Ohm resistor is dificult to acquire…. Any alternatives?
    Best Regards.
    Nélio Abreu

  3. Would it be possible to make this able to charge 6x 4.2v Lithium ion battery’s separately with their own scr trigger while only using 1 lm317

  4. Good day sir
    How can I modify the circuit to connect 3 batteries in series 12v I basically need a 12v li ion battery charger

    John

  5. Dear Mr. Swagatam.
    Could you provide us the simple and cheap circuit to protected the Li-Ion batteries due charging cut off at 4.2V max and discharging cut off at 2.5V, thanks so much.

  6. This is some nice work, keep up the good work. However I am confused about using an E instead of Ohms to express resistance.
    Am assuming 1 Ohm = 1E.
    Also for simplicity I'm considering using this circuit to charge a 7.8v laptop battery as it's internal charging has failed to work, also will it be ok if I use the battery while it's charging. The laptops charger rating are 12v 3Amps.

    • i am glad you liked my site…thanks

      sometimes R and E are also used for representing Ohms as alternate symbols.

      the above article only discuses the concepts which may be useful for charging a Li-ion battery, however these might require some fine tuning until the concepts may look suitable for actual implementation.

      there are much better options discussed in this site which you can try using opamps.

      furthermore an Lm317 power supply alone could be just fine for charging a Li-ion batt if its output is restricted to 4V for a typical 3.7V cell, and the input current at 50% of the Li-ion batt's AH rating.

      for a 7.8V laptop batt, this could be set to 8.5V.

      yes both could be used together, but the charging current for the batt must be separately restricted to the above explained level for safe charging.

  7. but sir how these types of circuits can work efficiently ? please refer this charging cycle diagram of a Li Ion cell.

    https://www.google.co.in/search?q=charging+cycle+of+lithium+ion+battery&safe=off&prmd=ivn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiLoNCpt-DNAhUDso8KHa6tA4YQ_AUIBygB&biw=640&bih=279#imgrc=tlQylQep8HZyHM%3A

    according to the circuis that you are saying, the source stops charging the battery ( the current drawn by the battery from the source terminates ) when the battery voltage reaches the threshold of 4.2 V. but according to the charging cycle of a Li Ion battery charging continues even after reaching the threshold point.. After this voltage and current begins to decrease.. and we can say the battery fully charged when the charge reduced as below as 0.1 C and voltage remains stable in the value of 3.9 V.. so what is the point of making a voltage sensing types of chargers ? that is why i have asked you to suggest a circuit based of current sensing

    • sorry I could not understand the following sentence:

      "according to the charging cycle of a Li Ion battery charging continues even after reaching the threshold point"

      the voltage threshold cut-off method is the correct method and is employed by all standard chargers and for all types of batteries, however for extremely efficient charging process a stepped current charging is often recommended, but is not strictly required if the voltage cut-off is precisely adjusted

      to ensure a enhanced life for the Li-ion cell you can reduce the cut off threshold to 4V instead of 4.2V which might help to reduce the stress level on the cell although that would also mean charging the cell to only 85%
      another method is to reduce the charging current to 1/10th of the cell mAH, but this might require a much longer charging period…

  8. sir while testing the charging of a Li Ion battery taken from an old Sony Xperia phone using only a 7805 voltage regulator the charging current was initially 390 mA and voltage across the battery terminal rised directly from 3.7 V to 4.1 v . . after 1 hr the voltage was 4.2 V and current reduced to 250 mA.. this went on and after 4 hours the charging current reduced below 10 mA and voltage was shown as 3.9 V… so if i am using a circuit to terminate battery charging at 4.2 V how can it charge the battery fully… in my case the charging was not even complete when the battery voltage reaches 4.2 V .. also this cut of voltage reached very soon after connecting to the charger… so please suggest a better circuit which will be able to work according to the current drawn by the battery from the source while charging… i.e, making a circuit which senses the amount of current drawn by the Li Ion battery and will disconnect the charging supply from battery when that current reduces below 10 mA… i think that circuit will be good compared to the voltage sensing ones…… ot is easily understood from the charging cycle of Li Ion battery…

  9. You do realise that this will only charge the battery to about 75% as you dont have the final constant voltage charging mode that is supposed to continue at the voltage you shut the system off at until current reaches .1C? ie it is only the first thalf of a CC/CV charger.

  10. Sir , i have tried the 2nd ckt many times, with scr 1k pot& 3.9v zener.
    Whenever i connected the fully charged batt from mobile(3.7v) and adjusted pot led never lights
    when i disconnected connection between the led and pot terminal its lighting the led.
    actually one the 2 terminals of pot(left + adj) is connected to -ve and the 3rd terminal or right
    is connected to led's positive lead so its a chance of short circuiting?

  11. Hello sir, I'm a student and currently doing my final project. What would be charging circuit if i want to charge a 4.2V,30000mAH lithium ion battery with overcharge and over discharge protection too. i hope you can help me. Thanks.

  12. Hello;
    What would be the changing required in circuit if i want to charge a 4.2V,1200mAH cylindrical battery with overcharge and over discharge protection.Over discharge in sense if battery is connected with circuit and their is no power supply attached the battery will drain through the circuitry..
    Also it should have 3 status LED's for charging yellow,full charge green and for fault indication red if battery is dead or short circuited.Please reply i am hoping for good.

  13. Thanku Swagatam. I just realized one more thing, Once i trigger the SCR its not possible to switch off my SCR. Which means i cannot charge my battery again with out disconnecting my battery. ( I am trying to use this system as a nano UPS to power up Hard disks). Is there any other better way for me to do this??
    Thanks
    Yeshwanth Kumar J

  14. Hi Sawgat,
    I am trying to implement the battery charger circuit for a 12v 2.8Ah, (though i will be using 3 batteries in parallel to achieve the required voltage).
    say by selecting a 10 V zener wil i be able to achieve the same??
    Thanks
    Yeshwanth Kumar J

    • Hi Yeshwanth, yes it can be implemented by using a 10V zener and setting the preset appropriately, also the 700 ohm resistor will need to calculated and replaced for getting the required 14V output for charging the 12V batt.

      The input current must not be much higher than 1/10th of the battery AH for correct response.
      The circuit has not been verified practically by me, though.

    • thanks Swagat
      I have another doubt what is the use of 240 ohm resistor that i connected from the output to the scr? And can you help me out with the calculation of the 700 ohm resistor? If you can just guide me through it will be of great help to me.
      Thanks
      Yeshwanth Kumar J

    • Thanks for your prompt response
      I have a few doubts You said adjust the 1k to preset? what do you mean by this? Also can you help me out with the calculation realted to the selection of 700OHm resistor?
      Thanks
      Yeshwanth Kumar J

    • Yeshwant, 240 ohms is as per the datasheet of the LM317, it's mandatory and fixed.
      For finding the 700 ohm resistor replacement, you can use any online LM317 calculator software.
      Just enter the 240 value and the required output voltage in the given slots and press calculate to get the value in question.

    • Thanku Swagatam it will be of great help to me, to understand my working better. Once again thanku waiting for your response
      Regards
      yeshwanth Kumar J

  15. Is the second one to be declared stable now? I need it to connect my 3,7 Lithion battery to a 6V solar panel.
    I want to minimize the components, so if I can remove the pot and change the zener to a 4007 it would be great. Also removing the LED and connect the SCR to the ground. Right?

  16. Hi Swagat,
    Can we modify this circuit for charging 11.1Volt, 6600mAh Li-Ion battery. Or do you have any recommended circuit for this.
    Thanks

    • Hi Arun,

      yes the above circuits can modified for the specified application…… the second design by selecting the zener diode suitably.

  17. Hi Swagatam
    Thanks for the interesting simple yet safe Lithium battery charger circuit. Recently I acquired some 4.2 volt 4200 mAh lithium ion batteries (size TR18650) but am planning to build my own charger. Your max voltage goes up to 3.9v, but how does one go about safely charging the newer 4.2 v cells ? Any modifications needed to your circuit to achieve this ? Regards, J.

  18. BT169 is a standard ScR very popular one, you will get it.

    Please first confirm the above circuit and then we can go ahead with the LED indications….please note that this circuit is not confirmed yet, so the functions may not be guaranteed.

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