Lipo Battery Balance Charger for Charging of Series Connected Lipo Cells

The post discusses a relatively easy lipo battery balance charger circuit which is designed to continuously scan and charge the connected cells of the battery.

The idea was requested by Mr. Schindler and Mr. Emil Jan Thomas Baticulon.



 Charging 6 Li-Po Packs

The concepts are very well written, concise and clear. Thank you so much for the deep coverage of the charging subject.

Have you encountered the need to charge several identical lipo packs regularly? I have that very need, it is time consuming to recharge 6 high power packs containing 4 cells each every few days.

I propose a single cell charger that scans all cells via the balance plugs and serves up the requirement per need during a partitioned interval of the scan period.

Arduino sketch, shift registers, discrete coupling and a plan to stitch it together... there is where I bid you to guide me to a viable implementation. If you'd be so kind?

Charging 18650 Li-Ion Pack

Good day,

I just recently found your blog and upon further reading your post it's very helpful with or without electronic background and i appreciate your work.

I have a project in mind but I am stuck with it, My idea was how can I charge 13pcs 18650 li-on battery in series connection with balancing charger?. Can you help me with it and add this to your work?

Thank you,

The Design

As shown in the following diagram, the proposed lipo battery balance charger circuit can be implemented rather effortlessly using a couple of IC stages.

Let's try to understand how the circuit is intended to function:

The second diagram below is the relay driver stage which needs to be repeated 10 times and the base of the BC557 associated with the red spots of the relevant BC547 stages from the first circuit below.

Control Circuit Diagram

Lipo Battery Balance Charger Circuit

I forgot to show resistors on the bases of the transistors, make sure you put them, 10K would be OK...

Relay Driver Schematic

If the cells are 3.7V rated, the opamp preset is adjusted such that its output pin#6 just goes high when the charge level across the cell reaches around 4.2V.

How to Set up the Balance Charger Circuit

For setting this up, a sample 4.2V may be fed at the shown preset's upper lead, and the preset slider adjusted to make pin#6 of the opamp just high (positive).

  1. With all the positions connected as depicted in the diagrams and power switched ON, let's assume that at the onset pin#3 of the IC4017 is high which in turns activates the associated BC547, BC557 and the connected relay contacts.
  2. Cell#1 now begins charging, which drags down the supply voltage across the preset pin#3 of the opamp to may be say 3.4V or whatever may be the initial discharge level of the cell#1.
  3. While this happens, pin#3 of the opamp experiences a lower potential than it's pin#2 ensuring a low signal at its pin#6 and the pin#14 of the IC 4017.
  4. As cell#1 of the lipo battery charges, the terminal voltage of this cell slowly increases until it reaches the stipulated 4.2V mark.
  5. As soon as this happens, pin#3 of the opamp also is subjected at this voltage forcing its output pin#6 to go high, which in turn prompts the IC4017 to shift its pin#3 logic high to it next pin#2, toggling the driver stage of this pin into action.
  6. The above shift activates the charging of the second cell of the lipo battery in the same manner as it did for the first cell.
  7. The process now continues and repeats itself by scanning and charging the cells in steps continuously.
  8. Thus the lipo battery cells are maintained with optimal charging level through the above explained lipo battery balance charger circuit as long as the circuit remains connected with the lipo cells.

Parts list for the above diagram:

All resistors are 10K, preset is 10K, all transistors are BC547, zener diodes are 3V/ 1/2 watt

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26 thoughts on “Lipo Battery Balance Charger for Charging of Series Connected Lipo Cells”

  1. As the lipo batteries get closer to the 4.2 volt level are we not supposed to reduce the constant current as it gets closer to this level. Is there a way to include that in this circuit. Or is that not so important.

    • constant current does not mean the current would be forced at that level constantly, no. Constant current means, the specified maximum amount of current level will never be allowed to increase by the source, or and will be restricted at the specified level..
      It is the load which decides how much current it has to consume from the source, provided the voltage is maintained at the right level.

      Here since the input is fixed at 4.2V which is compatible with the Lipo rating, the current is automatically shut down by the Lipo when it reaches the 4.2V mark.

      The constant current could become dangerous if the input voltage is increased above the 4.2V mark.

      Therefore current reduction is not required, the battery will take care of it automatically, however once 4.2V level is reached the supply must be immediately switched OFF

      • Many thanks I now understand. So once voltage reaches the value set by potentiometer at pin 3 then voltages are both equal to 4.2 so no more current is then able to flow. Makes perfect sense. As it always needs a voltage difference for charge to flow.

        So initially on charging at the very start the current is at a fixed rate below a specified level, in other words not say 3amps or something too high. Hence is current limited by the opamp?

        • Glad you found my reply useful….yes that’s exactly what I wanted to explain.

          Whether it is at the start or at the end, the charging current consumption rate is determined by the battery’s internal chemical reactions, the constant current feature just makes sure that under any circumstances the current is never allowed to exceed the safe limit, let’s say if the current is selected to restrict at 3 amps then this value will be never exceeded by the CC source, but within this 3 amp value it is the battery which decides how much current it needs to consume throughout its charging period.

  2. I have a 6s Tattu 16000mah (6 cells connected in series) Li-Po Battery with 2 main +ve and -ve terminal and balancer connector. Do above circuit can charge my battery? The battery’s balance connector should connect to which part of the circuit above?

    • Yes circuit will charge your battery but this circuit is for those who have advanced knowledge of electronics and opamps, because adjusting the preset and setting up the circuit is crucial here….

          • a sample 4.2V may be fed at the shown preset’s upper lead, and the preset slider adjusted to make pin#6 of the opamp just high.

            Can u further explain these 2 sentences? From my current understanding, at initially the 4.2V (from power supply) will feed to the pin3 of the opamp, and the “slider” will adjust the 4.2V to further higher volt/lower volt in order to make the pin6 of opamp output a high signal. What is the slider? a potentiometer?
            ————————————————————————–

            What mean by Li-po charging voltage source (that feed to the pin3 of opamp)? Is it a lab bench power supply with voltage fixed to 4.2V? This charging source can be non-constant output power source such as solar generator which its output depend on current light intensity.
            Is it the Li-po charging voltage source is the same source with the Li-Po charging voltage feed into the cell#1.

            • Teoh, your understanding is mostly correct regarding the mentioned specs.

              4.2V should be from a constant voltage and constant current source, as per the battery specs.

              the slider means the moving shaft of the preset connected with the opamp, which is moved by the screwedriver

              the 4.2V supply source to the opamp and the relay are the same….and should be taken from the same supply source.

  3. hello Swagatam, i am trying to get a battery balancer for 12v battery is series in a 24v circuit. When charging with the inverter,it should be balancing both batteries at same voltage. i dont understand how to use this circuit to get this done..what values of resistor am i using? can i still use this mosftets and transistors?

    thanks

    • hello kentro, the above circuit is intended for balance charging batteries in series, I could not get how many batteries you have used in your battery set. you are saying “a 12V battery in series with a 24V circuit” sorry I could not understand this statement, please clarify this.

      do you mean a 12V battery in series with a 24V battery?

      • OK, for two batteries also you can use the above circuit with a slight modification. Use the outputs at pin#3 and pin#2 and eliminate the remaining outputs.

        to ensure automatic changeover disconnect pin#15 from ground and connect it with pin#4 of the IC

        integrate two relay stage with the BC547 BJTs attached with the above mentioned pinouts, finally configure the contacts with the respective battery terminals.

  4. Hi

    Awesome circuit! Question:
    – For any channel, when it’s counter-pin goes hi, its BC557 turns on, which turns on its BC547, which turns on its relay. Correct?
    – Is it possible for the BC557 to drive the relay directly, and remove the BC547 from the circuit?
    thx!

    • Hi Thanks,
      actually it’s the opposite, during a high state of the 4017 output pin, the relevant BC547 is triggered ON, which in turn triggers the associated BC557, therefore BC557 cannot be directly used with the 4017 outputs.

  5. Hi thank you for your post.
    I couldn't understand how to choose battery for op-amp noninverse input. Dont we need to set a circuit for switching between batteries.

    • Hi, thanks, the switching between the cells is done by the relay….please read the article completely for the details.

      the source voltage at the non-inverting pin is supposed to be from the selected charger power supply source.

  6. Gday Swagatam,

    Just a query on this and your article on ((Lipo) Battery Charger Circuit).

    It seems you have it so only 1 cell at a time is getting charged with this, would it be possible to adapt this to have all cells charging at the same time?

    Otherwise, following your other article, would this work;
    Supply 4.2v to each cell and have a cutoff trigger of 12.6v, checked against the whole pack assuming 3s?

    • Hi Daniel,

      charging all the cells together might not be feasible, because the positive of one cell is the negiative of the next cell, so charging them together can cause short circuit acroos the terinals, so according to me that's not possible and not recommended.

      charging the whole unit with a single supply is the ordinary method that people usually implement, but that has severe drawbacks and that's why the above circuit was designed to ensure individual charging of the cells and ensure max efficiency and long life for the cells

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