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I have been reading about BMS systems and currently looking for open-source projects to analyze them and learn more in practical application.

The schematic is this and my interpretation is: -The same circuit is used for each cell Battery 1...4 which consist mainly on comparator for V+ and V- references, npn bjt transistor, phototransistor, resistors, zener diode and potentiometer. (But I don't know what is the function of such circuit, is that a balancer?)

-The another circuit would be the one which implies the irf540n mosfet at the right of the schematic, is that one for balancing as literature says? or in this case that is not correct? enter image description here

Circuit 1 (Cell circuit):

enter image description here Is pretty similar as stated in literature:

enter image description here

So, question 1 would be: could somebody help me with the correct interpretation?

Question 2 would be: What functions perform this BMS? (balancing? over/under voltage? over/under current?) (as I could see in the literature this design seems to be a monitor and monitors does not balance each cell voltage) Many thanks

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1 Answer 1

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This is a balancer!

The 120 Ohm resistors are the balancing resistors.

1N472B is a Z-Diode with 4.1V, this is used as voltage reference for the comparator LM311.

If the voltage is higher than 4.1V, then the comperator is going to high and switching the LED inside of the optocoupler on. Then TP16 is on GND level.

This means that Q2 is off and Q1 is switching on. But it must do the opposite. Q1 should go off if one cell is reached 4.1V so that the loading process is interrupted!

If TP16 would be connected with TP11 with a bridge, then it would work as over-voltage battery protection. It would stop the loading if one of the cell have reached 4.1 Volt.

Balancing-process: The LED of at least one PC817 is active too. There is a current flow of 26mA parallel to this cell in this moment, this means that the cell-voltage will be reduced slowly and after a while the loading process starts again if the voltage is low enough. This makes it possible that all cells get loaded to the same voltage.

There are several problems in this circuit. The voltage reference is not good and it is not doing what it should. The quiescence current of the LM311 is high, around 5mA. There is a current flow out of every cell, even if the loading of the battery is stopped. The LM311 is consuming energy and he can not be deactivated.

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  • \$\begingroup\$ Thank you much for your help on the interpretation @MikroPower I agree with all you stated but I have a doubt with the TP16 you are refering to, since I looked in the schematic by command and I am not able to find it Which element is TP16 for you??? Thank you so much again \$\endgroup\$
    – imt_blake
    Commented Mar 10, 2023 at 19:19
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    \$\begingroup\$ I have make a mistake and corrected my answer. It is a cell-balancer. \$\endgroup\$
    – MikroPower
    Commented Mar 10, 2023 at 19:21
  • \$\begingroup\$ Thank you for your detailed explanation! Could you tell me which element is TP16 for you? since I can not find it in the schematic by command and by eye \$\endgroup\$
    – imt_blake
    Commented Mar 10, 2023 at 19:23
  • \$\begingroup\$ What is TP16??? please! since there is not TP16 in the schematic, please help, thank you @MikroPower \$\endgroup\$
    – imt_blake
    Commented Mar 10, 2023 at 19:31
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    \$\begingroup\$ TP10 (not TP16) is only a Test-Point. There you can measure the voltage if you want to debug the circuit. This are only little cooper points on the PCB to have a easy electrical connection. - - - The problem is: If the voltage of each cell is under 4.1V, then there is a current flow over R31 -> Q1 is active and pulls the gate of Q2 to Ground. But this would stop the current flow over Conn_01x02 into the cells. This does not make any sense. But if you connect TP10 (not 16, it is a low image resolution) with TP11, then this will work like desired. \$\endgroup\$
    – MikroPower
    Commented Mar 10, 2023 at 19:50

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