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I've never really investigated cell balancing too much, so I got a two cell protection and balancing board to explore.
The module has three distinct circuitry sections. A two cell protection circuit with charge/discharge MOSFETs to protect against overcharge and over discharge of the cells, where the first cell to reach 4.25V while charging or 2.5V while discharging disconnects the cells from the charger or load.
The other two sections are completely separate and work to bypass low level current across a fully charged cell to allow the other to keep charging.
It's really important to note that the balancing must be done at low current - less than 60mA in this case. A higher current can still overcharge the cells.
The balancing concept is very simple. When a cell reaches 4.2V a MOSFET clamps a resistor across it to prevent the cell voltage from increasing further. That allows current to keep flowing in the circuit to keep topping the other cell up until it also reaches its full charge. The MOSFET turns back off when the voltage drops back down to 4.19V.
The charge current shouldn't exceed the level where the voltage across the bypass resistor can exceed the upper threshold voltage of the cell, as otherwise it will keep charging it until the protection kicks in at 4.25V. It means the charger will have to be limiting the current as the voltage nears roughly 4V per cell.
My thoughts of using an over-simplified three wire charging system would involve the cell protection circuit cutting in when the first cell reached its upper cutoff threshold and then trickling current through the cells directly to allow the balancing circuit to match the voltages. Not ideal, but a lot simpler than a sophisticated charger. The resistor could not be left in circuit for normal use as it bypasses the protection MOSFETs.
A nicer system, and probably the correct one, would just involve the charger putting out marginally above 4.2V per cell, with a series resistor to limit the current progressively as the pack charged to allow the balancing system to do its job.
These are just random thoughts and not intended as a guide to the correct way of using this module.
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15 thoughts on “Lithium cell balancer reverse engineering with schematic”
  1. Avataaar/Circle Created with python_avatars kapegede says:

    You explained it so well! Thank you very much. So this is a passive balancer (the 68 ohm resistor reduces the capacity a bit until the weaker cell catches up). Furthermore there are active balancers out there, too. And I'm keen to know how they work exactly, too.

  2. Avataaar/Circle Created with python_avatars PicoNano says:

    I was wondering if partial charge and discharge of a cell, counts as a charge/recharge cycle?
    After googling for hours, I can't find a definitive answer to that. Only opinions. No research papers!
    For this reason, I don't charge my moped after each little trip to the store. My range is 40Km, and I recharge every 30Km

  3. Avataaar/Circle Created with python_avatars PicoNano says:

    Even my Daly 100A 48V LiFEPO4 BMS for my scooter, only balances at 50mA. Charges at 500W until the first cell hits the threshold, then it stops and goes into trickle charge mode @~14W.
    Daly sells an active balancer with Bluetooth, but the price was the same as the BMS, so I just went with the BMS and it's 50mA limitation.
    I thought this value was too small, but after graduating from Google university, I found pretty much all brands are like that.
    Thankfully, I had to balance them once by leaving the charger on for 24Hrs and let it trickle charge all cells @50mA, to the 3.65V maximum per cell.
    Once balanced, they all seem to charge and discharge the same amount and don't require balancing. I contribute that to matched internal resistance and capacity of each cell.
    I don't know if that will change with time or not. I figured the worst thing would be that one cell dies sooner and shuts off the BMS and when that happens, I'll let it on the charger for 24hrs until all cells are full and balanced once again.

  4. Avataaar/Circle Created with python_avatars Simon Tay says:

    Current can flow through a MOSFET in either direction without resistance when turned on? I had always assumed MOSFETs had to be connected so current flows from drain to source. Ive never actually tested it. I didn't know that. So does that mean you could use a MOSFET like a triac to but when turned off you would just have half wave AC?

  5. Avataaar/Circle Created with python_avatars Ed R says:

    I use these on the 2S 18650 battery pack on my bicycles automatic shifting. I got them for the protection and didn't believe the balancing part of it. However the packs have stayed perfectly in balance and maybe this is why. I have never balance charged the pack and it receives a lot of charge from a front hub dynamo.

  6. Avataaar/Circle Created with python_avatars Bruce Reichert says:

    I wonder if this would work on the large solar lawn lights, that take 2 18650 batteries. Because those are built so cheaply that one battery always seams to suffer more than the other.

  7. Avataaar/Circle Created with python_avatars mark1138 says:

    you need to build ssd bigC approved

  8. Avataaar/Circle Created with python_avatars Assassinlexx says:

    So it is a dab circuit.

  9. Avataaar/Circle Created with python_avatars Teh Freek says:

    ISTM that you could do the charging via B+, BM, and B- with a smart three-terminal charger, and use P+ and P- for output only (or brute force quick-charge, of course). That would involve more wires, but would maximize balancing effectiveness.

  10. Avataaar/Circle Created with python_avatars Tiger Cat says:

    Typically a Li-Ion charge controller like TP5100 is used with a 2S balancing BMS, which limits charge current properly and works well. But buyer beware – many "chargers" (e.g. eBay "8.4V 1A US plug 1000mA charger adapter for Lithium Ion Battery Li-ion LiPo 2S") sold are just 8.4V power supplies with no Li-Ion charging logic.

  11. Avataaar/Circle Created with python_avatars William Petty says:

    You missed how these actually work. Each circuit operates independently but not necessarily at the same time. You are correct that the large mosfets will disconnect when one cell overcharges, but the balance circuit stays operating and will slowly discharge the high cell. Once the voltage of this high cell drops to a preset voltage, the main mosfets reconnect and charging resumes for both cells. This cycle repeats until the lower cell catches up to the higher voltage cell. With each cycle the lower cell gets an additional amount of charge equal to what was discharged from the higher cell while disconnected. This does not require the charger to limit current or know anything about the current state of charge of the cells.

  12. Avataaar/Circle Created with python_avatars t1d100 says:

    Interesting. Thank you.

  13. Avataaar/Circle Created with python_avatars jlucasound says:

    I thought the ideal voltage for storing these cells was 3.8 volts. The ultimate circuit in a BMS would be to take the (individual) cell to 4.2V and if it wasn't discharged by use in a determined amount of time, the BMS circuit would discharge the cell(s) down to the optimal storage level of 80%. I know. Only in a perfect world! 😄There of course would need to be something in place that would allow reset so you don't start out with 80% charge when you need 99%. Also, in industrial usage, these cells would be constantly in use. I guess I am thinking of consumer applications, such as an electric chainsaw used seasonally.

  14. Avataaar/Circle Created with python_avatars AgW says:

    On a related note, I would love to see you do a deep dive on the budget/cheap spot welders used to build lithium packs with, like the 10-20£ wish 12v ones. I often thought of buying one but are worried that they may do damage to my pack or my 12v battery/source. Could you see if a regular iup battery and/or a car battery charger (6 Amp) could work as a source of power to run it, would a super capacitor be better/required to make it work/not kill my 12v lead acid battery?

  15. Avataaar/Circle Created with python_avatars Mark Rice says:


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