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I want to control FET to discharge battery cell with 5V logic from microcontroller with common grond with battery as shown.

schematic

simulate this circuit – Schematic created using CircuitLab

What is the proper to turn on/off the mosfet with difference source voltage?

schematic

simulate this circuit

This schematic above is my initial idea. it seem like when the bjt conduct, the gate voltage will be negative and not the same voltage for all cell. This might destroy MosFET if it connect with higher cell so I searching for better solution. Note: this diagram came from TMS570 enter image description here

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    \$\begingroup\$ Step 1) Choose a FET with specified Rds(on) with Vgs at the lowest cell voltage, like 3V. Step 2) Use optoisolators to drive Vgs for each FET, powered from the cell it connects to. \$\endgroup\$ Feb 18 '21 at 19:31
  • \$\begingroup\$ @BrianDrummond Optoisolator is good idea. but I don't see this thing on BMS ICs can it be something else? \$\endgroup\$
    – M lab
    Feb 18 '21 at 19:52
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    \$\begingroup\$ @Mlab Use the cell's own voltage to power the floating driver drives the MOSFET that discharges it and use an opto so the microcontroller can control it with it without sharing the same reference voltage. In otherwords, just connect an opto to the gates of each MOSFET and power the opto off the cell. \$\endgroup\$
    – DKNguyen
    Feb 19 '21 at 5:06
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    \$\begingroup\$ There are voltage translator tricks that are however somewhat complicated and usually only used in ICs. In short you pull down a current from the upper rail and you develop that in a gate voltage referred to the source you are driving. It's used all the time in the upper gate drivers for bridges. \$\endgroup\$ Feb 19 '21 at 7:33
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    \$\begingroup\$ @Mlab Exactly, which is why I suggested the opto. \$\endgroup\$ Feb 19 '21 at 15:41
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You'll need to take care not to discharge the cells too much. With a cell voltage of 4.2, I'm going to assume that you are using some lithium chemistry batteries. If you do not have proper discharge protection, you can destroy those batteries by discharging them too deeply. When you try to charge them again, they become a serious fire hazard. I agree with Brian that your component selection is not going to work. The N-Channel MOSFET you specify requires the gate to be driven positive relative to the source to conduct and your circuit is taking it below the source voltage when Q1 is biased on in every case except the last cell that is referenced to ground. This circuit will also take the gate of the top MOSFET outside the recommended operating range which is +/- 20V. The MOSFETs will likely be conducting when the circuit is disconnected from the logic source (or it's turned off) since the gate is tied to the drain through a resistor and the threshold voltage is 2-4 volts. The gate essentially has the cell voltage applied to it if Q1 is not conducting. Maybe this is the behavior that you want, but again, you need to have some under-voltage protection for lithium chemistry batteries. Taking them below 2.5 volts can damage them. Here is a circuit (possibly similar to what Brian was thinking) that can operate with 3.3 to 5V logic and provides gate control that is independent of discharge rate. This circuit may be dangerous for batteries since it can discharge below 2V. Discharge Circuit You may be able to tune the values of R2 and R4 to get the minimum discharge voltage that you want.

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  • \$\begingroup\$ The mosget in my schematic is just for conceptaul (not specify yet) my plan is to use microcontroller function as BMS. Maybe using optoisolator is optimal way to do this. \$\endgroup\$
    – M lab
    Feb 19 '21 at 4:42
  • \$\begingroup\$ Depending on the balance current, the optocoupler could even replace the FET as the main switching element. \$\endgroup\$
    – Hearth
    Feb 19 '21 at 5:39

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