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    MOSFET as switch to cut power to a BMS in a 4S Li-ion?

    Hi,
    I am building a power-pack from old Nissan LEAF modules. To make sure not to destroy any of the cells in the 4S configuration I am using a "1-8S Lipo Li-ion Fe Battery Voltage 2IN1 Indicator Tester Low Voltage Buzzer Alarm For RC Car Boat LED 3.7-30V Dual Speaker" since they are very cheap, and very light. (I am using an external balancing charger).

    My problem is that I can not turn this device off unless 4 out of 5 pins are disconnected. Even if I cut the ground the device will still be active. I want to just push a switch and turn the whole ting off.

    Since this device will "buzz" at a set voltage, any significant voltage drop will cause it to not work as supposed. It is however possible to change the the setting on when the buzzer goes off, but not very much since the cells are nominell 3,7.

    As of now I am using 5 relays to cut all power, but that should not be needed since only two of them are cutting the output power, and 3 cuts power only to the buzzer-device.


    So is it possible to use MOSFETs to replace the three relays? If so, witch one?

    •   Alt6th December 2017, 19:07

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    Re: MOSFET as switch to cut power to a BMS in a 4S Li-ion?

    Hi,

    It's a shame the link for what you're using has no user manual or schematic diagram or anything, but they are really cheap, aren't they? :)

    The only point to using a relay would be for isolation, I assume, and it doesn't sound like you need that to turn off an LED and a buzzer, I guess.

    You should look for (as) low RDS on (as you can find) MOSFETs and I'd look for low VGSth devices, or (ultra) low Vce BJTs - which also can have low equivalent RDSon values, because you'll always have a voltage drop with that method - which it seems you do not want.

    Hold on... "Since this device will "buzz" at a set voltage, any significant voltage drop will cause it to not work as supposed. It is however possible to change the the setting on when the buzzer goes off, but not very much since the cells are nominell 3,7. " ... Do you mean the buzzer will mess with the external balancing charger or with what? It's not clear to me.

    But back to the low RDSon and VGSth MOSFETs or low VCE BJTs, presumably the devices that will do to replace the relays.



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    Re: MOSFET as switch to cut power to a BMS in a 4S Li-ion?

    Thanks for your reply d123,
    I am not a native speaker of English, so some might have been "lost in translation" in my post. Sorry. And not being an engineer I ask dumb questions. And on top of that I tried to simplify my problem.

    The "buzzer" is the device in the link. It is not really a BMS since it can not direct more current to any cell. I do not know what to call it. It just gives the voltage on the stack/battery, and on the individual cell and it will "buzz" when any of the cells dips below a set voltage value. I want use that "buzzers" output to cut all power so that the whole battery pack shuts down, including the "buzzer".

    I have the pack operational, but is very bulky.

    Since each transistor in this setup will operate at different voltage, I am uncertain if it is even possible without some sort of relay. I can accept up to a 0,05 v drop. More than that will make the "buzzer" be pointless since it will show the wrong voltage.



    •   Alt7th December 2017, 21:16

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    Re: MOSFET as switch to cut power to a BMS in a 4S Li-ion?

    Hi,

    Not sure, might be possible with PMOS devices, but as I said before you'd want low RDSon and low VGSth. The gates would be on the buzzer power/high line. Theoretically, and very loosely, They'd pass the voltage until the buzzer line went high, then they'd turn off.

    What voltage(s) do you have for the transistors (/relays)?

    Really, if this has several different voltage levels that are supposed to trigger several of these buzzers at different times, and you can provide a separate supply voltage if necessary, then something like the LM3914 is used for this.

    I think that thing is basically a voltmeter with a buzzer that does a couple more things related to batteries/voltage levels, but more than that I couldn't tell you.

    Lastly, to paint the worst picture, if the transistor version has more than a 50mV drop, you may find it becomes a little more complicated regarding number of parts needed, involving comparators or op amps and possibly more resistors and ditching the MOSFET version, and may find the bulky relay version will do - but it's a thought about comparators instead of transistors, depending on how much current the buzzer needs to buzz its little heart out.

    - - - Updated - - -

    Hi again TorC,

    To be honest, on reflection, I haven't got much of a clue what you're doing, I think. To invert the saying about pictures painting a thousand words, unfortunately words paint a thousand pictures...

    You seem to understand your circuit well, fortunately. This will probably be much quicker to resolve if you upload a picture (using the insert image icon on the toolbar), pen and paper and a block diagram thing should be enough, including the actual wires/connections you're interested in; and a list of the voltages you have e.g. the voltage to power that BMS thing and the buzzer, any significant currents drawn that seem important to how what you're doing works or not if there are any and if you know that, and the voltages you need to sense for the cut-off points.

    This way a more able member might be able to suggest something appropriate quickly.

    That way a more



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    Re: MOSFET as switch to cut power to a BMS in a 4S Li-ion?

    [Click image for larger version. 

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    Thanks once again. I tried to simplify. Even though the picture in my head was very clear, others can`t see it....

    I have uploaded a basic drawing (made in Word....).

    The "buzzer" (Low voltage alarm) have 5 cables. 1 is ground, 2 is 3,2-4,2V, 3 is 6,4-8,4V, 4 is 9,6-12,6V, 5 is 12,8-16,8V.

    As of now all are controlled by 5 NO relays. The buzzer will be partly active even if ground and cable 5 are isolated. All 4 positive cables should have the same voltage drop, and I will need the relays on two mains out. So the alternative is using transistors on all as switches, and keep the two relays since they are handling a lot of current.

    Maybe it just a whole lot simpler to keep the 3 relays.



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    Re: MOSFET as switch to cut power to a BMS in a 4S Li-ion?

    Hi TorC,

    It's not you, I'm a bit slow at things like this, frankly, so don't worry.

    If this is correct: Several specific voltages need to sensed and when sensed something has to turn off three lines as two are already handled by relays.

    Rhetorical: How big are the relays that could be replaced by semiconductors, like bricks or sugarcubes?

    I don't know much, anyway. If it weren't for the 50mV voltage drop limit on each line, I'd still consider using a quad comparator IC followed by (unfortunately) an inverter IC followed by a quad analog switch IC. That would leave one comparator, one analog switch and three inverters unused from all three packages, but if the relays are big then I guess this circuit would take up less space.

    Quad comparator suggestion: LM339
    Inverter suggestion: CD4049
    Quad analog switch: CD4016

    All three are available as through-hole ICs, and SOIC. There are analog switches with far lower throughput resistance, but from the quick look I had they are all low voltage devices.

    But, as you can't use this due to that 50mV, with my limited knowledge I'd return to the quad comparator controlling three PMOS MOSFETS. A very quick search of ONSemi makes me think maybe the FDD4141 might be suitable. If you want to browse the OnSemi PMOS low RDSon selection yourself (recommended), here is the link.

    - - - Updated - - -

    ...

    As you appear to have a 12V supply, the LM3914 would (without doing the maths about voltage division per battery voltage sensed, please look at page 8 to see how it works on the inside) seem a simple solution to drive the MOSFET gates as comparators would need reference voltages set, which this device already has because of the resistor ladder inside, basically.



    •   Alt8th December 2017, 21:40

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  7. #7
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    Re: MOSFET as switch to cut power to a BMS in a 4S Li-ion?

    Hi,

    The images are of simulations I did to see how that P-Channel MOSFET might work for you - looks good r.e. the 50mV limit.

    The simulations use LV devices as that's what's available in the simulator, but the LMV339 is the same as the LM339 except the LM339 operates from 2 to 30V, so that's not an issue. I was only able to emulate battery 1 (3.2 to 4.2V) because the LMV339 is only up to 5V supply. TLV431 is 1.24V reference, in comparison TL431 is 2.5V reference; there are plenty of ~1.25V references out there. As mentioned, LM339 is 1 package with 4 comparators.

    FDD4141 cost around 1USD/1GBP/€1.50 each, whatever currency is in your country. But... smallest amount I saw was a pack of 10. They only seem to come in DPAK-3 (SMD), but that's easy to solder.

    I haven't simulated this with the batteries in series, so guess the resistor dividers would need to be changed, perhaps the whole of that side of the circuit but I was just trying to give an idea of one way to do what I think I have understood you are trying to do.

    One nuisance may be having to get a load of resistors you may only use once, maybe the relays are a simpler choice, however, the MOSFETs will last longer, worthwhile bearing in mind.

    Click image for larger version. 

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    - - - Updated - - -

    If what you want to do is this:

    Click image for larger version. 

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    It won't work. The MOSFETs need a reference value of their own for the gate voltage to work, which comes from Source to Drain voltage. These will all have different voltages so either the top one or the bottom MOSFET will not turn off when sent the same buzzer signal as the other two... Relays don't care what the voltage on the other side of the coil is, MOSFETs do, unfortunately.

    Again, for that battery stack I think what's needed is differential measurement of each battery voltage, followed by a comparator, then the MOSFET. The more I think about it, the more I think that if the relays work, they just take up more space than liked, I'd stick with them as from the little I know it might turn out to be a rather complicated circuit (that ironically emulates the 96 cent device in function).

    Sorry, TorC, I've tried to think of a semiconductor solution, but can't come up with anything better.



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