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ground issues with switching batteries

Discussion in 'Power Electronics' started by cyelis1224, Aug 26, 2016.

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  1. cyelis1224

    cyelis1224

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    Aug 26, 2016
    i am attempting a project in which i need to be able to switch using several 4v cells into series and obtaining a higher voltage using a microcontroller, a relay, diodes, and some mosfets to make the switch happen. i planned on using diodes to stop the current flow from the grounds of the batteries from becoming a higher ground potential than the power supply i am using to switch the mosfets. the output of the power supply would be controlled by a simple relay that would be connected to a microcontroller. is this possible? am i going about this the right way? i cannot simply use the mosfets to put the batteries in series without the diode because as my ground potential on my batteries gets higher than my supply the mosfets fry and the ground wires burn up. i was thinking a diode would stop the reverse current flow from the batteries making the mosfets only capable of seeing the ground that i need for them to switch and not the ground of the overall set of batteries. any pointers would be greatly appreciated. image attached but it is poor quality and some of the symbols may be wrong as i have very limited understanding of electrical engineering. thanks!
     

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  2. BobK

    BobK

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    Jan 5, 2010
    What exactly is it that you are you trying to do? Why can't you simply connect the batteries in series and then have a single switch between them and the load (either high side or low side). Are you trying to switch the batteries from being in parallel to being in series?

    Bob
     
  3. cyelis1224

    cyelis1224

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    Aug 26, 2016
    Yes I am trying to take a set of batteries and automate putting them in series to be used and taking them out of series to be charged. I'm really just asking if using diodes in the way I described will keep the mosfets from seeing the ground potential of the series so that a short does not occur.
     
  4. BobK

    BobK

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    Jan 5, 2010
    No, they will not. Your circuit has 24V shorted to ground through a forward biased diode. Guaranteed to smoke the diode.

    What type of batteries are they? Some types can be charged in series quite effectively.

    Bob
     
  5. BobK

    BobK

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    Jan 5, 2010
    One way to solve your problem would be to use opto-isolators and logic level MOSFETS that could have the gate driven by the 4V batteries themselves. That way, no common ground is required.

    Bob
     
  6. cyelis1224

    cyelis1224

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    Aug 26, 2016
    Didn't think about that. Thanks bob
     
  7. cyelis1224

    cyelis1224

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    Aug 26, 2016
    Won't the gate of the mosfets close once the batteries start to get used? I need the batteries to stay in series until they have around 20% left. As the batteries drain wouldn't they split back apart when the voltage drops? And also I'm gonna be drawing a lot of amps across the system as well, if the gate is not wide open won't my wires get hot and come un-soldered?
     
  8. BobK

    BobK

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    Jan 5, 2010
    You would need a MOSFET that can pass the needed current at the minimum battery voltage.

    Here, for example, is a MOSFET that can handle 10A with a 0.25V drop at 2.8V Vgs That sounds like about the worst case you might see.

    http://www.mouser.com/ds/2/302/PSMN9R1-30YL-843401.pdf

    Bob
     
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