12V Battery Shutdown

Discussion in 'The Projects Forum' started by anotheruser1, Dec 10, 2011.

  1. anotheruser1

    Thread Starter Member

    Dec 6, 2011
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    I have a bank of batteries that i want to shut off at around 12 volts. The normal running voltage with generator running will be approx 13.8. I have looked at anything i could find here that was close, but nothing matches what i need. Anyone have any diagrams of such would be appreciated
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    How much money can you spend?

    How much current does your load draw at most?

    What hysteresis do you want? Off at 12V and On at ...?
     
  3. anotheruser1

    Thread Starter Member

    Dec 6, 2011
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    4
    Hi, I have a ton of different ic's and things I got from a plant shutdown, so hopefully I have everything already. It is not a super critical thing, but I would like it to cut off at 12 and cut back on basically anything above 12, so for numbers ill just say cut on at 12.8. As for current it will be controlled by separate relays because it is a 400 amp load. Thanks for helping
     
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Something like this would work:
    [​IMG]
    It turns on the output voltage (relay) when the input voltage makes a transition from <12.9V to >12.9V. It turns off the output voltage at about 12.2V. (high to low transition)
    The p-channel MOSFET could also be substituted by an PNP transistor with some changes to resistors made.
    The ON transition is determined by the resistor divider 41.2k and 10k. The hysteresis (OFF transition) by the 41.2k in parallel with the 750k resistor and the previously mentioned 10k resistor.

    The small relay would have to switch your 400A :)eek:) contactor.
     
    Last edited: Dec 10, 2011
  5. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Normal operating voltages of 13.8V From a 12V battery is not practical as this is the float charge voltage. As soon as this voltage is removed the battery will drop quickly, especially under a load down to in the ballpark of 12.7V.

    Since Lead Acid Batteries begin to build sulfate on their terminals at about 12.4V I would not recommend having the lower set point much lower than that.
     
  6. anotheruser1

    Thread Starter Member

    Dec 6, 2011
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    4
    Thank you I will give it a try and see what happens.
     
  7. anotheruser1

    Thread Starter Member

    Dec 6, 2011
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    The battery bank is for a emergency back up system, when its in use there is a generator running. If the generator fails, I want it to shut off. So about 12 volts would be good for this purpose. Thanks for your input
     
  8. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Yes, you are right, it's worth a little sulfation for emergency power!
     
  9. anotheruser1

    Thread Starter Member

    Dec 6, 2011
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    i have one question, the diode with the arrow pointing to it (see attachment) where does that go? i have never seen a connection in the middle. thanks
     
  10. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    That's not a simple diode, it's a voltage reference, a very common one.

    Here goes the datasheet: http://www.fairchildsemi.com/ds/TL/TL431A.pdf

    When the voltage at the middle pin goes higher than 2.5V with respect to the anode then it conducts (cathode anode).

    It can be done with other parts, that's the easiest I could think of.
     
    Last edited: Dec 12, 2011
  11. anotheruser1

    Thread Starter Member

    Dec 6, 2011
    30
    4
    Learn something new everyday, thank you for your time
     
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