Circuit to disconnect battery from Short Circuit?

Discussion in 'General Electronics Chat' started by tglaria, Sep 2, 2014.

  1. tglaria

    Thread Starter New Member

    Aug 11, 2014
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    Right now I have a device that works with a 24V NiMH battery. This device has a connector where the charger plugs so the battery can be charged, obviously, the problem is that recently a piece of metal got into this connector making a short circuit and showing a big pretty shiny light (an electric arc) leaving a big nasty black mark on the connector.

    Now I'm having trouble finding a solution to prevent this. The idea was to put a protection circuit that disconects this connector from the battery when it the carger isn't connected.
    I tried a relay to use only enable the circuit when the charger is on (and powering the coil) but once the circuit energized, the battery keeps the coil energized so it never shuts down.

    Right now I have no idea which kind of circuit could disconnect the connector pins from the battery when charger is not plugged in.
    Ideas ?
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    how about a fuse or circuit breaker?
     
  3. tglaria

    Thread Starter New Member

    Aug 11, 2014
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    Yes and no.
    The problem with the fuse would be to replace it, the idea is to just prevent the short circuit but with the possibility to function after the short is fixed.
    The problem with the circuit breaker (as far as I know) is that it has a voltage drop which could prevent the battery to charge to its maximum capacity, this is why I was searching for a circuit with no voltage drop. Am I mistaken here?
     
  4. Lestraveled

    Well-Known Member

    May 19, 2014
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    I use AirPax circuit breakers for my chargers. The 30 amp unit that I use has .003 ohms resistance (DC).
     
  5. tglaria

    Thread Starter New Member

    Aug 11, 2014
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    Do they exist with auto reset?
    I need it to be able to reset itself without human interaction.
     
  6. inwo

    Well-Known Member

    Nov 7, 2013
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    Post a picture of the plug and socket.

    Any room for a reed or prox switch to activate relay?
     
  7. Lestraveled

    Well-Known Member

    May 19, 2014
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    This is not true in practice. The first charger phase is a constant current bulk charge until the battery reaches the transition voltage. At this point the charger then goes in to a constant voltage mode until the current drops below a threshold (1/10 to 1/20 of max). Even though there is a voltage error due to IR loss, the charger will stay in constant voltage mode and as the current drops, the errors due to IR drops with it. Your charger IR losses should not be evaluated at max current, they should be evaluated at the "end of charge current, which is normally 1/10 to 1/20 the max current.
     
    Last edited: Sep 2, 2014
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    How about a self-resetting PolyFuse?
     
  9. Lestraveled

    Well-Known Member

    May 19, 2014
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    Here is a data sheet for the AirPax CBs I use. You can get a remote trip feature but I do not see a remote reset feature.

    MikeML - A polyfuse is worth looking at.

    Tglaria - What is the max current?
     
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  10. Theagentxero

    New Member

    Aug 26, 2014
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    I would agree with everyone here so far.
    And Say you would be well served with a PTC Resettable Fuse.
    Digi-Key has them under the breadcrumb trail of: Product Index > Circuit Protection > PTC Resettable Fuse

    They go from 14ma to 50A trip current.
     
  11. tglaria

    Thread Starter New Member

    Aug 11, 2014
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    The socket is this one:
    [​IMG]

    I don't see space to put something in it, tough I'm willing to evaluate any option.

    The battery is a 24V 4.2Ah NiMH one, during charge, the current measured with a multimeter is around 4A.
    Since I haven't been able to find an auto reset circuit breaker that would work between 24-40V, would a simple diode work in this case?
    I'd have a constant voltage drop and should work with 4A through it, but other than this voltage drop, acording to what you wwote and understand, it should still work ok, right?

    I'll check it out, don't know them.

    I couldn't find a auto or remote reset, if all else fails, I guess I could go with a manual reset one, but it's not the ideal.
    The battery is a 24V 4.2Ah one, last times I charged it, the current measured with a voltmeter was around 4A during charge.

    I'll check them out, thanks.
     
  12. inwo

    Well-Known Member

    Nov 7, 2013
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    I'll look at the data sheet.

    Is the design fixed?

    Could two shorted pins be added for relay activation? ie. 5 pin.

    Oh wait. This is three pin. Do you use all three?
     
  13. tglaria

    Thread Starter New Member

    Aug 11, 2014
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    The connector is a three pin one and all three are used.
    The idea was originally to not change the connector (I was thinking in using 4 pin connector, have two of them connected to the power line and use one to activate the realy coil and the other one to feed the battery, keep the ground pin as shared one).
     
  14. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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  15. inwo

    Well-Known Member

    Nov 7, 2013
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    If possible, the extra pin seems a good idea. +1
     
    tglaria likes this.
  16. tglaria

    Thread Starter New Member

    Aug 11, 2014
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    Apparently it's possible, I'll go this way.

    I'd still like to know if there's an alternative using a circuit rather without this extra pin.
    Oh well, apparently not.
     
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