Great marine electrical article in ..

Discussion in 'OnBoard Electronics & Controls' started by rwatson, Nov 14, 2011.

  1. thudpucker
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    thudpucker Senior Member

    There's a long dissertation on 'thermal Runaway' but it means only an over loaded circuit, and one component get's hot, the current flow increases, more heat, etc and then the Smoke escapes.
     
  2. pdwiley
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    pdwiley Senior Member

    OK, so how does this differ from what happens in any other electrical circuit?

    Standard practice is to size the circuit breaker to protect the wiring not what's attached to it. If you've done this, you can't get the above scenario as the breaker trips out. If you haven't done this, you're an idiot and deserve to suffer the consequences of something totally predictable.

    PDW
     
  3. thudpucker
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    thudpucker Senior Member

    Thermal runaway was the original term given to the Early Transistors when the Heads popped off during circuit use.
    There are several types of Transistor Circuits that don't have Circuit breakers or fuses.

    "Thermal Runaway" may have been in use long before that too. I just don't go back that far.

    I don't know if "It" does happen differently in any other circuit.

    Also, you may not know this but the MOPAR's of the 60's were often accused of engineering circuit and components to protect the fuses.
    Very often a Component would go and the fuse would still be OK.

    As well as.....the Chinese fuses we got in the early 70's were a little like Circuit Breakers in that they would open under a load and close when the load was gone.
    Them were buggers to find till we got on to them and replaced them all with American fuses.
    "Sizing Circuit breakes?" We dont need no sized circuit breakers.

    Or; "here, use this breaker that's been in the barn for a lotta years!"
     
  4. rwatson
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    rwatson Senior Member

    Not at all

    The article is at pains to point out a lot more than that.

    The problem is batteries reaching a high "charge-acceptance rate" when the battery very hot from is being charged under a high output situation, this is where that battery absorbs all the charging input, without increasing its own voltage, and the excess current creates dangerous problems even in AGM batteries.

    Its not the circuits its feeding that are the problem, its the heat in the battery that causes the charger to push the battery to a dangerous temperature.

    He recommends batteries that are worked hard only be charged through temperature-regulated charging systems for this reason.
     
    Last edited: Nov 25, 2011
  5. waikikin
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    waikikin Senior Member

    My paper copy just turned up, always some interesting articles.
    Great for you to raise awareness of the article & magazine, 'cos more knowledge is good for every one.
    All the best from Jeff.
     
  6. michael pierzga
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    michael pierzga Senior Member

    Seems runaway battery charging and fire is becoming a common occurance. Makes me nervous. Im sailing with a 1500 amp hour 24v bank. Im curious..How do you fight a battery bank fire .
     
  7. thudpucker
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    thudpucker Senior Member

    " Not at all"?
    Watson we are saying the same thing. I just simplified it.

    Some part of a circuit is overheating, suddenly more current is flowing.
    Whether it's the Battery or a Transistor, it's in "thermal Runaway" or heating beyond its Wattage capacity.
    All this while the source is still energizing the circuit.

    Older lead acid batteries exploded the Top off the cells. In all the years I worked on Automotive and Marine systems, I never heard of or saw a Battery burst into flames and threaten the Vehicle.

    In the case of a Transistor overheating, the transistor normally burns open long before any circuit breaker or fuse would sense the overload.
     
  8. rwatson
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    rwatson Senior Member


    I think its a different thing he is discussing. Nothing to do with " Chinese fuses we got in the early 70's were a little like Circuit Breakers ". In your very valid scenario, a circuit is receiving too much juice - hence the need for fuses, circuit breakers and the like.


    This point he is making is about the battery overheating from charging current - impossible to guard against except with a temperature sensing charging system.

    This point he is making is about the battery drawing excessive power, due to its higher than normal operating temperature caused by a combination of high current draw, location in the hull etc etc.

    The heat in the battery increases until some part of the battery ( not the circuits either side) fails dramatically.

    You may not have encountered it, but he points out that it has happened to him during battery testing in a barn in midwinter.

    He might be making it up, but then again, he might know what he is talking about.
     
  9. pdwiley
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    pdwiley Senior Member

    Yes, thanks for that, I can see the difference and I agree that it is a different animal entirely. One I'd never considered and I can see this happening, for example, with an electric anchor winch under heavy load pulling big current from the batteries while they're being fed from a running engine. The wiring is sized for the current & so is the circuit breaker, it's the battery that's the failure point.

    Something to think about...

    PDW
     

  10. michael pierzga
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    michael pierzga Senior Member

    I normally see the Positive terminal on the battery fail... explode...and physically detach from the battery. Optima brand red tops frequently blow the terminal
     
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