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Car Battery Problem

  1. Sep 30, 2010 #1
    Car Battery Problem!!!

    Hi all,

    I have to tell my story before actully asking the question. So, please be a bit patient!!

    I went overseas for a holiday for 3 weeks. Before I went, I washed my car. Unfortunately, I forgot to turn off my cabin light. Of course, I couldnt start up my car when I came back. I could not even turn on the dash borad. I guess since the cabin light blub consumes very small power, so it eventally drain all of the energy in the battery.

    I have ask another guy to jumper start (or cable start) my car. unfortunately, we reversed the polarity for about 20 sec.

    For the ease of explaination:
    The car with battery - Car A
    My dead car - car B

    The way we did is car A positive to car b negative, and Car A negative to Car B positive.
    Car A engine was on at all time.

    Of couse, car B would not start, and I saw the cable start smoking. and very hot. i quickly disconnected.

    after 1 min, we tried again with the right polarity. And my car finially started and everything was fine. i drive like normal now.

    I have a few questions out from this story. I hope any experience Physicists or Engineers may be able to answer..(by the way, i am an Mechatronic engineering grad)

    1) have i actually damaged any of my battery or electronics? - like ECU or generator etc. or car A.

    2)while it was in wrong polarity, can anyone explain what is happening in car B battery.
    -like reversing current in the battery, or any current is going to the Car B electronics.

    I have asked a mechanic in a garage. he said:
    - since Car B's electronics were still connected to Car B battery while the reverse polarity connection, it will not do any harm to Car B electronics
    - since Car B's negative terminal was connecting to Car B chassis. The car B's battery would not be harmed.
    - the only harm is to the cable and Car A generator, since the short circuit is drawing to much voltage.

    he didnt really said why, and i dont really understand why(the 1st two points he said).

    One other guy said it didnt harm my car was becasue my car battery was totally flat. if my dashboard was on while the reverse polarity connection. My car would be damaged.

    Can any of you explain the 2 questions in a more sciencitific or engineering way.

  2. jcsd
  3. Sep 30, 2010 #2
    Re: Car Battery Problem!!!

    There isn't really much to be described beyond what the mechanic said.

    1) No you wont harm your car. Becuase you short circuted the two batteries there is no circuit connected to your electronics.

    2) It's doing pretty much nothing to your battery. The negatve teminal is just acting as an earth, meaning no impedence to battery A. The same effect would be achieved if you had attached the positive from battery A to a peice of metal bodywork on your car.

    The only potential danger in this scenario is that the cable and Car A's battery melts/sets on fire. Also just a nitpicky technical comment, it would draw too much current (that's what causes fires) not voltage.
  4. Sep 30, 2010 #3
    Re: Car Battery Problem!!!

    More precisely it is power, voltage times current that causes heating and there is quite a bit of power available in a fully charged battery. As the resistance decreases with the short, the cables have a very low resistance, much less than one ohm, the clip-on connectors would probably receive the most heat but the voltage would drop considerably and the current would be enormous, maybe as much as 10,000 watts for a short time. I think you got lucky the cables didn't catch fire.
  5. Sep 30, 2010 #4
    Re: Car Battery Problem!!!

    Say wut now? Don't really know where you are getting that from, it's current that causes heating.

    You can shove 10kV down a 1mm cross section copper cable at low amps. It's amps you need cross section for. 10kV @ 1 amp will be taken by a small cross section cable no problem. 10kA @ 1V will destroy the cable in spectacular fashion.

    High voltage can electrically burn insulation when it spark though it.

    Don't get it confused because your lightbulbs/whatever show their rating in power.

    EDIT: See http://en.wikipedia.org/wiki/Ampacity
    Last edited: Sep 30, 2010
  6. Sep 30, 2010 #5


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    Re: Car Battery Problem!!!

    My personal experience with automotive batteries is that they must be respected. I've received some nasty surprises due to misjudgement. I once saw (in a "Hot Rod" magazine, rather than in person) the result of a fellow accidentally bridging the terminals with a 3/8" combination wrench (open on one end and boxed on the other). The bloody thing melted in the middle. :eek:
  7. Sep 30, 2010 #6
    Re: Car Battery Problem!!!

    We are not talking about 10,000 volts, we are talking about 12 volts so 10Kw at 12 volts would be almost a thousand amps. Why are you bringing up 10,000 volts? We are talking about low voltage circuits here and power is still voltage times current. You acknowledge that with the 10Kv @1 amp and 1 V at 10,000 amps is the same power rating. But for his purposes we are discussing inherently low voltages and high currents but if the voltage was dropped to 1.2 volts the current also drops or did you forget that part? At 1.2 volts the current drops to 80 amps or so, well within the capability of a standard battery jump-start cable. In your first example you cite 10,000 volts at 1 amp, a resistance of 10,000 ohms. In the second case you go to 1 volt and 10,000 amps, a resistance of 1/10,000 ohm a difference in resistance of 100,000,000 to 1. You are comparing apples to oranges. If you used the same circuit with 1/10,000 ohm at 10,000 volts you would be talking about 100 MEGAwatts. You can't have it both ways.
  8. Sep 30, 2010 #7
    Re: Car Battery Problem!!!

    Hmm.. Jolly good point, I didn't think of that.

    This is precisely the reason why I hate electrickary, it's terribly confusing. I'm a mechanical engineer, and becuase of my current job have been forced to learn some superficial electrical nonsense. All I know is, you can stick lots of volts down a very small cable, but hte high current needs a very large CSA.

    I just used 10kV as it was a test voltage used on an electical jumper I've just designed. It's all easy stuff, is X < cable rating, then use it.
  9. Sep 30, 2010 #8


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    Re: Car Battery Problem!!!

    I have seen electronics on cars fried because of hooking up a battery charger wrong. Jumper cables are no different. The mechanic who told you that shouldn't touch electrical systems on vehicles. Some vehicles may have protection for this sort of thing, and since I don't know your specific vehicle I can't say much else. I will say you are lucky though.
  10. Sep 30, 2010 #9
    Re: Car Battery Problem!!!

    Yep, we use high voltage probes to check out our 200,000 volt ion implant accelerator power supplies, big suckers:) I pretty much know about high voltages! My field is as you may have guessed, ion implant field service. Some of them go over a million volts. We use voltage dividers and accelerator rings with a lot of insulation between each ring and a 10 gigohm resistor between each ring so at 200,000 volts and a total of 100 gig ohms works out to about 20 microamps flowing through those resistors total. The beam itself sucks up a lot more power, there can be as much as 10 milliamps flowing in the beam, arsenic, boron, or phosphorous are the big three dopants used in the semiconductor world. 10 milliamps at 200,000 volts is 2000 watts so there is a fair amount of energy flopping around inside the vacuum chamber:) In this case the conductor is an ionized beam of one of those three elements I mentioned.
  11. Sep 30, 2010 #10

    Ranger Mike

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    Re: Car Battery Problem!!!

    i agree with you and chris xxxx
    in fact we used to hook up a 12 volt battery to melt out a rusted bolt stuck in a leaf spring perch, years ago...

    the car providing the "Jump" may have had the diodes shorted out in its alternator and suffer from a low charge /no charge situation because of this..
    the live battery JUMPER car is always the one in jeapody..thats why tow truck operators carry seperate jump batteries
    Last edited: Sep 30, 2010
  12. Sep 30, 2010 #11


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    Re: Car Battery Problem!!!

    And, in my neck of the woods, 20m cables. It takes a lot more than average juice to push through that length of copper and still crank an engine that normally draws over 900 amps on start-up.
  13. Oct 1, 2010 #12
    Re: Car Battery Problem!!!

    20 meters, that's a long cable! 60 odd feet. I guess that lets you get to the battery when you can't get close enough with the truck. I see some guys have a separate battery they just carry to the car and use that with regular 6 foot cables, take the mountain to mohammed so to speak. There would be a pretty good voltage drop with 50 foot or more of cable for sure. If the cable was fatter that could make up for some of it.
  14. Oct 1, 2010 #13


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    Re: Car Battery Problem!!!

    They aren't used often, of course. The regular cables are around 5 metres, but they carry "extension cords" that can be plugged in for extreme conditions. There's a lot of off-road activity here, both recreational and agricultural, and some of the areas just aren't accessible to anything other than a mud-bogger or tractor.
  15. Oct 2, 2010 #14
    Re: Car Battery Problem!!!

    A "Mechtronic grad" and you could not jump start a car? Sad!
  16. Oct 5, 2010 #15
    Re: Car Battery Problem!!!

    yeah!! it was a bit embarrassing!!
    The story was that. The guy helping me was an idiot and he had an expensive car. But the **** thing was there was no color or indication on his battery. And he kept saying he had done it for so many times and he could remember the terminial. I didnt want to damage his car and I trusted him. But he was wrong.

    that was it. it was embarrassing!!!
  17. Oct 7, 2010 #16


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    Re: Car Battery Problem!!!

    To start with, that would immediately screw someone who is colour-blind. All reasonably modern North American vehicles are 12VDC negative ground systems. Colour notwithstanding, there will be a "+" or "-" symbol on both the battery and the terminals to indicate "positive" and "negative". Negative is always the "ground" connection. The easiest way to make sure, therefore, is to check which cable from the battery is grounded to the chassis and engine block. The "hot" red line will go the the starter relay (on real cars; I don't know where the hell it goes on anything with a computer on-board).
  18. Nov 15, 2010 #17
    Re: Car Battery Problem!!!

    That's like saying it's the water flow, not the pressure that causes the tub to fill. It's both. You cannot have current without voltage, but you can have voltage without current. Power is heat dissipation. Power is voltage x current. Look at your light bulbs, they are rated in Watts, not current. Higher Wattage = hotter bulb.
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