JumpStart Car Electrical Question

Discussion in 'General Electronics Chat' started by raghavanunm, Mar 21, 2007.

  1. raghavanunm

    Thread Starter Member

    Mar 21, 2007
    14
    0
    Hi,
    I am having trouble understanding why Connecting the Red cable to + and + on both cars is essential.Why can't i connect + to - , I mean if anything i will be aiding the series connection of batteries won't I. Please help me understand
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    If you like interesting explosions involving splatters of sulfuric acid, by all means. Making such a connection is not a series connection, but a short circuit.
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,790
    A sealed lead acid battery needs a higher voltage than its nominal output to recharge. This higher voltage comes from the alternator of the car giving the jump. If you reversed the connections you would be putting a neagative voltage on the bettery being jumped assuming that it would even survive the connection.

    What you want is the alternator and two batteries in parallel. That's the combo that works.

    BTW you are smart enough to connect plus(RED) to plus(RED) and then connect minus(BLACK) to body steel instead of the battery post. Aren't you?
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    Electrons flow from - to +, and this flow is opposed by impedence. The ammount of curent flowing will be equal to the voltage divided by the impedence. The power dissipated as heat will be equal to the voltage multiplied by the current.

    If you connect the batteries as hypothosized (- to +) instead of as reccommended (+ to +), there will be only the tiniest impedence opposing current flow. The current will be tremendous. The power dissipated as heat will create the effects BeenThere described in the previous post. You will be unhappy.

    If you connect the batteries as reccommended, current flow will be limited by the internal resistance of the bad battery, and by anything in parallel with the bad battery (starter motor, windshield wipers, radio, whatever). Very little power will be dissipated as heat and much power will be usable by the starter motor of the disabled vehicle. You will be happy.
     
  5. raghavanunm

    Thread Starter Member

    Mar 21, 2007
    14
    0
    First of all thank you guys for the prompt response. I've got a follow up question ..if + is connected to + is the short circuit created because the current will not flow through the paralle paths to (Starter solenoid etc)...but then won't the current be limited by battery's internal resistance?
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Sure will - that's what generates the heat that will make the batteries explode.
     
  7. raghavanunm

    Thread Starter Member

    Mar 21, 2007
    14
    0
    A sealed lead acid battery needs a higher voltage than its nominal output to recharge. This higher voltage comes from the alternator of the car giving the jump. If you reversed the connections you would be putting a neagative voltage on the bettery being jumped assuming that it would even survive the connection.

    What you want is the alternator and two batteries in parallel. That's the combo that works.




    How is it a alternator and two batteries in parallel..isn't it alternator ,2 batteries in series?? can you please explain?
     
  8. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    The + of the good battery connects to the + of the bad battery connetcts to the + of the disabled vehicle's regulator connects to the + of everything else in the circuit.

    We call this "parallel" because the + points are all electrically common.

    Have a quick look here: http://www.allaboutcircuits.com/vol_1/chpt_5/1.html
     
  9. Papabravo

    Expert

    Feb 24, 2006
    10,144
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    What he said!
     
  10. alitex

    Active Member

    Mar 5, 2007
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  11. ohmbrian

    New Member

    Apr 4, 2007
    2
    0
    Hello everyone,

    About a week ago I had to jump start my car due to a leaky battery.This was around 2 a.m so my idiot friend connected the jumper cables to the opposite polarities on he booster car.Thats when the cables started smokin I knew what he did. Bought a new battery the next morning, week later my car won't start period...short circuit? Any advice would be mighty helpful.
     
  12. goodbyegti

    Well-Known Member

    Apr 28, 2004
    59
    1
    Chances are it could be something unrelated, unless your batteries are being drained by parasitic resistance somewhere. Does it crank over?
     
  13. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    Well,

    You could disconnect the battery and insert an ammeter to see what's being drawn with the car turned off.

    You'll probably see a little draw, especially if you have a clock in your radio, but that certainly won't draw down a fully charged battery in one week.

    Have you tested your charging system?
     
  14. ohmbrian

    New Member

    Apr 4, 2007
    2
    0
    The negative cable for the battery is still covered with that white chalky coat from the leaky battery, think it could be a contributin element to the problem?Yes, I disconnect the battery and check the car while turned off with a ammeter and it read zero voltage. I'm gonna to get it check out tomarrow, is it a possible the alternator just got fried and just gonna have to buy a new one?

    How do you test the charging syst?
     
  15. goodbyegti

    Well-Known Member

    Apr 28, 2004
    59
    1
    Get a stiff brush, clean the contacts and put a vaseline on them. Then measure the voltage across the red and black terminals. If it's less than about 12.5V your batteries flat. If it is then do what JoeJester said, remember to measure amps not volts. Wash your hand afterwards and don't use your toothbrush because that white stuff will have plenty of lead in it.
     
  16. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    Your battery voltage should be 12.6 volts. When you start your vehicle and it's running, the Charger should be applying 13.8 volts to the battery. Measure the VOLTAGE between the positive and negative terminals of the battery. Follow the proper polarity if your not using a digital multimeter.

    To check and see if your drawing excessive current with the vehicle off, disconnect one terminal, insert an AMPMETER between the terminal and the connector. Follow the proper polarity if your not using a digital multimeter.

    If those two concepts are foreign to you ... find someone who understands them to help you.
     
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