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Battery in the car?

  1. Jan 19, 2007 #1
    1. The problem statement, all variables and given/known data
    A motor car with a 'flat' battery can be started by connecting jumper leads from the battery to the leads of another car battery. To do this, the two positive terminals and the two negative terminals must be connected together.
    1) Why is this?
    2) Are the two batteries in series or are they parallel?


    2. Relevant equations
    none


    3. The attempt at a solution
    1) The dead battery has the positive and negative terminals at the same potential. So when a working battery connects its positive end to the positive end of a dead battery and negative to negative, a current will flow from positive to positive of dead battery. Current (using postive charge always, reverse for electrons) will flow from negative of dead battery to negative of good battery since the potental in the negative of dead battery is greater than potential of negative of good battery. Overall, this will mean higher potential at positive end of dead battery (because more charges are there now) and lower potential at negative end of dead battery (because charges which were there have migrated to the other good battery) meaning a restoration of the dead battery. However, this is done at the expense of the good battery as its PD is smaller as a result.

    2) The two batteries are connected in series?
     
  2. jcsd
  3. Jan 19, 2007 #2

    DaveC426913

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    Sounds good.
     
  4. Jan 19, 2007 #3

    chroot

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    No! The batteries are in parallel, not in series. A series connection would involve connecting the positive terminal of one battery to the negative terminal of another.

    - Warren
     
  5. Jan 20, 2007 #4

    DaveC426913

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    Doesn't that depend on whether you see the dead battery as a power source or a drain? When dead, surely it acts like a resistor.
     
  6. Jan 20, 2007 #5

    andrevdh

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    When connected in series the voltages of the batteries add up to give a greater "push" to the current (compare with the case when inserting batteries in electronic devices - flashlight, walkmans etc.). Since the electric systems of a car operates on 12 V this would not be a good idea. When they are connected in parallel the output voltage stays the same, but the total amount of current delivered into the load will be the sum of each supply (the parallel connections lets each supply add to the current finally reaching the load) - which is what is needed when the car is started - lots of current!
     
  7. Jan 20, 2007 #6

    OlderDan

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    A "flat" battery is more complicated than a resistor. The simplest way to model a dying battery is as a constant source of emf with an increasing internal resistance. A voltmeter with huge internal resistance draws essentially no current, so if the voltage is measured with no other load on the battery it will still read the same voltage. A battery tester uses a "typical" load resistance to draw current from the battery while the terminal voltage is being measured. If the voltage reads low, it is because of the voltage drop across the internal resistance of the battery. For a car battery, the internal resistance is very low, on the order of a few milliohms. It takes a lot of current to drive the starter motor, so if the internal resistance rises the terminal voltage drops and the battery cannot drive the starter. This is why the lights dim when you crank the starter, even though they may still be bright otherwise.

    A battery is not recharged by simply connecting it to another battery with the same emf. It has to be connected to a higher voltage source that will force a reverse current through the dying battery. Battery charges do this, usually with some control circuitry to regulate the reverse current. It is also done by the generator connected to the car's engine.

    Jump starting a car is usually accomplished by using a running automobile with a good battery connected in parallel with the weak battery in a second auto to run the starter motor and get the second engine running. The jumpers can then be removed because the running engine drives the generator, which will then recharge the flat battery. Even if the second auto is not started, the generator of the running auto will provide the higher voltage needed to recharge the flat battery in the second auto, so after being connected for some time the flat battery will be sufficiently charged to start the car.

    The batteries are definitely not connected in series. If they were, there would be a 24 volt emf in a circuit of very low resistance, and some very nasty things could happen.
     
  8. Jan 20, 2007 #7

    AlephZero

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    No, it still acts (chemically) like a battery, though it does have some internal electrical resistance as well.

    To charge a battery, you pass current through it in the "wrong" direction, into the +ve terminal and out of the -ve. If you pass more current through a fully discharged battery in the "normal" direction, you stand a good chance of wrecking it permanently - e.g. you may supply energy that causes irreversible chemical reactions, and/or damage its physical structure by overheating it.

    In the "flat car battery" case, you connect a fully charged battery in parallel with the "flat one". You don't break the existing circuit (battery + ignition switch + starter motor) and connect the charged battery in series with those components.
     
  9. Jan 21, 2007 #8

    DaveC426913

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    Hm. Is it too late to change my answer?
     
  10. Jan 22, 2007 #9
    ...connect the charged battery in series...? I thought it must be in parallel as everyone have been saying?

    It's in parallel because car batteries is usually connected in parallel so to recharge it, the new battery must also be connected in parallel. But could you argue that the car battery is a block of parallel connected emfs. How about modelling it as a single large emf so you now have two emfs connected in series?
     
    Last edited: Jan 22, 2007
  11. Jan 22, 2007 #10

    andrevdh

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    The "new" battery does not serve to recharge the "dead" battery in such a situation. It effectively "replaces" the dead battery as OlderDan explained. Since the dead battery will be at a slightly lower potential than the new battery the new battery will push some current through it in reverse, thereby starting to recharge it, but with most batteries this is a timely process and the dead battery will therefore act as an additional load on the new battery throughout the jumper start.
     
  12. Jan 23, 2007 #11
    I do not really follow what most of you are saying. Maybe a picture of the circuit will clear things up. I've drawn one. I think there are 6 chemcial reactions going on with a total of 2V in each so a total of 12V for a car battery. Is my diagram correct? If not please correct me. I was thinking of representing the 6 car batteries as 1 dead battery. So you have 1 good battery and 1 dead battery which looks very much is connected in series. Have I made some fundalmental mistakes.
     

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  13. Jan 23, 2007 #12

    andrevdh

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    One car battery (dead or alive) will consist of six individual cells connected in series like this

    1 battery:

    o === |- === |- === |- === |- === |- === |- === o

    o : pole connection on battery
    |- : one chemical cell in the battery
    === : connections between components
    | : positive side of a chemical cell
    - : negative side of a cell

    the left o will therefore be the high potential + pole of the battery
    the right o will then be the negative pole of the battery

    when a battery goes down one or more of its cells can become ineffective
     
  14. Jan 23, 2007 #13

    chroot

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    Forget about the internal cells that make up a single 12V battery -- they're completely irrelevant. Just consider connecting one 12V battery in parallel with another, and connecting them to the rest of the car's circuitry (including its starter motor).

    - Warren
     
  15. Jan 23, 2007 #14

    DaveC426913

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    Ah. Andrevh has an important point in post 10! The problem does not ask about RECHARGING the dead battery, it asks about STARTING the MOTOR.

    The new battery WILL start the motor, whether or not it also charges the dead battery.

    Thus, he is correct, the new battery is in PARALLEL with the dead one.
     
  16. Jan 24, 2007 #15
    Are my diagrams correct?

    Are you suggesting the (second) diagram on the right is correct? If so are you suggesting it is connected in parallel? It looks like in series to me with the single block.
     
  17. Jan 24, 2007 #16

    NoTime

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    If you need more current than a single battery can supply.
    How do you hook up additional batteries?

    What happens if you connect the positive terminal of a battery to the negative terminal?

    In the above question, does it make a difference if your battery is composed of multiple cells in series?
     
  18. Jan 25, 2007 #17

    DaveC426913

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    No.

    See my attached diagram.

    The circuit we are being asked about is BATTERY > MOTOR. The dead battery acts effectively like a resistor. It will charge, but you could replace the dead battery with a lightbulb and have effectively the same circuit.

    If both those batteries were working, they would be in parallel - powering the motor.


    I should defer to those more knowledgeable; this is just my understanding.
     

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  19. Jan 25, 2007 #18
    Dave, your diagram seem to make sense. I didn't factor in the motor. The battery's job is to start the motor working only isn't it? So if the battery in the car is dead, all you need to do is install a new battery in the circuit to start the motor up.

    If you are going to recharge the dead battery, you wouldn't do it in the car, or else during the recharge the motor would be on and a lot of electricity is wasted into powering the motor. You would take the dead battery out of the car and connect it with the good battery in series. Like what I showed in my diagrams.
     
  20. Jan 25, 2007 #19

    NoTime

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    DaveC426913 has the right idea.
    But , yes u are basicly just replacing the battery.
    The existing one doesn't make much difference.


    Err No. Definitly not. This is just wrong.
     
  21. Jan 26, 2007 #20

    andrevdh

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    The dead battery will draw very little current from the new battery during the jumper start. So it acts as like large resistor in effect - like Dave says.

    When you recharge a battery it is common practice to disconnect the leads to the car circuit. It is not connected to the electric components of the car at that stage (when the car is turned off no components is connected to it or draws current from it - except maybe the security system). But the main reason is to protect the alternator diodes which could be damaged by the recharger due to the large reverse voltage applied by it.
     
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