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Homework Help: Does a car battery charge and deplete at the same time while running?

  1. Jan 4, 2012 #1
    1. The problem statement, all variables and given/known data

    Obviously I do not believe this to be the case, but when running a car the battery is I assume having current drawn in, reversing its normal means of depletion so that the potential chemical reaction can take place later on. My question is when you turn on the radio while you are driving (and the current generated by the magnets in your wheels is charging the battery) is this excess current from the induction? (I assume that is how it works)..It can't be that the battery is working and charging at the same time...

    Thanks for helping out, I will be back in a few hours so if I do not reply don't let that be a sign that I don't care or something.

    2. Relevant equations

    electricity only flows one way

    3. The attempt at a solution

    wiki page on car batteries leads me to believe that just the cranking (starting) mechanism is the only function of the battery and that the induction from the wheels provides all the DC power after conversion.
  2. jcsd
  3. Jan 4, 2012 #2
    When running, the voltage regulator reacts to current draw to keep the battery charged. So when you turn on the lights or blower, the regulator reacts to maintain approvimately 13.5V across the battery terminals. The voltage across the terminals of a fully charged disconnected battery is only slightly over 12V. Therefore it is continually being charged, sometimes more, sometimes less depending on its state of charge.

    Cars have alternators that are turned by a fan belt. The voltage regulator, these days, is an integral part of the alternator.
  4. Jan 4, 2012 #3


    Staff: Mentor

    Car batteries have 12v ratings but car generators charge them at 14v adding in the radio would reduce the v across the battery somewhat but not below 12v. That would be bad consider the dome light being left on for say 8 hours, its enough to drain the battery so you cant start your car.

    the big drain on the batt is the startup, basically its running a motor when you turn the key. A solenoid connects it to the engine and the motor turns the engine shaft while the spark plugs are fired while gas is injected hopefully starting up the engine. the solenoid quickly disconnects the startup motor once the engine starts up. The car engine then spins the car generator that in turn recharges the batt. (and you thought computers were so complicated)

    draw a circuit diagram of car gen + batt + radio is it a series or parallel circuit? then evaluate what happens when the radio is turned on and off and that should answer your question.
  5. Jan 4, 2012 #4


    Staff: Mentor

    one other thing cars have voltage regulators that control the amount of voltage across the batt. sometimes these circuits fail but you'll never know until your car starts seem to take a bit longer or it just up and fails to start.

    the first inclination is old batt / bad batt replace it then it fails again and then you realize its something else.

    there's a cool gadget that measure the car voltage using the cig lighter outlet and you can use it to see if you're putting out the necessary 14v to charge the batt.

    just another car talk moment.
  6. Jan 4, 2012 #5
    This was a great help. I was starting to take you up on drawing it but I couldn't come up with a good reason for choosing a parallel circuit. The only thing I can think of is to say that since there exists a regulator that is trying to maintain a steady voltage parallel circuits would be in demand because in that case the current is additive and not the voltage which is constant. I am very interested in the correct schematic and I have lt spice to test it on!
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