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12v car batt paralleled with 9v

  1. Feb 9, 2008 #1
    I have a circuit that can be powered by either a 9V battery or a 12V battery.
    Can I connect a 9V battery in parallel with a car cigarette lighter 12V battery plug, so that the supply's are now in parallel with one another and they can both power the circuit?

    I'm a bit scared because 12V outputs in a car are generally regulated and can spit out enough current to cook the 9V battery...
  2. jcsd
  3. Feb 9, 2008 #2
    Can you say BOOM? Do not do this!
  4. Feb 9, 2008 #3


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    Although wth a couple of diodes you can arrange that it would be powered by the 9v when the 12v is not connected and vice-versa.
  5. Feb 10, 2008 #4
    Ok so what I did was place a simple protection diode from the +ve part of the 9V to the +ve rail, and just connected the +ve part of the 12V cig. lighter plug to the rails too. This should prevent ANY current going through the diode into the +ve 9V right?
  6. Feb 13, 2008 #5
    .....just a thought for safety. Try using a proper adapter because a car charging system works on voltage differentials tocharge the battery. say a battery at 12v is being charged and the charging circuit is operating at 16 volts. Your cigarette lighter output will be at 16 volts . sorry i changed this
    Last edited: Feb 13, 2008
  7. Feb 13, 2008 #6


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    If your car is hitting 16v you need a new battery or regulator (or both).
    However, it is normal to approach 14v.

    If the device in question was made for 12v battery operation, then having it plugged into a running car could possibly overheat and damage it. This may not happen immediately, so unless the manual specifies auto operation it would be best to use an adapter as noted.

    Using the diode will keep the 9v battery from exploding but can cause a shorter effective battery life on the 9v as there will be a voltage drop associated with the diode.
    If this turns out to be a problem then you can use a power jack that disconnects the 9v when the external power cable is plugged in.
  8. Feb 14, 2008 #7
    quick question for diode advisor

    why did you not advise a resistor in series with the battery to protect from overcurrent.
    i would advise him to have a qualified person verify his circuit
  9. Feb 14, 2008 #8


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    Homework questions.
    1) If the OP circuit draws 5ma in idle mode and you pick a resistor to drop 2v.
    What will be the voltage drop across the resistor when the devices active mode current it 50ma.
    2)Why did I pick 2v.
    3) Why did I pick 5ma and 50ma?

    1) The OP knows enough electronics to modify their circuit with a switch diode.
    That makes him/her somewhat qualified in their own right.
    2)I did warn them that they could damage it due to the car charging effect.
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