batteries in parallel and/or series

Discussion in 'General Electronics Chat' started by Hurt_it_Circuit, Oct 20, 2012.

  1. Hurt_it_Circuit

    Thread Starter Member

    Oct 2, 2012
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    How would you connect as many 1.5V AA batteries as needed in series and/or parallel to provide close to 1V to a 25Ohm speak? Include the number of batteries and any resistances associated with those batteries in your explanation.

    That is the question being asked for a lab. My internal resistance in my 1.5V AA battery is 1.57Ohms. I have been trying to put batteries in all sorts of combinations and then try and put these combinations into a voltage divider equation. I don't understand why I was given this question because it is bad practice to voltage sources in parallel. But their is no other way to lower the current to be .04Amps (1V/25Ohms). Any ideas where to begin?
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    You don't need to connect any batteries in parallel.

    Assuming that the battery truly behaves as an ideal voltage source in series with a fixed resistor (it's a bad assumption, but is probably the one they want you do use for this question), what is the effective circuit of two AA batteries put in series, but with opposite polarity?
     
  3. Hurt_it_Circuit

    Thread Starter Member

    Oct 2, 2012
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    like the drawing in the attachment? But wouldn't the current be pushed from two different directions into the speaker. Using KVL wouldn't that give me a voltage of zero across the speaker?
     
  4. WBahn

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    Mar 31, 2012
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    Ah, but what happens if you add a third battery?
     
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  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Put a resistor in series with the 25 Ohm speaker that will drop 0.5V in line with 25 Ohms.
     
  6. WBahn

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    Mar 31, 2012
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    He doesn't get to use any resistors other than the internal resistance of the batteries. I don't like the question, but it is what it is.
     
  7. Hurt_it_Circuit

    Thread Starter Member

    Oct 2, 2012
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    Ahhh I see. Just one last question what happens when batteries of opposite ends face each other? Does this overheat the battery cause you have two currents of the same value meeting at one location (like 3V source pointing one way and another 1.5V source pointing the other way)?

    In the picture above imagine that one of the 1.5V sources is a 3V source going in opposite direction of the 1.5V source.
     
  8. WBahn

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    You are mixing concepts. You do not have two currents flowing in opposite directions that somehow collide. There will be a single current flowing through both batteries. If the current is flowing out the positive terminal of a battery, then that battery is supplying power and is discharging in the process. As it discharges, the terminal voltage drops. If it is flowing into the poisitive terminal, then that battery is absorbing power and is charging in the process. As t charges, the terminal voltage increases. That, as well as some other things, is what makes an exercise like this little more than a paper curiosity.
     
  9. Hurt_it_Circuit

    Thread Starter Member

    Oct 2, 2012
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    So can I then assume that my current is flowing from my 3V source through the equivalent resistance (speaker + all internal resistances of batteries combined in series combination) and into my 1.5V battery which will get charged and have a higher voltage. Will my current just be the (3V source)/(equivalent resistance)?
     
  10. WBahn

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    Mar 31, 2012
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    No. what happens to the voltage as you go through the 1.5V battery that is in the opposite direction? Consider why you had 0V across the speaker earlier. And I think you have to ignore the charging/discharging issue for this problem. Assume that the question only concerns the initial voltage and current when first connected.
     
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  11. Hurt_it_Circuit

    Thread Starter Member

    Oct 2, 2012
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    The voltage across both batteries should be the same but if i measure it at the terminals i will get the difference between both batteries as the voltage. I just don't see how I will get close to 1V if my difference between the sources is going to be multiples of 1.5V.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    Why are you using a dead battery cell? A new Energizer AA alkaline cell has a maximum internal resistance of only 0.3 ohms. Its voltage is 1.5V so it can deliver a voltage of 1.48V to a 25 ohm load.
     
  13. Hurt_it_Circuit

    Thread Starter Member

    Oct 2, 2012
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    It is just a question that is being asked in our lab for circuits. I have to use the calculated value of internal resistance from previous task in lab.
     
  14. WBahn

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    As you add pairs of batteries that cancel each other out in terms of voltage, the resistances will still add.
     
  15. Hurt_it_Circuit

    Thread Starter Member

    Oct 2, 2012
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    But my current will stay the same. Unless I put them in parallel I still have to decrease teh current
     
  16. bountyhunter

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    Sep 7, 2009
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    Nope. Alkaline batteries do not recharge by forcing reverse current through them and they can overheat if the forced current is high enough. Every battery maker I know of warns against trying to recharge alkalines.
     
  17. WBahn

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    Please point to where anything in the problem statement says anything about alkaline batteries. All it says is that they are 1.5V AA batteries.

    The point I was making is that if current is flowing backwards in them, they are absorbing power. The use of the term 'charging' was generic and, probably should have been replaced with some other term, though off the top of my head I don't know a better one, other than perhaps just saying it is acting as a load. But I didn't do that because the point was also that you do not want to run them this way because the characteristics will change.

    Please keep in mind that this is a paper design problem (and a rather poor one, in my opinion). We need to stay within the framework of the problem as given. Notice that I specifically pointed out that the effects of running current the wrong way through the batteries should be ignored in this paper problem. If nothing else, there is nowhere near enough information to take anything else into account.
     
  18. WBahn

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    How will putting them in parallel decrease the current?

    Go back and look at the circuit you attached in post #3. Now imagine adding a third battery in series. What is the total voltage of the sources? What is the total resistance? What is the current?

    Now add two more batteries in series oriented in opposite directions. Answer the same questions.
     
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  19. Hurt_it_Circuit

    Thread Starter Member

    Oct 2, 2012
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    Oh my god I can't believe it took me this long to see that. It is so simple thank you for all the help.
     
  20. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    My point was that unless it is a specific type of battery, reversing them is dangerous. They would not be absorbing power, they would be dissipating power.

    I think this whole problem is the height of stupidity, and I would encourage the OP to convey that to the idiot who gave it to him and that is a good reason among others. The real problem here is the OP needs to just go directly back to whomever made up the assignment and ask him to clarify it instead of having a forum speculate on what it should have been.
     
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