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Adding Voltage in parallel?

  1. Jul 28, 2010 #1

    I wired a 24V battery --which were actually two 12V in series-- parallel with a 12V battery.

    The Voltage turned out to be about 18Volts when I measure the 12V Battery side. When I measured the 24V side, it turned out to be about 22.2Volts.

    What is going on? Why does the voltage change when different sides are measured? How would I go about calculating voltage when the batteries are in parallel and have different voltages?
  2. jcsd
  3. Jul 28, 2010 #2
    Maybe the wire you use to connect the terminals have some resistance causing the voltage drop.
  4. Jul 28, 2010 #3
    If batteries were ideal voltage sources, it would be undefined to connect batteries with different voltages in parallel, because the meaning of any components being in parallel is that they have the same voltage across them. With real batteries, the difference between the battery voltages is the voltage that will appear across the battery internal resistance and the wiring resistance, so be careful of the wires getting very hot. Another effect with real batteries is that the battery will fail to put out the advertised voltage, because the voltage produced by a battery is a function of the current. The voltage across a battery is approximately constant if the current is small or zero, but as the current increases the voltage drops.
  5. Jul 28, 2010 #4


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    You also have resistance in the wire, so voltage will depend on where on the wire you attach your voltage probes.

    By the way, you did create a short-circuit. I hope you didn't leave batteries like that for long.
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