Basic cicuitry help

Discussion in 'General Electronics Chat' started by insomaniacal, Aug 7, 2010.

  1. insomaniacal

    Thread Starter New Member

    Aug 7, 2010
    Hello everyone,

    I need some help with some weird stuff that's been going on with one a very basic "circuit".

    A picture is worth a thousand words, so here is a schematic, along with what I did.
    Basically, I connected two 9V batteries in series (+ to -) and vice versa of each battery. From the first one, I soldered on two more wires to the terminal, to be able to connect them to anything, and also make it easier to measure.

    The weird thing is, when in series, the voltage should be doubled, correct? However, the voltage went down. Both batteries were showing a ~8.15 V rating. When I connected them in series, and took the voltage, it always fluctuated between ~7.5 to ~6.0. I was puzzled, and started measuring the voltage around the terminals. When I did this to the battery with the red circles (the second one, with only one wire connected to the terminals, see picture if you like), I heard a loud popping noise, and the voltimeter instantly dropped to 0.0. Not wanting to damage anything or have a battery blow up, I cut the soldered wires connecting the first battery to the second.

    However, I noticed the first battery (the one with two sets of wires) was extremely hot.

    What bothers me most is why the voltage didn't increase, but decreased. I realize it probably wouldn't actually double. (Cheap copper wires ripped from some cables, not "new" batteries) I'm guessing there would be resistance, but I was expecting a voltage between 13-15. I used the "20 V" setting on the voltimeter, because there was no way I was going to get over 20, even if both batteries were giving a steady 9V.

    This is puzzling me, could someone please offer an explanation, along with suggestions.

    Thank you.
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    You are placing the batteries in anti-parallel, not in series. This will kill the batteries.
    In series:
    Load + to battery 1 (+), battery 1 (-) to battery 2 (+), battery 2 (-) to Load (-)

  3. insomaniacal

    Thread Starter New Member

    Aug 7, 2010
    That actually makes perfect sense, and makes me feel like a total moron.

    However, thank you for the speedy reply, I appreciate it!
  4. Ghar

    Active Member

    Mar 8, 2010
    By connecting the batteries you like that you create this loop:

    Vbattery + Vbattery + Vwire = 0

    So, the batteries do add up to 18V... but across a wire.
    9 + 9 = I*Rwire

    Rwire is tiny, a few milliohms.
    This makes 'I' huge, thousands of amperes. This can't actually happen though because batteries aren't ideal, they have a built in resistance and other factors which limit the current you can get.

    The internal battery resistance is much, much greater than the wire resistance. This means that most of the voltage actually gets dropped inside the battery.
    You actually should read close to 0V when you measure this but because one battery will be stronger than the other, one will drop more voltage than the other and you will read some fraction of 9V, such as your 6V. This resistance will be drastically changing as the batteries heat up and discharge. Doing this will drain the battery noticeably even after a short time.