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A Parallel AND a Series circuit

  1. May 21, 2012 #1
    I have a boat that runs a 12v domestic system to power things like lights while I am away from the shore-power. I want to know if I can use three batteries to give me 12v and 24v at the same time.

    Allow me to walk through my thoughts;

    If I connect battery A & B together in parallel, I get 12 volts. Now the question I have is can I now hook up battery B and C together in series to give me 24v at the same time.

    If this is possible, how do I hook up the/a battery charger to keep them all charged up?

  2. jcsd
  3. May 21, 2012 #2


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    I'm not sure about the charging, but you can certainly get 12/24 as you have proposed.
  4. May 21, 2012 #3
    Do you have batteries in parallel already? You need to make sure your alternator can handle the extra battery load.

    Also, if A and B are in parallel, then connecting B and C in series to get 24V will mean that A and C are in series as well.

    As far as charging, I'm not sure what your system has, but it may be designed to only charge a 12V system.
  5. May 21, 2012 #4
    That is an answer I wasn't expecting. Thanks. It certainly saves me worrying about either getting covered in molten lead or burning a hole in the bottom of the boat.

    Big thx
  6. May 21, 2012 #5
    On the boat I want to separate the two systems completely. This will give me a battery to start each engine (x2) and a second system to run everything else on the boat.

    To expand on my opening gambit; I replaced the starter batteries for the engines on the weekend and the previous owner hooked everything up to the starter motor batteries. This concerns me because I would rather have the radio die on me than not being able to start the engines when I'm out at sea.

    Now to add a fly into the ointment, I want to install a super loud fog horn that runs on 24v. To have two batteries set aside purely to run the horn seems a bit extravagant to me.

    Hope this helps to clarify my fuzzy logic
  7. May 22, 2012 #6
    the only way i can think of to simultaneously get 12v and 24v at the same time... is to use some electronic circuitry to constantly and rapidly switch between serial and parallel connections of the batteries, and between 2 positive-negative pairs of output terminals, one positive-negative pair for each voltage... and then have a large capacitor at each output terminal to smooth out the switching... you can leave the 24v 'phase' as A and B in parallel , and in series with C , without worrying about unequal discharge, since all the batteries will be parallel in the 12v 'phase' and they should equalize their charge
    the problem with this is that you are effectively halving your maximum power output since you are switching off the current for half the time to each terminal....
    if you need the maximum power through the 12v or 24v terminals, you will need abit more electronics to sense the voltage on the terminals, and bypass the switching so that the battery is directly connected to that terminal when the voltage drops too low due to excessive current(power) draw from the capacitor, until the power spike is over ....
    your other terminal will be powered solely by the capacitor on that side during this period tho, so it will take some prudent power management on your end , dont use anything power hungry on both the 12v and 24v outputs at the same time... and your electronics must prevent both the serial (24v) and parallel(12v) circuits from bypassing the switching at the same time, because that's a short!
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