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Connecting car alternators in parallel

  1. May 31, 2009 #1
    What happens if I connect two car alternators in parallel? Can I connect three or four alternators in parallel? I just want a cheap way to increase power. I need 5.5kw for recharging a battery pack.
  2. jcsd
  3. May 31, 2009 #2


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    I'm pretty sure that this is a bad idea; alternators do not output perfect DC, but rather, a rectified AC waveform that has a frequency that depends on how fast the engine is turning. It's hard enough to synchronize (phase) two generators that output nearly the same frequency (and voltage) but imagine doing that with two car alternators that have frequencies that vary quite a bit!

    5.5 kW works out to 46 A (using 120 V) or 23 A (using 240 V)--even if you could find a charger, that's a lot of current (never mind what the charger's efficiency between wall to output is). You could probably use a big gas generator, but you'd still need something to convert the 120 V output to the appropriate voltage to charge the battery pack.
  4. May 31, 2009 #3


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    Is the battery pack only 12 volts total? This would be the only reason you would run the alternators in parallel with each other. And as MATLABdude pointed out, there are problems with that configuration.

    If the pack is a group of 12 volt batteries in series then it would be feasible to place a single alternator in parallel with each battery. Though each alternator body would have to be electrically isolated from the others, as every alternator I've ever dealt with grounds the body.

    If any of the alternator bodies were to come into contact with each other, it would generate a direct short circuit across a section of the battery pack, and major destruction, death, and/or putting your eye out, might ensue.
  5. May 31, 2009 #4


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    I believe that 2 alternators with internal regulators of the same manufacturer and size can be paralelled without any problems. The fact that they don't output pure DC is irrelevant. The outputs are diode fed so nothing can feed back into the alternator. The battery which IS pure DC doesn't feed back into the alternator between pulses, so another alternator wouldn't either. I've seen more than one machine with dual alternators that came that way from the factory. The main thing to remember when charging battery banks with alternators is to NEVER disconnect the battery while the alternator is turning. Make sure the alternator has come to a stop before disconnecting.
  6. May 31, 2009 #5
    In theory that sounds reasonable. But alternators are also belt driven. You would want the belt tensions balanced as well, using the same pulleys, etc.
  7. May 31, 2009 #6
    What sort of battery packs are you trying to charge?
  8. May 31, 2009 #7
    I suspect that it can be done, with care. In an alternator, the rotor has a dc excitation, and the stator output is ac, which is rectified by two diodes. Feedback is taken from downstream of the two doides and fed back to the rotor to control the output voltage. If you connect two alternators in parallel, the two feedback loops will interfere with one another, unless you isolate the two alternator feedback loops by putting one additional rectifier diode per alternator downstream of the feedback loop. This extra diode drop will require that the regulated alternator output voltages be increased by the same amount.
  9. May 31, 2009 #8
    Thanks for your answers!
    It may sound crazy but I,m planning(in a near future) to test this connection in an electric vehicle:

    1- DC electric motor with a special pulley adapter for connecting two high output alternators
    2- alternators to DC/AC converter
    3- converter to charger(ev charger)
    4- charger to battery pack (96vdc)

    Im just trying to extend the driving range!
  10. May 31, 2009 #9


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    Eeegads..... Just add 6 more alternators. Batteries are like people: they are all different. A dedicated alternator for each battery, powered by regenerative braking, will optimize each batteries lifespan. All these converters will just add unnecessary time and expense to your project. KISS.

    But then again, I'm poor.... :cry:

    Good luck. :smile:

    Solar rules! :cool:


    Yes that's my freakin solar powered boat... Do you think I could make this stuff up?
  11. May 31, 2009 #10
    Can I do that and still connect all the batteries in serie to supply the 96 volts needed?
  12. May 31, 2009 #11
    I don't see the motor...
  13. Jun 1, 2009 #12


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    But as a reminder:
  14. Jun 2, 2009 #13
    OmCheeto, this a draw I did representing the picture in my mind about what you are saying. It is correct? If not explain, please...
  15. Jun 2, 2009 #14
  16. Jun 2, 2009 #15


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    You could make up a shaft with several pulleys on it and drive an alternator from each pulley and charge a battery from each alternator.
    Each alternator could have its own regulator like this.

    So, you might have a big solid frame with a motor at one end and a bracket with a bearing on it at the other and a pulley shaft between the two. Then have alternators on each side of the shaft with their outputs going to batteries.

    If the batteries were in series, you could leave them connected, but you would have to insulate the alternators from each other and put them in a closed box for safety.

    If the batteries were in parallel, you would have to remove them for charging.
  17. Jun 2, 2009 #16


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  18. Jun 3, 2009 #17
  19. Jun 3, 2009 #18
    I can't wait until you explain me how it is posible. I'm always tempted to ask why things happen.
    So here goes the question.
    How it is possible to charge each battery(with its own alternator), while at the same time have them connected in series to power a controler(or electrc motor 96vdc)??
    Isn't suppose to make a short circuit.
  20. Jun 3, 2009 #19


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    As long as each alternator is electrically isolated from the others, the only path for charge to go is into its own battery. This assumes the batteries are not being used in a vehicle while they are charging, so there is no load on the batteries. It is charging from the mains.

    It then behaves like a car alternator should, with its own regulator to stop charging when the battery is fully charged and to stop the voltage going too high.

    Why do you see this as a problem?
  21. Jun 3, 2009 #20


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    Just pretend the alternators are batteries also. The circuit then becomes two sets of parallel series batteries.

    The alternators have an added feature of having diode banks on their output. This prevents them from consuming power if their voltages drop below that of the batteries.
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