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Power conversion logic. What am I missing?

  1. Mar 7, 2013 #1

    I have a project that involves the coupling of a 12V Alternator with a small engine to power a loaded 12V battery.

    The gas engine is 1.4HP (1043.979Watts) coupled to a 40 Amp 12V alternator which is charging a 12V battery under a 35 Amp load.

    The way I see it, the 35 amp load @ 12V = 420 Watts = roughly 0.563HP. Giving the engine an advantage of 623.979 Watts greater than the required draw.

    What I do not know is; what the RPM of the alternator is at is most efficient speed. However I am almost positive the engine can meet and exceed it.

    Given a basic look at this, do these numbers seem to make enough sense to provide sufficient power for the system to continually work under load? Should I go with a 60A alternator instead? I know there will be losses due to mechanical inefficiency/friction But do these figures seem proportionate enough to actually put to use? What should I look out for?

    Any advice greatly appreciated
  2. jcsd
  3. Mar 8, 2013 #2


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    Gold Member

    You are asking a relevant question which has no simple answer afaik.
    Generator sets are usually built as an integrated unit, with gasoline motor and alternator tuned to get the desired performance. Your setup should work as is, simply because you do not have any big starting loads such as compressor motors or pumps and it is amply sized for the steady state load. Hopefully you have provisions to avoid frying the battery by overcharging it.
    However, the efficiency is indeterminate. It may be close to or far from optimal and only experiment will tell.
  4. Mar 8, 2013 #3
    Yeah, you'll have to benchmark it. There's no easy answer to this.
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