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safe use of car alternator?

Discussion in 'Home Power and Microgeneration' started by Robert Morein, Aug 4, 2004.

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  1. Because my Chinese generator is late in arriving, I want the capability of
    recharging my battery based home backup system from a car alternator.

    The battery bank will not be connected directly to the car alternator. The
    setup is thus: a 1.5KW/4KW surge AIM inverter is connected to the car DC
    terminals. The AC output of the AIM connects via 100' power cord to a 24V
    25A switching power supply. The power supply connects to an Outback MX-60
    charge controller, which charges a 660 ah 12V bank, at up to 50a, limited by
    the rating of the switching power supply and the (as yet) unknown safe
    operating parameters of the car alternator.

    The MX-60 allows me to set charging current limits. Thus, I can tightly
    control the amps drawn by the AIM inverter from the car DC system.

    How many amps can be drawn, at engine idle, from a typical car alternator --
    in this case, a 70 amp Bosch -- without damaging the unit?

    The car motor is a 5 cyl diesel, and thus relatively immune to overload.
  2. SQLit

    SQLit Guest

    100 feet is a long way to go with an extension cord. Unless you build one
    out of 8 gauge wire.

    I used 24v dc single phase as the voltage, 100 feet, and 25 amps.

    I suggest to you it would be better to get/rent an 120 volt generator and
    move it to an location close to the equipment. Until the genny comes in.
    The car will need to be held at something higher than a idle to get 25 amps.
    The cost of wear, tear fuel and increased maintenance costs on the 5
    cylinder engine is not worth it.
  3. You can get a rough measure of the load on the alternator by measuring the
    voltage. 12.2 would be the absolute lowest to allow, most alternators are set
    from 12.6 to 14.5 volts. Alternators also generally aren't doing constant duty
    unless there is a lot of airflow to cool them. By blocking the gas pedal and
    getting a little bit above idle speed you might be able to get 30 amps
    relatively safely for the short term, but at a huge cost. That setup is almost
    guaranteed to damage the alternator.

    If your backup system has any size at all, using this type of charging is
    analogous to trying to fill a swimming pool by pissing in it. You are also
    having to overcome engine friction and parasitic load losses, line losses,
    conversion losses on both ends, and probably some other losses I'm forgetting.
    As has been suggested, renting a generator or finding some other method to
    charge makes more sense.
  4. Please re-read my post. I bought a generator.
  5. Pete C

    Pete C Guest


    I'd work out how many amps the lights draw when on and use that, the
    alternator should be able to power them at idle.

    Would also be worth opening the hood some, the alternator has it's own
    fan but heat will build up round the engine when stationary.

  6. I have a RAMCO diesel generator coming. It's delayed by the importer due to
    missing parts. The question relates to emergency use.

    The engine cannot overheat at idle. It's a 5 cyl Mercedes diesel. These
    engines run so cool there's an accessory water pump to make sure the
    passenger compartment gets enough heat. Diesels idle with 1/2 the fuel
    consumption of an Otto engine, and are not stressed the way Otto engines are
    at idle.

    Your statement that the alternator is designed to merely float the battery
    does not conform with the realities of a modern automobile. The older
    Mercedes are famous for conservative design. In this car, the Bosch
    alternator must be able to supply, at idle:

    headlights: 110 watts
    front radiator fan: 100 watts
    interior AC ventilation fan: 100 watts
    Total: 310 watts = 26 amps.

    To give an idea of the quality of the electrical components, every
    electrical connector in this car uses silver plated, soldered, screw-machine

    My question is related to emergency use only.
  7. Your perspective is very interesting, because it contradicts every other
    comment on the list. You seem to be saying that an alternator does not have
    to be limited to 1/7 of the nominal rating to avoid cooking it.

    In my case, (please review my original post), a voltage limit imposed by the
    regulator is not a problem, because the proposal does not connect the
    alternator directly to the bank being charged. It goes through several
    intermediate steps.

    Your statement that there is an artificial cap on amperage, I would simply
    point out that this is in contradiction to the nominal spec of the
    alternator, which is 70 amps at > battery float voltage.
  8. Guest

    Not unless Mercedes does it different than everyone else. The
    regulator regulates voltage - period. The alternator puts out whatever
    current the batery/batteries will accept at that voltage, up to the
    nameplate current of the alternator.
    There are MANY alternators out there that will supply 80% rated output
    all day, without problems. The small frame GM is NOT one of them.
  9. Guest

    IMHO, the additional stages are where the problem may well come in.
    If you are a long ways from the battery, transmitting the power at
    high voltage IS a good idea - just make sure you use a GOOD inverter
    on the vehicle. Then the extention cord is not terribly critical - as
    72 amps at 13 volts is 936VA, which at 120 volts is only 7.8 amps.
    A good 16 guage tech would be adequate - 14 even better.
  10. Thank you for the correction.
    However, it seems to me you're getting full voltage at headlight amperage.
    That's not the same as full power output, which I think requires more rpms.
  11. [snip]

    I wonder if alternator frame temperature can be used to determine maximum
    safe draw. I could put a thermcouple on it.
  12. That sounds like an idea with merit. You still haven't mentioned the size of
    the battery bank, though.
  13. Sure I did, in the original post. Please reread.
  14. All this is for emergency use only. My house is on the grid. While I have
    nothing but admiration for you alternative energy guys, I just can't justify
    a massive off-grid system.

    I am very interested in creating a system with a very high degree of
    built-in redundancy. The plan has two battery inverter-charger systems. If
    one breaks down, the parts can be used to maintain the other. There is a
    cheap one cylinder diesel generator. There are four 110 watt solar panels,
    and an MX-60 charge controller. All these systems use compatible parts. The
    diesel car is an unutilized power source that I want to integrate into the
    redundant plan. Hence the question of this thread.

    A wind generator would be nice. However, it requires a very visible
    permanent installation that would not be well accepted in my suburban
    neighborhood. The other elements of this installation are invisible. The
    solar panels will not be deployed unless the diesel generator breaks or the
    fuel supply is depleted.
  15. Guest

    I'd recommend reversing that. Use solar as main source, with generator as
    supplement. Example: today's forecast is for partly cloudy this morning,
    becoming cloudy this afternoon. My battery had 73AH discharged from it this
    morning; I ran the generator long enough to replace half of that (being
    first charge in, it was at maximum load, hence best efficiency), trust there
    will be enough solar to finish the charge and supply the day's load (usually

    A motor-generator at low or variable load is not efficient because fixed
    losses (friction, windage, etc.) use a significant portion of the engine's
    output. Note throttle position at light load compared with no load - little

    Tom Willmon
    Mountainair, (mid) New Mexico, USA

    Power corrupts. Absolute power is kind of neat..........

    Net-Tamer V 1.12.0 - Registered

  16. I have no idea how long you are considering running this motor for, but I
    have seen things stating that running unloaded diesel engines for long
    periods will cause a carbon build up in the engine.
  17. Buy American
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