How do you limit amps for charging?

Discussion in 'OnBoard Electronics & Controls' started by Raftman1979, Oct 20, 2008.

  1. BHOFM
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    BHOFM Senior Member

    A 3 1/2 hp motor will not produce 78 amps at 14V!
    Find a 55 amp alternator with a built in regulator.

    I think the motor will only do about 30amps and you
    need to spin the alt. at 2X engine speed.

    We built these little set ups for boosters for race cars.
    A 5hp is the preferred engine. with a 2X drive, it will
    almost turn 3600 at full throttle.

    You can easily limit charging by adjusting the rpm of
    the engine. You just need a simple amp gauge in the
    line.
     
  2. thudpucker
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    thudpucker Senior Member

    We want some photos!

    We'd sure like to see some photos of your pretty good sounding effort!:)
     
  3. BHOFM
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    BHOFM Senior Member

    I have no photos. We tried the smaller motors and they
    pulled down badly.

    We were spending, in the late 70's about $200 on these.
    The 5hp was still light enough to handle on a small two
    wheel dolly with the alt. mounted and a small battery.

    These were to boost race cars, not really to charge batteries. They were very high compression engines.

    We did find that turning the alt. too slowly caused some
    over heating problems. We never had the problem when
    we went to the 2X drive.

    This was before the days of the booster pack you can
    carry around.

    Check this:

    http://www.delcoremy.com/Alt28SI.aspx
     
  4. pistnbroke
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    pistnbroke I try

    alternator starts to charge at about 1500 rpm producing about 8 amps...full output at about 6000 up max rpm usually about 12000 so 2 to 1 up gearing on your briggs should be fine ...or try it direct axial drive as you will not acheive more than 25a and falling charge current however big your altrernator is ...that is if you want it to charge a battery rather than supply lighting loads 100 A at 14v is 1400w thats 2 hp and at 50 percent efficiency you will need 4 hp to drive it ....forget the weed eater even if it works it will not last and makes a lot of noise...

    The guy on the raace cars is not trying to do what you want ...he had the voltage regulator shorted out and was only interested in amps to drive the race car starter ...

    you must not connect or disconnect anything when its running or you will f44k the alternator and you always need a battery connected

    Forget the idea that a 75 A alternator charges a battery at 75A it does not ...about 25A falling for the battery and the rest for the lights air con etc on the vehicle ...
     
  5. thudpucker
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    thudpucker Senior Member

    OK, I think I see the light now. :)
    Thanks all and especially Piston broke for the technical data.
     
  6. pistnbroke
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    pistnbroke I try

    Thanks for the rep thudplucker .....If you have a 3.5 briggs from a mower ..mount the alternator under the mower engine straight onto the shaft ,,makes a very compact unit or run the belts in a box under the motor with the alternator along side ,,you can make a trial set up using some 7/8 ply and blocks of wood at the ends to keep it off the floor ...gets your dimentions and pulley set up right ...
    use a standard alternator then you can strart the briggs and switch on the ignition to bring on the load ... if you can get a crank and camshaft gear from a small engine you will have 2 to 1 and could use a toothed belt .
     
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  7. DaveJ
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    DaveJ Senior Member

    With my limited knowledge in this field (I'm avionics, jack of all trades, master of none) i have to agree with alot of what pstnbroke has said, albeit not the method of how it was delivered, but i do get frustrated as well when people make opions not totally knowing all the facts.

    Not seeing the schamtics for a car alternator, its hard for me to comment whats really happening inside, but understand engineering principles, and i would have hoped that they design failsafes into the retifier/regulator, so that in the event a terminal jumps off the battery it will be able to deal with this and not selfdestruck, as i have disconnected the positive terminal to prove the charging system is working on afew occations without any adverse effects. So wondering if i just fluked not destroying the alternator or not. The current rating for the alternator is its max output current that it can provide, once again i would hope they have in place current limitors to protect the alternator from exceeding this current, and i do see it, friend has a large sound system in his car that when he cranks it up, the sounds system starts to steal all the power from the alternator and starts to drain the battery (hence thats why they tend to run a second alternator and battery system). The indication he gets is the charge light comes on.

    I think alot of people are forgetting, or not sure what is happening in a battery, in theory a lead acid (any lead acid) will last forever, but we all know in practical terms this not not true. I notice some of the number being quoted are based on theory, and practicality proves these numbers wrong, you have to remember, these things are man made, anything man made is dodgy.
    A cell in a lead acid battery will provide 2 - 2.2 volts, thats how it is designed the combinatoin of the metals in the plates and the sulphuric acid determin this. You shove 6 cells in series and you get your 12 v battery (well dah).
    Its this a chemical reaction that happens where the lead is removed or replaced into the the lead plate, where you get the power from a battery, but because it is man made the reaction deteriates over time and stops, your battery becomes dead.
    For the chemical reaction to happen there needs to be enough overvoltage to charge the plate with lead again, this is roughly 12 - 13.2 plus the voltage for the electrolysis action needs to make the chemical reaction to happen. So if the plate has no lead in it, the extra voltage required for the electrolysis to happen is low (this is seen as low internal resistance), as the plate charges up with lead the required voltage increase and the internal resistance increase until the internal resistance overcomes the avaliable voltage and the charging cycle stops. The gassing is the electrolysis action happening, the amount of gassing is based on the current that is used for charging, if there is too much current the reaction will happen too fast and build up too much heat. But because the reaction is happening fast the eletrolysis can't keep up so the plus voltage for this action increases raising the resistance to a point of equriblim (now i know there is an uproar, i hear people yelling out thermal run away) Yes thermal run away is a problem, but only if the voltage and current are allowed to esculate out of control, but because the alternator is locked at 14.4 v this occurance doesn't happen. This is why i agree with pistnbroke about you don't need anything to charge the battery, as this happens all the time in a normal car.
    There are alot of factors that make up the battery, the materials used in the plates, how much sulphuric acid is in the electrolite and what not, which determines the charge voltage and current, which is all aim around getting the longest life out of a man made product.
    The amount of current is proportional to the surface area of the plates, the bigger the plates, the more current can be provided also the more current that needs to charge it. Also the quality of the materials used in the plates manufacture determines the lenght of service.
    But what i believe the biggest thing that is overlooked alot of the time is the wires connecting all this crap up. You can have the best battery and alternator combo but if your cables old and tarnished, this can really effect the charging cycle of your battery. I use a product on the terminals that keeps the mositure out, stop the electrolite build up on the terminals and keep good contact between the terminals and terminal lugs. I believe this is major reason i got 10 years of out a marshal battery for my commodore.

    Dave,

    P.S. Now i know i'm not the be all and all about this topic, but hopefully i have shed some light on whats going on. Of course, if i'm mistaken, i'm all too happy to recieve criterism.
     
  8. thudpucker
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    thudpucker Senior Member

    Well I cant be the one to throw the first stone. I have an AA in electronics. I'm amazed at what other people know that I dont.

    I had one more little tid bit from my Dad who never understood electricity at all.
    When he thought he had an electrical problem, he hung a Hacksaw blade in front of the Generator or Alternator.
    If it was charging, the blade was attracted to the armature. Now aint that simple?

    DaveJ I had occasion to put some intercoms into a few C-150's. I was pleasantly amazed to find everything in those old airplanes was in great shape.
    Because in Aircraft, no hackers, no cheap Chinese wires or terminal ends are allowed. Everything is certified and with that, nothing is intermittent unless it has mechanics tracks written on it.
    Not like our boats at all. :)
     
  9. pistnbroke
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    pistnbroke I try

    Well there is no protection for the diodes so if you pull a lead off and the charge current is high its fineto baby

    The output current is regulated by the cross section of the core of the stator ...usuallly quoted as the alternator is self limiting for current ...

    Whats an AA in electrice ....I know what a degree is and HNC and all those city and guilds in automoblile diagnostics because I got them and more but pray whats an AA ???
     
  10. DaveJ
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    DaveJ Senior Member

    Accually Thudpucker, i do remember that, but with a screw driver.

    As for aircraft, yes there is way more control on what goes into them, I guess alot people don't know that when rating the size of wire for an application on the ground totally changes when you go to put it into an aircraft, as you go up in altitude, the current handling of the wire drops off. When designing wiring looms, you try to make the wires one complete lenght as much as possible, putting splices in (even the sealed type that are used in the aircraft) is introducing another failure point.
     
  11. thudpucker
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    thudpucker Senior Member

    AA: Associate of the Arts in Electronics Technology.
    It means I have a 2 year degree in Electronics.
    I have been exposed to everything from AC and DC physics, to the (1977) latest in Busy-chip development and even some machine level programming in Intel's 8080 processor.
    Among the most interesting years in my life was the three years it took me to get that two year degree. I didn't finish all of it though. "Report Writing" and "public speaking" was part of what I did for a living at the time so I skipped that.:p

    We'll never get rich doing what we do, but on the other hand, we'll never have a boring day as long as we have work!
     
  12. lamiataspaska
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    lamiataspaska New Member

    I have a similar problem like Raftman1979. I have a sailboat with 6 gel batteries 80Ah each. I had them all connected in parralel and for a while I thought that everything is working fine. One day I found the batteries almost melted, they really looked as if they've been in a fireplace for a few minutes, two of them were almost round. The alternator also shows 17V.

    Now I have them replaced with new ones, but I am a bit afraid to put them in the boat with the same setup.

    My engine alternator is rated at 50A, my electronic charger is rated 25A. When I used to charge them with the charger, I noticed that the charging never stops (maybe 480Ah is a bit too much for the charger to detect when they are "fully charged"). When I charge them with the alternator (which was very rare) I didn't use any amp limiting devices.

    So the question is if the "too big" battery bank burned my alternator and then it overcharged the batteries? Or did my charger do something wrong to them?
    In general is there any problem connecting 6 batteries? Should I separate them in two groups?
     
  13. pistnbroke
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    pistnbroke I try

    for a 480 ah pack alternator should be rated at 4.8 x 25 A which is 120 A . this will give you the shortest charge time . So a smaller alternator takes longer . Your 25 A charger is littlte more than a trickle charger in this situation.
    Your problem arrises from the failure of the voltage regulator in the alternator which is normally set at 14.4v. You should do two things ,,,,always check that the warning light is fully out on the alternator ( no glow) and fit a voltmeter so you can always be sure your batteries dont go over 14.4v

    I would also check with the maker what is the max voltage for these batteries as some are 13.8v. You could fit a second alternator or a bigger one .It is only a problem leaveing the batteries connected in paralled if they are not charged for long periods. I would be charging them monthly if they are not in use .
     
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  14. lamiataspaska
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    lamiataspaska New Member

    If the batteries have too low resistance and draw too much amps, can this damage the alternator or the volt regulator, or is it just slow charging? The batteries are for 13.8, but I thought it is not such a big difference between 13.8 and 14.4 (learned that I was wrong the hard way). If I disconnect the batteries from the alternator with a switch and the engine is still running, is it ok with the alternator going to keep running without any load?

    I can change the alternator to a bigger one, but on the other side it charges the starting battery as well and it is only 60Ah. Isn't it going to fry it with 120A?
     

  15. Fanie
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    Fanie Fanie

    Even a low current will overcharge a battery if it is charging with overvoltage all the time. This is because the internal resistance of a battery goes higher at rated voltage, hence allow the battery voltage to keep on going up.

    The alternator should supply to it's max current untill the battery requires less current. I doubt they should get damaged.
     
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