Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Auto Charging system logistics

  1. Oct 18, 2011 #1
    Hello this is my first post, I just joined, I hope I am posting in the correct place. I belong to some computer forums and some biker forums but not sure how these technical forums work.

    I am not a student but an enthusiest (spelling?). My current subject study is:

    In automotive charging systems it appears the Alternator is wired in parallel with the car or motorcycle battery and main circuit. The voltage regulator keep the alternator from burning up the battery with too much increased potential.

    My question is can one test for total system amps but simply putting an amp probe around one of the battery cables ?

    Or does the charging system have an effect (with it's increased voltage?) on the system amps ? And if so is there any way to separate the charging sectors input from the system amps ?

    I understand that batteries wired in parallel still supply the same voltage (12 volts) to the system but what happens when a charging circuit increases this potential ? I.e. I see that when I test the battery voltage while running and not running I get different amounts.

    Would appreciate any info or pointers that would help. Or please direct me where I should post this type of question.
     
  2. jcsd
  3. Oct 18, 2011 #2

    vk6kro

    User Avatar
    Science Advisor

    Current probes only work on AC current because they have a small transformer in them.

    Batteries do not get constant current from an alternator, though. The current flows into the battery when the alternator output is greater than the battery voltage and this occurs in pulses from the three phase generated output after it is rectified.

    So, you would get some reading on the ammeter, but it would probably not be the actual DC current.

    Any wire can only have one current flowing in it at a time, so, for example, the current going to the head lamps of a car may be coming from the battery or from the alternator (or both) and you can't tell the difference unless you put individual meters in the wires leading to these components.

    Both the battery and the headlamps of a car might be getting current from the alternator if this output is greater than the battery voltage.

    When a battery is being charged, its terminal voltage rises to be the same as the charging voltage. A conductor can only have one voltage on it at a time, so this has to happen.
    A useful test to see if a battery is being charged is to compare the battery voltage at low engine revs with the voltage at high engine revs.
     
  4. Oct 18, 2011 #3

    Averagesupernova

    User Avatar
    Science Advisor
    Gold Member

    There are non-contact DC current sensors available. They are hall-effect devices.
     
    Last edited: Oct 18, 2011
  5. Oct 18, 2011 #4

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    i picked up in a junkshop a handy antique ammeter that's akin to a compass.

    you lay it over the wire, there's a slot on the back to position it.
    The needle indicates current and direction. Obviously the needle is attached to a magnet that is deflected by current through the wire. I'm going to guess by style of numbers on face that it's from 1920's.

    what a handy gizmo for working on cars. I carry it in glovebox.
    somebody should re-invent that, they'd sell....
     
  6. Oct 18, 2011 #5

    Averagesupernova

    User Avatar
    Science Advisor
    Gold Member

    I have a starter current indicator like that. So long since I looked at it I can't recall if it will measure low enough to indicate anything useful for a charging system.
     
  7. Oct 18, 2011 #6

    vk6kro

    User Avatar
    Science Advisor

    I have used precision tangent galvanometers that work like that. They compare a local magnetic field with the earth's magnetic field by deflecting a compass needle away from its usual north south alignment.

    Some of the instruments from those days were works of art and constructed with craftsman skill.

    I wondered about Hall probes, but the thickness of the wire insulation would have a big effect on the reading, I guess.
     
  8. Oct 19, 2011 #7
    Wow thanks for the excellent replies. vk6kro what you said goes along with many of my imaginations of the situation, although I did not know that about the amp probe on DC. I did realize though the current from the regulator had been rectified and was DC.

    I have a little cheap meter that has an AMP part to it. But I will have to check, maybe that setting if for AC only. I was wondering (if it's also for DC amps) if I could set it to a safe level (say 50 amps, since the manual says my system amps are somewhere round 35) and just hook it between the Red battery cable and the battery and crank the bike ?

    Also there is something that has always puzzled me along this line of though. I know basic Ohms law stuff but have never been able to wrap my head around how (or why) you can have the same voltage but have a greater AMP potential in a given scenario. In other words like when you jump a 12v battery (or hook two 12v batteries together like some old tractors had) you will still have only 12 volts on the line (since they are in parallel) but you have twice the amp potential.

    Now I know it all sounds logical on paper but I keep having this hangup that current is DIRECTLY related to the voltage potential and resistance. In other words (in my mind) current is nothing more than the resultant electron flow that occurs when you have a larger number of electrons accumulated at one section of a circuit (or atmosphere with lightning) than at another. The 'imbalance' of the electron accumulation (in my mind) IS the voltage. In other words (in my mind) voltage is nothing but pressure resulting from electron buildup and when you give it path of condusive resistance the current starts attempting to equalize the balance, (slow on a battery circuit, very fast with lightning).

    So having said all that, how can you have the same voltage with a larger amp capacity? Or maybe a more precise question is, what exactly does it mean when you have IDENTICAL circuits, both with 12v source and both has a sort of reostat hooked to an inductive load (a motor). BUT one circuit has twice the amp potential (two 12v batts in parallel) than the other (only one 12v batt).

    As you simultaneously switch both circuits on and dial their resostats exactly the same, As it would appear, the lower amp capacity circuit would turn the motor at a slower rpm as the same physical load on each circuit increased. What does that mean when one shows more amps flowing than the other. How can more electron flow occur without more voltage showing ? I have a real mental block in this area and it affects how I troubleshoot circuits.
     
  9. Oct 19, 2011 #8

    vk6kro

    User Avatar
    Science Advisor

    All batteries have the equivalent of an internal resistance in series with a voltage generator.

    I know this sounds a bit weird, but it works out OK even though there is no actual resistor.

    So, if you put an ammeter across an alkaline AA cell, you would read something like 6 amps. Yet, the battery has 1.5 volts and the ammeter has very little resistance (maybe 0.1 ohms) so the current should be 15 amps? (1.5 volts / 0.1 ohms = 15 amps)
    But what happens is that the battery has an internal resistance of (1.5 volts / 6 amps minus 0.1 ohms) or a resistance of 0.25 ohms minus the 0.1 ohms resistance of the meter gives 0.15 ohms.

    If I did this to a very old battery, then the current would be a lot smaller because this internal resistance increases with age and battery usage.

    If I put two similar AA batteries in parallel, then the short circuit current would be doubled because each can deliver the same current and the result is double the current.

    Car batteries have a very low internal resistance and they can easily deliver hundreds of amps.

    If you wanted to measure the current flowing while your battery was being charged, you could touch the leads of your multimeter together while the motor was started and then separate them while it was running.
    Or you could leave the meter in the circuit that goes to the alternator, but not to the starter motor.
     
  10. Oct 19, 2011 #9

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    ""I have a starter current indicator like that. So long since I looked at it I can't recall if it will measure low enough to indicate anything useful for a charging system.""

    mine is +/- 30 amps full scale, perfect for car work. It'll move needle discernibly at two amps, one if you're young and not in bifocals. I treasure it. Keep your eyes peeled in junkshops and auctions.

    There exists a Fuke clamp-on voltmeter accessory that's battery powered, reads DC or AC up to two khz, gives 2V out for full scale current which is switch selectable 20 or 200 amps. Takes 4 AA cells.
    VERY handy with an oscilloscope or hand-held voltmeter for non-intrusive current measurement. I used them at work for years. A bit expensive for most hobbyists but i've seen them on Ebay. Search for "clamp on current probe"
     
  11. Oct 20, 2011 #10
    Thanks guys for the excellent replies. I just now got back on the forum.

    vk6dro I found some good reading on batteries since I posted last and I now understand about the internal resistance thing that you explained. And how the voltage is limited to 2 volts per cell (acid car type) of a battery inherent in the chemical ability to generate. The amps are inreased (as you said) by manipulating the internal resistance with is done by cell size (area) which includes larger cells in high amp batteries and sometimes wiring two batteries in parallel. And how the individual cells in a car battery are wired in series to give the 12 volts output.

    However you kinda lost me when you said if I wanted to measure the current flowing while your battery was being charged, you could touch the leads of your multimeter together while the motor was started and then separate them while it was running.

    What exactly do you mean by touch the leads of the meter together ? Do you mean while on the circuit conductor also ?

    And when you say OR I could leave the meter in the circuit that goes to the alternator, but not to the starter motor. I 'think' I understand you here but as to where. I understand the starter cable is subject to rotor inductive states, but as to putting the amp tester between the positive battery cable and the battery OR putting it on the Alternator cable I'm still kinda fuzzy on what's really being measured here.

    I.e. IF the starter is not currently turning (off) would an amp test between the pos batt terminal and the pos bat cable show system amps ?

    And on a separate thread of conversation, by nature of it having to go thru the voltage regulator first, I don't understand exactly what testing the alternator cable would do except to tell you the output of the alternator also, which would be much higher than the system current would it not ?

    Please excuse my ignorance but would really like to learn the low down on this stuff.

    Also Jim Hardy I did find a AC/DC amp probe but they were over $200 bucks. I happened along something that I want to try instead and that is a Shunt Resistor that I ordered for $5 that has a design voltage and resultive amps that according to what I read can be used in conjunction with a volmeter to measure my line amps.

    This shunt is designed such that the voltage drop across it is X mV when the current flowing through it is X Amps.

    Therefore one can calculate the 'design X value' ohms resistance of this shunt to be design voltage divided by design current = XmV / Xamp = X design ohms. (typically to within ± 0.25% accuracy).

    Therefore if doing an in circuit test with this shunt, a voltage drop . . . . of say 28mV is measured using a standard multimeter, we know (by ratio math) that the current flowing is 0.028/'X design' ohms = Test Scenario Amps. Correct ? or did I totally miss this one ?

    Again you guys have been a great help to get my mind wrapped around the stuff to a point where I feel more confident testing stuff.
     
  12. Oct 20, 2011 #11

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    you have it exactly right i think.

    one milli-ohm (0.001 ohms) shunt will produce one milli-volt per ampere

    ohm's law: volts = amps X ohms; volts/amps = ohms; volts/ohms = amps .


    0.028volt / 0.001ohm = 28 amps

    be aware your starter shold pull a couple hundred amps so feel of your shunt after a very short crank and make sure it doesn't overheat.
    Hopefully your shunt is one of the real stout ones, heavy brass ends connected by flat blades sorta like a venetian blind, about 3-4 inches long overall.

    One practical point - the shunt should have four terminals.
    Apply current between outsiide two, measure millivolts between inside two.
    That way your millivolt reading does not include contact drop across your current connections which would be an error.

    3745.Jpg

    Yes those DC clamp-ons are expensive. I found one in a junkshop for $5 and to my good fortune when i got it home all it needed was the corroded battery terminals cleaned up.
     
  13. Oct 20, 2011 #12

    vk6kro

    User Avatar
    Science Advisor

    I have used a sensitive meter across the negative lead that goes from the battery to the body of a car engine.

    This gives a reading without interfering with the car's electrical system. It uses the existing wire as a shunt for the sensitive meter.
     
  14. Oct 20, 2011 #13

    vk6kro

    User Avatar
    Science Advisor

    However you kinda lost me when you said if I wanted to measure the current flowing while your battery was being charged, you could touch the leads of your multimeter together while the motor was started and then separate them while it was running.

    What exactly do you mean by touch the leads of the meter together ? Do you mean while on the circuit conductor also ?


    You don't want the meter to carry the full current of the starter motor (and the bike probably would not start with mutimeter leads in series with the starter motor), so you can connect the meter in series with the starter motor but short it out while the bike is actually being started.
    You do this by touching the multimeter probes together.

    Then you remove the short circuit and you can measure the normal currents of the bike charging system.
     
  15. Oct 20, 2011 #14
    Oh YES of course, Duh the red cable goes right to the starter. Sometimes I can be the dummy. I need to find a place in the circuit like maybe a jumper across the main amp (possibly) will have to investigate that possibility. The negative cable also sounds like a good option but for some reason (now that I think aobut it more) with the charging system running the cables going to the batteries seem like they will only read the amps going back into (charging) the battery in direct ratio to its weaker voltage than the Voltage regulator input. Course again there I realize it's probably pulsing back and forth between the two scenarios of regulator input and battery output, many times a minute so the plot thickens.

    I think I need to just take a good look at the wring to figure the best place to put this bugger. Also one last question. I'm wondering what kind of quality reading I can expect from the $4.50 shunt resistor that I've found? Would you think I need to find a more expensive sophisticated one to get any real accuracy ?

    Again you guys have steered my novice brain in the right directions and I appreciate it much.
     
  16. Oct 20, 2011 #15

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    great idea , should work quite well.
    just pick your contact points so you're measuring voltage drop along the wire not across the terminations...


    here's a handy table of wire resistabce for various sizes.
    http://www.powerstream.com/Wire_Size.htm
    The ohms per 1000 ft column would be same as milli-ohms per foot.

    i think ordinary battery cable is about #2.

    Be aware that older Fords have in the middle of their negative battery cable a little metal tab that looks like just a simple clamp to hold the wire to the fenderwell or frame.
    BUT in reality that little tab is the connection from battery negative to chassis.
    When somebody replaces their negative battery cable with a generic one lacking that clamp, the car won't start . When they jump-start it all sorts of trouble lights come on.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Auto Charging system logistics
  1. Charging systems (Replies: 1)

Loading...