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  1. #1
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    Ohms Law, using test lights to diagnose parasitic battery drains, short circuits

    ...
    --------

    before springing for a new battery you want to check the system for a current draw. Easiest way to to wire a taillight bulb is series with the ground lead and see if it lights up when everything on the boat is switched off and you connect it.

    Don't turn anything on or the bulb filament will vaporize

    takes a big draw to flatten a group 65 sized battery in a week..if the battery is really old, just replace it.

    ...
    Last edited by K447; 05-08-2014 at 11:05 PM. Reason: Moved the Ohms Law discussion away from the OP battery thread


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    Ohms Law, using test lights to diagnose parasitics drains, short circuits

    Quote Originally Posted by nmpeter View Post
    ... Don't turn anything on or the bulb filament will vaporize

    ...
    Why would the bulb blow? It should just light up at almost full brightness, no?

  3. #3
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    Quote Originally Posted by K447 View Post
    Quote Originally Posted by nmpeter View Post
    ... Don't turn anything on or the bulb filament will vaporize

    ...
    Why would the bulb blow? It should just light up at almost full brightness, no?
    the bulb will fail as the current draw (through) the bulb if you hit the starter is far larger then the 3amps (typical 35w stop lamp) it can typically pass when it's connected from the hot side of the battery to the ground.

    keep in mind that the setup for the "current draw" test is the 12v bulb in series with the hot or ground lead.

    that's why the bulb filament will simply evaporate, the same way a fuse does when you dead short it.

    I've been testing for and demonstrating small draws on repair jobs since I took my first electrical theory course in high school and figured out it was really good money, since:

    1-most folks are afraid of electricity ( and for good reason)
    2-most shade tree techs don't have a clue, unless they've done the bookwork and have a hard grasp of ohms law.
    Last edited by K447; 05-08-2014 at 11:08 PM.

  4. #4
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by nmpeter View Post
    Quote Originally Posted by K447 View Post
    Quote Originally Posted by nmpeter View Post
    ... Don't turn anything on or the bulb filament will vaporize

    ...
    Why would the bulb blow? It should just light up at almost full brightness, no?
    the bulb will fail as the current draw (through) the bulb if you hit the starter is far larger then the 3amps (typical 35w stop lamp) it can typically pass when it's connected from the hot side of the battery to the ground.

    keep in mind that the setup for the "current draw" test is the 12v bulb in series with the hot or ground lead.

    that's why the bulb filament will simply evaporate, the same way a fuse does when you dead short it.

    I've been testing for and demonstrating small draws on repair jobs since I took my first electrical theory course in high school and figured out it was really good money, since:

    1-most folks are afraid of electricity ( and for good reason)
    2-most shade tree techs don't have a clue, unless they've done the bookwork and have a hard grasp of ohms law.
    Current draw through the incandescent light bulb is limited by the filament resistance of the light bulb.

    When a 12 volt light bulb is directly connected across the 12 volt battery posts, it just lights up, no?

    If some low resistance (such as a starter motor or solenoid coil) is in series with the light bulb filament that additional resistance cannot increase the current flow through the light bulb.

    Fuses and light bulbs are not interchangeable things. One is designed to have (or tolerate) significant voltage across the terminals (at least up to the rated voltage of the light bulb), the other is not.

  5. #5
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    it's pretty much the same deal as a fuse except the light bulb give you the visual indication of the battery being drawn down. You cannot pull 50 amp thru a light bulb.

    If you've every blown up a cheap voltmeter that can only handle 10amps on it's current test, you'll find a pretty heft bit of copper in the meter has a huge break in it, with copper splatter everywhere.

    The difference here is that the bulb doesn't have one side connected to ground and the other to the hot side, we're working it "like a fuse" to show the very low current draw when you don't have a proper meter handy.

    The bulb is between the ground lead and the battery, or the hot lead and the battery..so ALL of the draw for the electrics is thru that bulb, which is why pressing the starter button make it go poof.

  6. #6
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by nmpeter View Post
    ...
    The bulb is between the ground lead and the battery, or the hot lead and the battery..so ALL of the draw for the electrics is thru that bulb, which is why pressing the starter button make it go poof.
    I simply do not accept this. Electricity does not work like that, per Ohms Law.

    There is no such thing as 'draw', there is merely voltage and resistance, which combine to allow current flow. The test light bulb itself defines the minimum circuit resistance and the battery defines the available voltage. Hence the current flow through the light bulb cannot exceed what the light bulb resistance alone will allow.

    ... we're working it "like a fuse" to show the very low current draw ...
    The light bulb is 'working like a resistor'. The bulb has internal resistance. This is the key factor. Fuses and ammeters have extremely low internal resistances, usually tiny fractions of an ohm. Very little resistance to current flow.

    The simple light bulb has an inherent and well defined resistance. In addition, for a typical tungsten filament the resistance increases as it gets brighter and reaches resistance and current stability when faced with the full bulb spec voltage.

    That the test bulb has resistance is the reason why the size/wattage of the test bulb must be matched to the resistance of the electrical 'drain' that is being diagnosed. If you want to reveal a few milliAmps parasitic drain you cannot use a 55 watt headlamp bulb, the bulb will not even begin to glow. You must use a tiny light bulb which only needs a few milliAmps itself to light up.

    If you want to power up the entire electrical system in a watercraft but need to diagnose a short that is blowing fuses, then the headlamp bulb in series with a battery cable might be perfect. It has only a couple ohms internal filament resistance so an amp or so will not produce a big voltage drop across the bulb. Hit the starter button or trigger the short condition and the bulb itself is suddenly the majority remaining circuit resistance between battery posts, so it glows at near full brightness.

    As would the tiny test light bulb. Neither would go poof regardless of the load or 'draw', no matter how heavy.
    Last edited by K447; 05-08-2014 at 11:14 PM.

  7. #7
    captain slow Turbo Retro's Avatar
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    emmm..

    the bulb in series with negative? and it glows would indicate that there is a short from the + side of the electrical system?

    when i fixed my 04 ecu, i was suprised to find that the ecu pins switch - and all + is on the mpem. completely different to the 07. this is on seadoo 4tecs of course



    Quote Originally Posted by nmpeter View Post
    ...
    --------

    before springing for a new battery you want to check the system for a current draw. Easiest way to to wire a taillight bulb is series with the ground lead and see if it lights up when everything on the boat is switched off and you connect it.

    Don't turn anything on or the bulb filament will vaporize

    takes a big draw to flatten a group 65 sized battery in a week..if the battery is really old, just replace it.

    ...

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