1992 Marathon lighting questions

[gmtilt]

I have a 1992 Marathon (electric) that I'm hooking up headlights on. I know you can't use the chassis for a ground. But that means you have to find headlights that are not grounded thru their own brackets? The headlights I have do have separate ground wires but are grounded thru the headlight brackets. I've got a 12 converter on the way. Will I be able to run the positive and negative wires to the headlights without any feedback from the chassis ground?
And why do I pick up voltage thru the chassis ground (volts depending on which battery?
Thanks for any help or suggestions!

[WPI_91]

I'll be interested in hearing a response from the veterans as well.
I would have thought making the chassis the 12V ground (connect converter ground to chassis) would be fine.
None of the batteries are electrically connected to the chassis, so why can't it be 12V ground?

[LVCJ]

Well,,,,first off, I'm not an electrician, but here are some main reasons why its not recommended to use the carts chassis as a ground....especially in an electric vehicle:
"Aside from protection against arcing, there's also the issue of the high currents. Having all the high currents go through nicely protected and shielded bus bars is a safer and more reliable design compared to shunting several hundred amps through the chassis; which could result in all sorts of potential EMI issues and induced voltages on other nearby low-voltage cables running next to the chassis. because of the Chassis's irregular shape, these problems become somewhat unpredictable"......found this on the web..

So basically its much safer to keep isolated the high current circuits from the low current circuits. It is safer to isolate the high voltage from the chassis to reduce the risk of short circuits, especially in the event of a collision.

Interference and Safety are the main reasons.

Someone with more Knowledge on this will chime in to help shed a little more "light" on your questions. (pun intended)

[JohnnieB]

There has likely been several master theses (So the spelling police don't get their knickers in a wad: theses is the plural of thesis) and perhaps a few doctoral dissertations based on why competent engineers don't run amps of electrical current through vehicle frames, but the bean counters in the automotive and other industries managed to convince the ultimate decision makers to defy sound engineering practices and use the less conductive metal in the body and frame to complete the circuits, thereby saving about half of the wiring costs.

Each battery is potentially a bomb that sprays sulfuric acid when it explodes, as well as a arc welder. In a car or truck, you only have one place to light the fuse (Positive terminal), while in an electric golf cart, you have a half dozen or more.

Also in a electric golf cart, the motor case is connected to the frame and as the brushes wear, a layer of carbon covers everything inside the motor, creating a myriad of conductive pathways. If the frame was connected to B-, the cart's performance and run-time would decrease as the carbon buildup increased. It still happens with an non-grounded frame, but not to the same extent.

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Most, if not all, DC to DC converters are non-isolated ground systems, meaning that the battery pack negative input is electrically connected to the negative 12V output. Therefore, if you use 12V accessories that grounds the -12V to their chassis, you are, in fact, grounding the cart's body/frame to the main negative terminal of the battery pack.

If possible, avoid using 12V accessories that ground the -12V through their chassis.

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As for why you can measure a voltage on your cart frame, your DVM is just too darn good.

Perfect insulators do not exist, or at least not outside a physics laboratory and the high input impedance DVM's of today can measure voltages that barely exist. In fact, you can almost say voltages that don't exist.

Use an old analog voltmeter (20KΩ/V) or connect a <100KΩ resistor between your DVM's test leads and the phantom voltage will disappear.

Instead of measuring from the battery pack to the frame, measure from one of the battery terminals to the clean dry top of the same battery. You will read a voltage. (If the battery top is dirty or damp, you will read a higher voltage.)

What you are reading is a point that is connected to the voltage source through a very high resistance (less than perfect insulator) with a voltmeter that draws very little current from the circuit to make the reading (aka High input impedance), so very little of the electrical potential (voltage) is dropped across the high resistance between to point of measurement and the voltage source when the DVM is a making the measurement.

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FWIW: When I was teaching troubleshooting, I had my students try to troubleshoot a multi-staged vacuum tube cascode amplifier with multiple mega-ohm resistors with a 20KΩ/V analog voltmeter. The objective was to teach them that the test equipment becomes part of the circuit under test and the circuit under test becomes part of the test equipment.

Again for the spelling police: a cascode vacuum tube amplifier has the plate of one stage directly connected to the grid of the next stage, while a cascade amp uses capacitors to decouple the DC voltages between stages.

[WPI_91]

Originally Posted by JohnnieB

...the motor case is connected to the frame ... conductive pathways.
...DC to DC converters are non-isolated ground systems

Thanks JohnnieB.
Those are great reasons.
I was thinking the 12V output would be isolated from the 36V system.