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I'm about to install a voltage sensitive relay which will be in between my car battery and my auxiliary battery in my van. When doing research for this, I find diagrams like this:

VSR Diagram

What confuses me here, is ground, and if I should be grounding my aux battery to the chassis, and what would be the purpose of this?

In this schema, does ground just mean that the negative poles should be connected? I understood it like just a shortcut, instead of having to close the circuit in the schema, you can just use the ground symbol, which means the negative poles should be connected to each other. Or does it actually mean grounding to the chassis?

Sorry if this is a duplicate. I've done research for this but I haven't been able to grasp it yet...

Thank you.

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    \$\begingroup\$ The chassis of a car or van is typically used as the return path for most circuits. \$\endgroup\$
    – user80875
    Oct 4, 2020 at 14:38
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    \$\begingroup\$ Have a look in your van and you should see that the existing battery negative strap is bolted to chassis which acts ac a return conductor for almost everything else including starter, lights, fans, etc. \$\endgroup\$
    – Transistor
    Oct 4, 2020 at 14:41
  • \$\begingroup\$ One should not be building a custom vehicle electrical system until there is a much better understanding of things like this, and far more important issues not being sufficiently considered. \$\endgroup\$ Oct 5, 2020 at 15:38

2 Answers 2

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The chassis, metal body on a monocoque car is used as the negative wire, or ground.

This saves on needing two cables (supply and return) to each device or light etc. The accountants save on the cost by reducing the number of cables and often by minimizing the cable size used to each device.

So each battery negative is connected to the chassis, you could control the negative instead of the positive but it does depend on the total of what you want to achieve and not with that particular relay which has an override function.

Just remember to ground or provide a return for devices or lights on cars that have fibreglass panels :)

Also take care as many cars before about 1965 were positive earth...

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  • \$\begingroup\$ Hmm, so the whole chassi becomes the negative pole? Isn't this dangerous? Or are all the appliances also grounded within the car then? \$\endgroup\$
    – mtnptrsn
    Oct 4, 2020 at 14:54
  • \$\begingroup\$ So this is only done to save on cables, not on stabilising voltages and for security purposes? \$\endgroup\$
    – mtnptrsn
    Oct 4, 2020 at 14:55
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    \$\begingroup\$ @mtnptrsn more danger from poor or drunk drivers than connecting the chassis / body to the negative pole... \$\endgroup\$
    – Solar Mike
    Oct 4, 2020 at 14:55
  • \$\begingroup\$ @mtnptrsn the voltage goes form about 12v to 15.3V so any device that needs a stable supply tends to be connected via a voltage stabiliser, which when they failed allowed the revving of the engine to give you more fuel as the gauge would move as the engine revs increased., \$\endgroup\$
    – Solar Mike
    Oct 4, 2020 at 14:58
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    \$\begingroup\$ "So I don't need to ground my aux battery to the chassi then?" Nobody said that and if you don't do that how will current return to the battery? You really do need to think about safety. The word is chassis, not chassi. (chassis (US: /ˈtʃæsi/, UK: /ˈʃæsi/; plural chassis /-iz/)) \$\endgroup\$
    – Transistor
    Oct 4, 2020 at 15:06
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The earthing symbol, in an automotive electrical system schematic, does signify actual connection to chassis.

The path from the power source to the loads, in an automobile, is through copper cables whereas the return path is through the low resistance steel chassis.

The advantages of such a system are:

  1. Cost savings in copper cables to the extent of 50%.

  2. Reduction in vehicle weight to the extent of 50% of the total weight of copper that would be otherwise required.

  3. Space savings to the extent of 50% of the total volume of copper cables that would otherwise be required.

Towards this end the negative terminals (generally) of all electrical sources and loads in a vehicle are connected to the chassis.

Likewise, this concept is equally valid for the electrical system in a caravan.

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