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Normally in a car's DC installation the negative is on the car's body. Is it just to make it cheaper by not having to provide two wires everywhere, but just one and use the car body as negative, or is there another more important reason it is done that way?

I ask, because I want to make a secondary DC installation in the car, with another battery, different voltage, to be used for my electronics, computers, etc. The second installation will have a separate battery, but that battery will be charged off the main installation - not directly, but through a charge controller, basically a DC-to-DC converter with current limit, which will have separate positive and negative terminals for both input and output.

How do you recommend to wire the negative of this secondary DC circuit, should I keep it separate, isolated from the car body, or should I also connect its negative wire to the car body, so use a common ground for both installations?

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  • \$\begingroup\$ Pretty solid ground. Both batteries should or will be grounded to chasis too. \$\endgroup\$ – kenny Mar 30 '11 at 19:02
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You can connect the grounds in one point or leave them separate. As long as the only common point between the two circuits is the isolated power supply you are free to do as you wish.

If OTOH you want to connect them somewhere (to use the car antenna or sth.) then you should carefully choose the connection point (or even points) to avoid ground loops and ground noise.

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    \$\begingroup\$ I got an an answer from the manufacturer: wattflyer.com/forums/showpost.php?p=797303&postcount=11 of the DC-DC converter that the grounds of both sides can not be connected together, so I'll have to keep them isolated. Maybe I'll isolate the car body from the primary circuit too, put thick negative wire to all car's devices and be completely sure nothing will make the grounds touch. \$\endgroup\$ – miernik Mar 31 '11 at 12:19
  • \$\begingroup\$ So this means that your converter is not in fact isolated and what's more it has a current sensing resistor in the ground path (so ground potentials will vary a little with the charging output current). As far as isolating your car is concerned I would rather isolate the secondary circuit and leave the primary as is. \$\endgroup\$ – jpc Mar 31 '11 at 15:12
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The battery negative being attached the the body is for a common ground. If you add a second battery it would probably be best to add it's negative to the body as well. If not you could have issue with a floating ground. This can cause circuit noise and shocks/discharges.

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  • \$\begingroup\$ But the second battery will be connected to the main circuit using the "FMA PowerLab 8 V2" device, and normally it is not meant to be used with a common ground of input and output. I am wondering will it even operate correctly that way. \$\endgroup\$ – miernik Mar 30 '11 at 19:10
  • \$\begingroup\$ As long as the input and output sides are totally isolated from each other it shouldn't be an issue. But if your second circuit isn't grounded with the first there could be a voltage potential difference between the two ground circuits. \$\endgroup\$ – Matthew Whited Mar 30 '11 at 19:19
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    \$\begingroup\$ Hook the device up as the instructions say. The second battery ground should be connected to the car's ground though the negative on the power source side. \$\endgroup\$ – Matthew Whited Mar 30 '11 at 19:23
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    \$\begingroup\$ "But if your second circuit isn't grounded with the first there could be a voltage potential difference between the two ground circuits" There are voltage differences between different points in a single ground circuit. Metal is not a perfect conductor and any currents flowing through (cars have large, spikey currents) will create voltage differences from one point to another. Make sure this isn't going to cause problems if the two circuits share a ground path. Generally, you want the grounds of different systems connected together at a single point, not two. \$\endgroup\$ – endolith Mar 30 '11 at 19:49
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    \$\begingroup\$ @Matthew: "While having a second grounding connection should just reduce the overall resistance on the ground." How is this different from "multiple connections between the grounds"? \$\endgroup\$ – endolith Mar 30 '11 at 21:20

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