The advice on always grounding a connection

[Bru9]

Why is this always suggested over just going to the batteries negative terminal? say with spotlights & cable runs for fridges.

All parts of the vehicle exposed body have paint & are not bare metal, so there must be some significant resistance from the layer of paint. My bonnet pressure switch did not work while touching the metal frame, only in a pre existing hole where some of the paint was not present i think.


In my bonnet the main ground site looks to be attached to a worn area with no paint. or maybe an area that is rustless steel?

maybe I'm wrong about the paint? please correct me if so.

thanks

[jimsutt]

Why is this always suggested over just going to the batteries negative terminal? say with spotlights & cable runs for fridges.

All parts of the vehicle exposed body have paint & are not bare metal, so there must be some significant resistance from the layer of paint. My bonnet pressure switch did not work while touching the metal frame, only in a pre existing hole where some of the paint was not present i think.


In my bonnet the main ground site looks to be attached to a worn area with no paint. or maybe an area that is rustless steel?

maybe I'm wrong about the paint? please correct me if so.

thanks

It's better to go back to battery negative than try an connect to vehicle body... If you check out the existing wiring you will see negatives run to all lights etc.

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[NFT5]

As you've noticed, manufacturers ground to body, rather than running everything back to the battery negative in individual, separate circuits. Reasons for this include cost (less cabling) and reduced weight.

More importantly though, the body represents a much lower resistance than individual returns, so lighter cabling can be used on the positive side to still get minimal voltage drop in any circuit. Less cabling also reduces the likelihood of failures due to cable damage. Done properly the possibility of creating ground loops is also reduced.

What is essential, though, is the quality of the connection. Grounding to body won't work if you just drill a hole in a light gauge panel and pop a bolt and nut through it. If you look at the factory grounding points you'll see that they usually use a welded nut on the back of lighter panels. Since this is attached before painting there is a good connection to the panel and the electricity can flow through the bolt and the threaded section where there is no paint. The use of toothed washers can also improve the connection, biting through the painted surface to the metal underneath.

In later model vehicles with "smart" alternators grounding to body also enables the current flow to be monitored (through a single cable connection to the battery negative) so that alternator output can be adjusted to compensate for the load.

[Dicko1]

As you've noticed, manufacturers ground to body, rather than running everything back to the battery negative in individual, separate circuits. Reasons for this include cost (less cabling) and reduced weight.

More importantly though, the body represents a much lower resistance than individual returns, so lighter cabling can be used on the positive side to still get minimal voltage drop in any circuit. Less cabling also reduces the likelihood of failures due to cable damage. Done properly the possibility of creating ground loops is also reduced.

What is essential, though, is the quality of the connection. Grounding to body won't work if you just drill a hole in a light gauge panel and pop a bolt and nut through it. If you look at the factory grounding points you'll see that they usually use a welded nut on the back of lighter panels. Since this is attached before painting there is a good connection to the panel and the electricity can flow through the bolt and the threaded section where there is no paint. The use of toothed washers can also improve the connection, biting through the painted surface to the metal underneath.

In later model vehicles with "smart" alternators grounding to body also enables the current flow to be monitored (through a single cable connection to the battery negative) so that alternator output can be adjusted to compensate for the load.

Totally correct. I have found that nearly all accessories I have purchased and requiring wiring do state that scrape off any paint and dirt from a surface before bolting/screwing earth fixture to body work. Same as a battery terminal...clean and shiny is required for an excellent join between terminal and connector. Dirty connections lead to high resistance connections that will not allow accessory to function correctly.

[traktor]

I found these articles useful

http://www.w8ji.com/negative_lead_to_battery.htm

http://www.w8ji.com/battery_wiring.htm

It explains why you should use ground connections instead of the negative post.
Also one can find description and photos of proper ground connections.
Here is an extract from the article:

Sharing the negative battery lead to engine bolt with anything else or connecting directly to the battery negative post with anything except the block and chassis grounds is a terrible idea. (Connecting electrical devices or hardware directly to a battery negative post is a bad idea (no matter who tells you to do it) unless the negative connection is 100% ground isolated at the electrical device.) When an electrical device is directly connected to the negative post, if the negative post to block or chassis connection opens up or develops excessive resistance, the battery negative post will divert alternator or starter current through whatever is attached to the negative post. This can be hundreds of amperes! Very few devices and wiring will suffer a fault like this without permanent damage. It is also a fire risk.

[nj swb]

Sharing the negative battery lead to engine bolt with anything else or connecting directly to the battery negative post with anything except the block and chassis grounds is a terrible idea. (Connecting electrical devices or hardware directly to a battery negative post is a bad idea (no matter who tells you to do it) unless the negative connection is 100% ground isolated at the electrical device.) When an electrical device is directly connected to the negative post, if the negative post to block or chassis connection opens up or develops excessive resistance, the battery negative post will divert alternator or starter current through whatever is attached to the negative post. This can be hundreds of amperes! Very few devices and wiring will suffer a fault like this without permanent damage. It is also a fire risk.

Balderdash.

[traktor]

Balderdash.

Perhaps, you need to read the full article ...

[nj swb]

Perhaps, you need to read the full article ...

OK - I take it back.

He's worried about making a connection at the battery post, and again elsewhere in the vehicle, creating a parallel path through the ground wire. I read that he was worried about the hundreds of amps flowing through the equipment itself.

I apologise.

[traktor]

The auther builds race cars, i'd imagine his requirements are tougher then for 'normal' cars due to safety concerns. I learned a few tricks from him.

Particularity I use dialectic grease for all electrical connections, specifically when crimping.

http://www.w8ji.com/dielectric_grease_vs_conductive_grease.htm

http://www.w8ji.com/images/Cars/Battery/09980.pdf

[Bru9]

Thanks all.

Forgot to add: what about rust between the connector & body? Since the site is not stainless. So if you scuff the paint back wont rust inevitable develop between the connection?

[06PAJ]

You will always get corrosion with dissimilar metals
Use the grease for connections to avoid corrosion for a little while

[Turorit]

I found these articles useful

http://www.w8ji.com/negative_lead_to_battery.htm

http://www.w8ji.com/battery_wiring.htm

It explains why you should use ground connections instead of the negative post.
Also one can find description and photos of proper ground connections.

Thanks for the links, very interesting reading.

How has everyone wired their winches? Mine included a negative lead long enough to reach the battery negative post, which is what I've done.
I feel like I should cut this to a shorter 40cm lead and go straight from the winch to the chassis rails/similar?

Similarly I've run 4m of double-sheath twin 6B&S to the rear for my fridge & compressor. Negative lead straight to the battery post...

Fire hazard(s) waiting to happen?

[Dicko1]

Thanks all.

Forgot to add: what about rust between the connector & body? Since the site is not stainless. So if you scuff the paint back wont rust inevitable develop between the connection?

Cover the connection in enamel paint, plastic paint, correct type grease or quality battery terminal spray...simples.

[nj swb]

Fire hazard(s) waiting to happen?

The fire hazard comes if there is a ground connection at each end i.e. Connection to battery terminal at one end and to chassis at the other end.

If this happens, and the primary connection from battery to chassis goes high resistance/open circuit, your fridge ground wire can become the return path for your starter motor and/or your alternator.

Under these conditions, fire wouldn't be an automatic consequence, but it couldn't be ruled out either.

[Pajshomoneroguntero]

I’ve run the winch return to the bigass chassis ground just below the battery. Seems to be the primary chassis ground. With the newer smart alternators I wanted the earth to pass the current sensor just before the -ve post. I’ve wired the earth from the second battery in the rear tub to one of the M14(might be M12) bolts holding the steel rear seat carrier to maximise contact area, ie there is a path into the chassis via another two bolts.