negative-earth on body v battery

[thehanko]

Im refitting my air compressor into my new truck.

In the diagram it shows that the heavy negative cable needs to run direct to the neg terminal on the battery.

How is that any different to simply earthing out to a solid point on the chassis or body?

Basically the cable wont reach my battery and i want to earth it to another point, neater cabling and less contacts to the battery terminals.

Is there any difference?

[chimpboy]

No, not if you have suitably beefy connections.

[zuffen]

By running the earth direct to the battery you eliminate any risk of a dodgy earth between the compressor and body or the body and the battery.

If you careful and make sure there is no paint or corrossion where the cables mount to the body for both the compressor and battery it won't make any difference.

[thehanko]

excellent to know.

Thanks
Ben

[festy]

How is that any different to simply earthing out to a solid point on the chassis or body?

Don't just assume that your chassis is earthed - a common setup is battery earth to the body and engine only.
Some chassis appear to be earthed, but have a moderate resistance that causes problems when you draw a couple of amps. The chassis might only be getting earthed via the steering box or something.

Providing your body earth connections are good, you're better off earthing the compressor to the body rather than running a wire back to the battery. The body should have a lower voltage drop over the distance than your average wire.
Also, it eliminates the chance of your compressor becoming the earth path for your starter motor if the earth strap fails. It also prevents ground loops which can cause problems.

[MrGrim]

simple lesson in dc voltage/current

current (amps) flows from neg to pos ...hence earth is more important than pos

voltage flows from pos to neg ...

you can have a small voltage drop with out effect ..
but a small drop in current will cause you problems ..over heating and resistance in wires fuses and also the unit
current drop in most cases is the reason for failure also a spike or loss of voltage due to resistance relavant to the loss of earth

[matto]

your right but also wrong

electron current flow is from - to +
conventional current flow is from + to -

the current or amps is determined by the voltage source and the resistance on that circuit (ohms law) if the voltage and resistance are stable then current will be stable too. more resistance will lower your current flow.

just make sure all electrical terminations are micky mouse and you wont have a drama

[festy]

wtf?

Current (electrons) flows from neg to pos, voltage doesn't flow.
Also both halves of the circuit are equally important, as together they form a "circuit" - A loss on either side is exactly the same.

[r0ck_m0nkey]

Also both halves of the circuit are equally important, as together they form a "circuit" - A loss on either side is exactly the same.

On that, depending on what the earth lead is like between the battery and the body, it may need to be upgraded in size. It's size was only ever determined by what was needed to run the vehicle and not any accessories you are going to attach to the vehicle.

[chimpboy]

simple lesson in dc voltage/current

current (amps) flows from neg to pos ...hence earth is more important than pos

voltage flows from pos to neg ...

you can have a small voltage drop with out effect ..
but a small drop in current will cause you problems ..over heating and resistance in wires fuses and also the unit
current drop in most cases is the reason for failure also a spike or loss of voltage due to resistance relavant to the loss of earth

omg is it still April 1st where you live?

[-Scott-]

On that, depending on what the earth lead is like between the battery and the body, it may need to be upgraded in size. It's size was only ever determined by what was needed to run the vehicle and not any accessories you are going to attach to the vehicle.

Depends on precisely how the connection is made.

If it's designed to carry start current (as some of mine are) then it will be fine to run a bucketload of accessories - which would not normally be running while cranking.

[3 cyl]

Volts = pressure Amps =flow Ohms (resistance)=the size hole you're trying to push the electricity through. So if the compressor runs at 25 amps and say a dodgy plug drops 2v across it then the dodgy plug heats up at 50watts (25A x 2V). If the dodgy connection was on your starter motor power lead and dropped 2V then the connection heats up at say 200A x 2V so 400watts. Make sense? V=IxR.

[Shadow]

pressure amps

flow ohms

W

T

F

[-Scott-]

pressure amps

flow ohms

W

T

F

Tell me again why punctuation doesn't matter?

Volts = pressure.
Amps = flow.
Ohms (resistance) = the size hole you're trying to push the electricity through.

So if the compressor runs at 25 amps and say a dodgy plug drops 2v across it then the dodgy plug heats up at 50watts (25A x 2V).

If the dodgy connection was on your starter motor power lead and dropped 2V then the connection heats up at say 200A x 2V so 400watts.

Make sense? V=IxR.