Chasis or Battery for Earth?
Submitted: Tuesday, Jul 11, 2006 at 15:11
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35732
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Bubbles
Hi,
Im just starting to install a fewe accesories into the car. I want to put a heavy duty, high'ish load (6-12amps actual draw) plug behind the centre console. Is it OK to use the chasis as earth or do I need to run both +ve and -ve from the battery to the plug?
Reply By: Mike Harding - Tuesday, Jul 11, 2006 at 15:14
Tuesday, Jul 11, 2006 at 15:14
It is better to go directly to the battery terminals - but _providing_ you get a good metal to metal (not paint) contact with the chassis it should be OK. 12A is not a particularly high current.
Mike Harding
AnswerID:
182943
Follow Up By: Member - Mike DID - Tuesday, Jul 11, 2006 at 16:31
Tuesday, Jul 11, 2006 at 16:31
If you are connecting to a bolt or screw that penetrates the outer skin, it is really important to seal the thread with paint or Lanolin Grease to prevent corrosion of the connection as soon as it gets wet.
Otherwise it will work for a few months and then start to go high resistance.
Usually you will find you can use an existing mounting bolt and put a lug under it - making sure you remove any paint.
Mike
FollowupID:
439433
Follow Up By: Mike Harding - Tuesday, Jul 11, 2006 at 17:24
Tuesday, Jul 11, 2006 at 17:24
You know... I was going to say that... then forgot... old age catching up :)
Good point Mike, otherwise the moisture in the air will encourage an electrolytic reaction at
the junction and cause corrosion. Any particular reason you suggest a lanolin grease Mike, I would have thought any grease would do? iirc I usually use silicon heatsink grease.
Mike Harding
FollowupID:
439452
Reply By: Michael Carey - Tuesday, Jul 11, 2006 at 15:53
Tuesday, Jul 11, 2006 at 15:53
It's a good idea to get your 12V directly from the battery (properly fused of course), but never, Never, NEVER connect the negative wire directly to the battery negative post. Connect it to an earth point close to where the battery's earth connects to the chassis.
In the off (but possible!) chance that the main negative wire from the battery to the chassis breaks or becomes disconnected (and the negative wire going to the engine block is a possible suspect as
well, it usually fails due to constant vibration)..... it's quite possible that the entire vehicles negative reference might be though that cable you have installed and then through any equipment you install.
If it's the only intact wire still connected to the negative of the battery and you have a UHF (for example) which is earthed to the chassis though the antenna cable, then everything in the vehicle will connected to the negative THROUGH the antenna coax, UHF and new power wire. How do you think it would handle a starter motors current? Remember that most people DON'T fuse the negative wire.....
It might never happen, but better safe than sorry!!
AnswerID:
182950
Follow Up By: Mike Harding - Tuesday, Jul 11, 2006 at 17:18
Tuesday, Jul 11, 2006 at 17:18
It's a good point you raise Michael and quite valid. It's also the reason why manufacturers of quality radio transceivers put a fuse in both the negative and positive supply leads. However I must confess my vehicle has the wiring connected directly to the bolt on the battery negative terminal but it would be better going to the chassis point where the main lead from the battery is attached.
Mike Harding
FollowupID:
439451
Follow Up By: Hairy - Tuesday, Jul 11, 2006 at 17:24
Tuesday, Jul 11, 2006 at 17:24
Mmmm.....
I think I might check my earth lead!!!
Good point!
FollowupID:
439453
Follow Up By: Member - Mike DID - Tuesday, Jul 11, 2006 at 17:35
Tuesday, Jul 11, 2006 at 17:35
Mike
Lanolin Grease is one of the most
water-resistant greases around - sheep test it 24hrs a day, 365 days a year (no, I am not from New Zealand. No, I have never lived on or worked on a farm. Yes, I am happily married)
Mike
FollowupID:
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Follow Up By: Mike Harding - Tuesday, Jul 11, 2006 at 17:38
Tuesday, Jul 11, 2006 at 17:38
You... errrr... seem to feel a strong need to justify yourself there Mike? :)
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Follow Up By: Member - Mike DID - Tuesday, Jul 11, 2006 at 17:40
Tuesday, Jul 11, 2006 at 17:40
I know how the minds of certain people around here work !
Mike
FollowupID:
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Follow Up By: Gramps (NSW) - Tuesday, Jul 11, 2006 at 17:45
Tuesday, Jul 11, 2006 at 17:45
hahahahahaha Thanks to all the Mike's for the info.
FollowupID:
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Follow Up By: Mad Dog - Vic - Tuesday, Jul 11, 2006 at 17:57
Tuesday, Jul 11, 2006 at 17:57
I agree with Mike and I did come from NZ and have worked on farms with sheep.
FollowupID:
439465
Follow Up By: Member - Mike DID - Tuesday, Jul 11, 2006 at 19:45
Tuesday, Jul 11, 2006 at 19:45
. . . . two hours later and still no sheep jokes ????
Is there something special on TV tonight ????
Mike
FollowupID:
439493
Follow Up By: Gramps (NSW) - Tuesday, Jul 11, 2006 at 19:48
Tuesday, Jul 11, 2006 at 19:48
Mike DID,
Ssshhhhhh ...... I'm trying to be good :))))))))
FollowupID:
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Follow Up By: Mike Harding - Tuesday, Jul 11, 2006 at 19:50
Tuesday, Jul 11, 2006 at 19:50
I suspect everyone is just feeling a little sheepish about it Mike :)
FollowupID:
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Follow Up By: Alloy c/t - Tuesday, Jul 11, 2006 at 19:58
Tuesday, Jul 11, 2006 at 19:58
So Mike + Mike ,Mad dog +and Gramps none of you will admit to owning gumboots ??
FollowupID:
439501
Follow Up By: Member - Mike DID - Tuesday, Jul 11, 2006 at 20:00
Tuesday, Jul 11, 2006 at 20:00
Yes, but Forensic testing will show no traces of wool inside them.
Mike
FollowupID:
439502
Follow Up By: Gramps (NSW) - Tuesday, Jul 11, 2006 at 20:01
Tuesday, Jul 11, 2006 at 20:01
or outside them for that matter :))))))
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Follow Up By: Mike Harding - Tuesday, Jul 11, 2006 at 20:12
Tuesday, Jul 11, 2006 at 20:12
Actually, I have some I brought with me from the UK which have wool linings!
FollowupID:
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Follow Up By: Member - Davoe (Nullagine) - Tuesday, Jul 11, 2006 at 20:18
Tuesday, Jul 11, 2006 at 20:18
Unless i am Mistaken the instructions for installing my Barrett recomended direct contact with the negative terminal
FollowupID:
439507
Follow Up By: Mike Harding - Wednesday, Jul 12, 2006 at 08:01
Wednesday, Jul 12, 2006 at 08:01
If the _both_ supply leads are fused at the radio it is OK to connect directly to the battery terminal. Although, personally, I'd still go to the first connection point off the battery and avoid the possibility of huge circulating currents around the radio body/electronics in the event of a bad battery connection.
Mike Harding
FollowupID:
439588
Reply By: Mr Fawlty - Tuesday, Jul 11, 2006 at 17:56
Tuesday, Jul 11, 2006 at 17:56
I have only one thing to say here, & that is unusual. I found the best way to protect the aditional circuitry that I installed for electric brakes,
fridge, aux van power etc was to use automatic circuit breakers. They cost about $15 each and come in a variety of amperages & are safe for kiddies & save you having to carry spare fuses.
Oh ok I will say that for reasons of personal pedanticism I ran both the 12vdc & 0vdc wires back to the battery and connected them on a busbar thing I got from supercheap. I had a Cabling Licence at one time & I just thought it looked neater....
AnswerID:
182971
Follow Up By: Mike Harding - Tuesday, Jul 11, 2006 at 18:00
Tuesday, Jul 11, 2006 at 18:00
>I have only one thing to say here, & that is unusual.
Good. Well that's enough from you then! :)
FollowupID:
439466
Reply By: stevesub - Tuesday, Jul 11, 2006 at 23:44
Tuesday, Jul 11, 2006 at 23:44
Back to the chassis real close to the battery earth cable otherwise you can get stray currents around the chassis that can cause electrolysis in modern radiators. Had a mate who had a radiator go on a Landrover through electrolysis and he determined it was his incorrectly wired earth on the spotlights.
On the old cars, it made no difference, espcially for low current stuff but high current spotlights could cause problems.
Stevesub
AnswerID:
183079