Battery to Chassis earth

Non-engine, eg: aerodynamics, gearboxes, brakes, suspension
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cantfindausername
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Battery to Chassis earth

Post by cantfindausername »

Hi all,

I was talking with someone at the weekend and they commented that the Battery to Chassis strap should be as short as possible, and ideally less than 8 inches.

This is the first time I've heard anything concerning this, so thought I'd come and ask the people who are most likely to know for certain.

Is there truth in what he was saying?

Thanks,
Ant
Guy Croft
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Re: Battery to Chassis earth

Post by Guy Croft »

In terms of voltage drop along it that is not true if the cable is the right thing (ie: copper), you could make it 3ft long and see none.
As far as the influence of the current on sensors I have no idea, sorry!

GC
cantfindausername
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Re: Battery to Chassis earth

Post by cantfindausername »

Thanks Guy. Think I need to investigate this further. Its all to do with my keyless ignition project as one chap said to put the battery master kill switch on the battery/chassis connection. Which is I do, is going to make it quite a bit longer.

Will update the post with my findings (if any).

Ant
djaychela
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Re: Battery to Chassis earth

Post by djaychela »

Ant

I would definitely ignore the direction to put the cut-off switch in the battery/earth connection. I've never seen a rally car wired up like that (and I've seen a few), and it makes no sense to do it that way - the idea of putting it in the positive means that usually the only run that will be significantly altered will be the main live feed to the starter/solenoid.

As for the earth strap, as GC says, if it's capable of handling the correct current then the length is immaterial. If it's of a higher resistance than it should be then the length would be an issue. I've found that using welding cable from a local supplier means affordable, flexible and well-insulated cable is available at a decent price, and you can get crimp terminals to fit it for pennies at an electrical distributor - a bit of ingenuity with a vice and the right metal formers and you can crimp them perfectly, too.

Darren
cantfindausername
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Re: Battery to Chassis earth

Post by cantfindausername »

Hi Darren, I'm definately going to be doing this on the live feed now.

I had done some searches on the internet for information about putting the cut off switch on the negative, and nothing had come up. I also noticed that all the installation instructions I'd seen had it wired up to the live, so figured thats the "right" way.

Ant
RedLexus
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Re: Battery to Chassis earth

Post by RedLexus »

I disagree... any rally car I've worked on , and that's a lot.. have had the battery negative broken by the the "kill switch" . And if it's voltage drop you worry about... it's a non-issue if good quality , proper guage cable is used . Think of all the MK2 Escort Rally Cars etc with the battery in the boot? Voltage drop in the positive from the battery to the starter doesn't appear to be an issue there , does it? And to me it makes more logical sense to put it on the negative line . I've seen cars with it on the positive stay running , on the alternator output , when the switch was turned off! also , heaven forbid , if the cable from the battery to the switch chafes on any metal , it'll cause a huge short , possibly a fire , not the case on a switched neutral . I know this should not happen , but in a rally car , in acrash... things can get messy .
Ford Sigma KA. GC_43
Guy Croft
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Re: Battery to Chassis earth

Post by Guy Croft »

The kill switch with alternator protection in my exp usually goes on the positive line. Here is an nice little 'how-to' article worth copying:

http://7faq.com/owbase/ow.asp?HowDoI%2F ... sterSwitch

What happens if you put it on the earth cabling I have no idea!

GC

edited by GC after reading the previous post more carefully!
djaychela
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Re: Battery to Chassis earth

Post by djaychela »

RedLexus wrote:I disagree... any rally car I've worked on , and that's a lot.. have had the battery negative broken by the the "kill switch" . And if it's voltage drop you worry about... it's a non-issue if good quality , proper guage cable is used . Think of all the MK2 Escort Rally Cars etc with the battery in the boot? Voltage drop in the positive from the battery to the starter doesn't appear to be an issue there , does it? And to me it makes more logical sense to put it on the negative line . I've seen cars with it on the positive stay running , on the alternator output , when the switch was turned off! also , heaven forbid , if the cable from the battery to the switch chafes on any metal , it'll cause a huge short , possibly a fire , not the case on a switched neutral . I know this should not happen , but in a rally car , in acrash... things can get messy .
If the car kept running because the engine electrics were being fed by the alternator then it should have failed scrutineering, and more importantly, the kill switch was wired in wrongly. The car's ENTIRE electrical system should be isolated with that switch, so the alternator should also be isolated from the car's electrics when the switch is used. There is provision for the alternator's output to be sunk to earth with a resistor so that it doesn't get damaged when the kill switch is used on a running engine. The FIA approval for the switch includes the installation, and that is specified as being in the positive connection in the wiring loom. It's quite possible for engines to be earthed by things such as throttle and choke cables (although they really object to it), so isolating the negative isn't such a clean-cut idea. If the positive is done correctly, then it's foolproof. The cable runs shouldn't be considerably longer than the standard runs anyway (and definitely when the battery is in the boot), so I think the "chafed cable" point is moot (and indeed the cable should be adequately mounted using something like insulated P-clips anyway for a decent build), and you can't account for every possible event that may occur in a crash.

As already mentioned, volt drop should never be an issue when adequately-rated cable is used.
Darren
RedLexus
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Re: Battery to Chassis earth

Post by RedLexus »

Thats fine Darren , thanks for giving a comprehensive answer . I've no problems admitting I'm wrong , when it's explained why!
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Guy Croft
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Re: Battery to Chassis earth

Post by Guy Croft »

MODEL POST!

Darren, well done.

Authoritative, lucid, accurate. Thanks.

As RedLexus indicates, we're all here to learn, me included!

GC
cantfindausername
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Re: Battery to Chassis earth

Post by cantfindausername »

More great information. Interesting you say about the throttle cable earthing, there is a "click of death" that ocurs with my make of car (Toyota MR2 mk2) and it normally happens when the main engine/chassis straps are left off. People have had glowing and melting throttle cables! Not much fun!
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