smart51
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| posted on 15/10/10 at 08:18 AM |
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Sleeving battery cables
The donor wiring loom for my single seater build has the battery at the back. I want to move it to the front for better weight distribution so
I've bought some battery cable. Now I want to run it under the floor for part of the run (not below the chassis rails so it's OK). Do I
need to sheath it in some way or is its insulation enough? One thing I've thought of is a long piece of heat shrink along the whole length.
What will the MSVA inspector want to see?
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02GF74
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| posted on 15/10/10 at 08:33 AM |
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battery cable that i've seen has pretty thick isulation - 2 mm or so .
providing the cable is fixed securely at correct intervals using insulated clips e.g. plastic P-clips away from any moving components then IMP that
should be fine.
Earth cable is less critcial (unless you have positive earth) since the chassis is at earth potential.
[Edited on 15/10/10 by 02GF74]
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matt_gsxr
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| posted on 15/10/10 at 08:45 AM |
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I'd go with something like this.
http://www.cbsonline.co.uk/split-sleeving-cable-protection-16mm-id-splt2-899-p.asp
I think my Audi has something similar. The insulation on battery cables is quite thick, but it is only PVC so can rub through. Lots of current and
fires if it does!
Earth doesn't matter at all, but that doesn't actually need to run front to back assuming your chassis is built from metal.
Matt
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02GF74
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| posted on 15/10/10 at 09:02 AM |
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quote:
Originally posted by matt_gsxr
Earth doesn't matter at all, but that doesn't actually need to run front to back assuming your chassis is built from metal.
I wouldn't recomment that - chassis will be steel with welds so will not be a good as nice chunky multi-stranded copper cable.
There may be high resistance spots, even a couple of ohms is too much when you have hundreds of amps flowing when turning the start over - resulting
in hot spots.
That is asking for a fire depending on what flammable item is adjacent e.g. petrol hose ... and there is no way of knowing where the high resistance
spots are without a lot of effort nor know if the will move as the chassis ages,flexes and rusts..
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britishtrident
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| posted on 15/10/10 at 10:14 AM |
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Normal practice is simply to earth to the chassis close to the battery.
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02GF74
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| posted on 15/10/10 at 10:53 AM |
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^^^ normal practise is to put the battery next to the engine, at least with the production cars I am familiar. I read the OP was putting the battery
at the other end to where the engine is.
It is a question of extra weight, cost but reliable starting versus breakdown and fire risk.
All IMO ofcourse. 
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dhutch
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| posted on 15/10/10 at 11:41 AM |
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quote:
Originally posted by 02GF74
^^^ normal practise is to put the battery next to the engine, at least with the production cars I am familiar. I read the OP was putting the battery
at the other end to where the engine is.
Thats it. That and the fact the on both the kitcar and the 306 although the -ve goes to the chassis from the battery, the -ve strap to the engine
attaches to the same stud.
Daniel
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coyoteboy
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| posted on 15/10/10 at 01:41 PM |
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It's be pretty simple to measure the resistance between the battery earth point and the engine earth point if no specific earth cable were used.
It's perfectly possible that any chassis that has been welded well has lower overall resistance than a relatively thin copper cable.
You're talking 4+ 1.6mm wall thickness tubes, running in parallel. That's like 60mm^2 per 1" tube, multiply that by 4. It would have
to be a seriously questionable weld to cause a problem, and if they were that questionable I'd be more worried about mechanical failure than
fire!
[Edited on 15/10/10 by coyoteboy]
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omega 24 v6
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| posted on 15/10/10 at 03:16 PM |
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If you are that worried then run it through a piece of garden hose.
If it looks wrong it probably is wrong.
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adithorp
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| posted on 15/10/10 at 03:47 PM |
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quote:
Originally posted by 02GF74
^^^ normal practise is to put the battery next to the engine, at least with the production cars I am familiar. I read the OP was putting the battery
at the other end to where the engine is.
It is a question of extra weight, cost but reliable starting versus breakdown and fire risk.
All IMO ofcourse.
So all those BMW's are doing it wrong... and the original Mini... and the Transits with it behind the drivers seat...and...
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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02GF74
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| posted on 15/10/10 at 06:55 PM |
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yes they are doing it wrong ... the only reason the battery is sited out of the engine bay is because there is no room for it, certainly is the case
for the Mini and I suspect transit.
Also was the case with the MGB with all manner of electrical problems, even worse as there were two 6 V batteries.
That is four models out of maybe thousands - who do you think got it right?
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adithorp
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| posted on 15/10/10 at 09:07 PM |
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This isn't about where the battery should be placed. It's about whether the chassis forms a good earth path. BMW and Ford think it does
and you don't.
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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omega 24 v6
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| posted on 15/10/10 at 10:10 PM |
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Chassis earth will be Just fine regardless of battery distance from starter etc. Work it out.
Main battery cables are 25mm csa
In most extreme cases 50mm csa
Area of any bolt/earth eye arrangement is around about 20mm dia so (10 X10)X 3.14 (area of a circle is pi r squared ) so 300mm csa.
even through 2 off 25x25x1.6 mm tubes you have an area of (25X1.6) X 4 X 2 = 320mm csa.
I cannot see why you think there is a problem TBH
If it looks wrong it probably is wrong.
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matt_gsxr
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| posted on 15/10/10 at 11:18 PM |
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FWIW my A6 has the biggest battery I have ever seen in a car (600Amp capacity) and has the battery in the boot for reasons of weight distribution.
One can deduce that the resistance between battery and engine is less than 0.02 Ohm (V=IR). Talk of 1 or 2 Ohm along a steel tube doesn't sound
realistic to me.
To get 1 Ohm of voltage drop along a steel tube of 1.6mm thick and 25mm along each side (i.e. 160 square mm, or 0.16*10-3 square meters)).
Assuming a resistivity of steel of 7.2 × 10-7Ohm m
You would need a 222m length of steel for 1Ohm; drop.
Matt
[Edited on 15/10/10 by matt_gsxr]
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Trev Borg
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| posted on 16/10/10 at 10:39 AM |
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As with any electrical circuit, the current going to the load from the live terminal will be equal to that returning through the earth path.
Any resistance in this path will act as a load itself and heat up like a heater element (not the best idea in the world, but you are only talking
about small load except cold cranking amps)
Simple ohms law
V =IR
P=IV
So, as long as the resistance of the chassis is good, then there are no problems at all.
My MGF has the battery in the front and the engine in the back. Buy saying that, there is a lot of steel making sure that the path is of low
resistance.
There is nothing wrong with running an earth lead to the engine area, and if I were building a light weight car with minimal chassis,I would do
this.
This does not mean that it is incorrect to use the chassis, just that you are eliminating any possible problems.
The Alfa 164 had the battery at the back and ran a huge live and earth to a set of terminals at the front. This is from a manufacturer that has
possibly the worst reputation for wiring in history.
All that said, I would run a live and earth wire to a terminal at the front, and I would use a split plastic conduit to do it.
6mm split conduit-convoluted tubing (5 metres) on eBay (end time 18-Oct-10 10:46:37 BST)
This is purely my opinion, the choice is yours.
Before you judge a man, walk a mile in his shoes.
By that time, who cares.
You're a mile away, and you've got his shoes
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02GF74
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| posted on 16/10/10 at 04:28 PM |
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25 mm tube, 1.6 mm wall, cross sectional area is 149 mm2
10 mm dia. copper cable, cross sectional area is 78 mm
Electrical conductivity of steel is 0.1 that of copper. (google it yourself )
So the steel tubing is eqivalent to 15 mm2, or 1/5 that of the copper cable (there is a reason why your mains cables and car wires are made from
copper).
That assumes the welding is perfect all the way from the stud in the chassis for the battery to the stud at the other end.
As to why BMW don't do that - ever heard of the bean counters? If they can save a tenner then that is a huge sum when several hundreds of
thousands cars are made. They could use an earth cable but can get away with it.
A BMW car is pressed steel monocoque that has the sheets pressed together and spot welded.
You are not comapreing like with like as a locost chassis is a load of steel tubes butt jointed and welded - my money is on the BMW having better
electrical condictivity from one end to the other than a DIY chassis.
Ron's chassis was designed to hold the suspension, engine and gearbox in place, it was not designed to have optimal electrical conductivity from
one end of the chassis to the other.
This is purely my opinion, the choice is yours.
[Edited on 16/10/10 by 02GF74]
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smart51
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| posted on 16/10/10 at 04:43 PM |
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quote:
Originally posted by 02GF74
Ron's chassis was designed to hold the suspension, engine and gearbox in place, it was not designed to have optimal electrical conductivity from
one end of the chassis to the other.
True, but I'm not building Ron's car. I was planning to run both black and red cables to the front but wanted to know if sleeving them
was a good idea. There's limited room so split convoluted tube is out really. I think I'll sleeve the +ve in heat shrink and leave the
earth as it is. The insulation is quite thick so it should be OK.
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omega 24 v6
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| posted on 16/10/10 at 11:02 PM |
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Bean counters are always a pain in the ar*e. but the debate is about does it work? and yes it does it is proven to work whether you believe it to be
right or not. As for the copper conductivity argument it is null and void given than gold is the best conductor IIRC. So BMW ( brakes might work lol)
are just a higher level of bean counters IMHO.
If it looks wrong it probably is wrong.
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iank
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| posted on 16/10/10 at 11:28 PM |
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The engine earth/starter motor earth on all cars I've seen is a steel braid about around an inch wide and 1/8th of an inch thick. Seems to work
just fine - losing the odd volt across the length of the car makes no practical difference as everything has wide tolerance's anyway. We
aren't dealing with precision electronics here. In the BMW/Mini case the conduction will be through a few dozen spot welds so total area will
be similar or worse than a properly welded spaceframe in all likelihood.
As an aside Silver is actually the best conductor (not including exotic superconductors) of both heat and electricity.
--
Never argue with an idiot. They drag you down to their level, then beat you with experience.
Anonymous
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