Moving the Battery to the rear.

[ZR1cliff]

Right i want to to the above to around a distance of 6ft at a guess of wire away from its present position,
1/do i need to make the wires thicker?,i heard this some where.
2/where can i get a kit to do this?

Thanks.

[Mikey G]

Yes you need to make the positive wire thicker, the Earth will still be a short run so no need to go dramatically thicker on that.
No kits available, it will depend on what car it is for. You will need to make sure all runs through bodywork are well protected to avoid shorting out. Also consider extra wiring needed for starter motor and charging circuit.

Most battery cables will go straight to the Starter and then branch out to the main vehicle circuits from there, other cars are a little more complicated with more than one wire coming direct from the battery to power the circuits. You would need to consider re routing these direct to the starter solenoid where the battery cable will connect to.

And dont forget before making alterations to your wiring, the biggest safety precaution you can take is to first.....
.
.
.
.
disconnect the battery

[denisb]

If you aren't using a dry cell battery of some form, make VERY sure it can't vent into the cockpit.

[ZR1cliff]

Thanks very much guys,thats good advice.

[Mikey G]

denisb said:

If you aren't using a dry cell battery of some form, make VERY sure it can't vent into the cockpit.

Dam good point, we had a problem with the alternator in our rallycar once which resulted in silly voltage going into the battery and cooking it, this also resulted in a very light headed and ill feeling driver and passenger.

[Mikey G]

After re reading my first post, just to clarify, the charging circuit does not need to go all the way back to the battery but only to the nearest point where the battery cable connects to the starter motor solenoid and other circuits.

[stevieturbo]

I used 35mm welding cable when I moved mine to the boot. Nice and flexible, and plenty big.

[Incorrigible]

I don't see why the wire needs to be thicker, it's not carrying any more current

[Nick_F]

It needs - under some circumstances - to be thicker because it's longer, so it's total resistance is higher unless it's made thicker to compensate.

I used arc welder cable on the Scimitar - works a treat. Bear in mind how important it is that the battery stays put in an impact when you decide how to clamp it in.

[eliot]

Should always make it as thick as possible/feasable. You will get a volt drop the longer it is.

I used the thickest stuff from here:
www.vehicle-wiring-products.co.uk/VWPweb2000/cables/cables.html#gen10

I think they sell battery boxes too.

[annodomini2]

Incorrigible said:

I don't see why the wire needs to be thicker, it's not carrying any more current

Electrical Resistance is proportional to Cross Sectional Area and length, if you increase the length you increase the resistance, unless you proportionally increase the cross sectional area.

If there is more resistance then the wire will get hotter and stands a greater chance of failing.

[Incorrigible]

I understand the physics of it, but carrying the same current along a longer length means the same voltage drop/m = the same temperature rise as the shorter wire shirley

OK so the voltage at the starter may now be 11.4V instead of 11.7V

Or is the drop much greater (genuine question)

That's why cables are rated by current, not a combination of current and length, unless I'm missing something obvious

[chassis 33]

Standard basic electrical theory that you're taught at school etc, assumes that wires in circuits have no resistance, hence there is no assumed voltage drop.

This however is not the case. If the wire is long enough or thin enough then the wire becomes a significant resistor in the circuit and will take energy/power out of the circuit, power consumed equals current x voltage drop. tThis power has to be dissipated and the most common energy release is heat, think of the electric bar heaters of old.

Wiring is rated by current for a particular application, eg in an automotive environment it is assumed that a nominal 12vDC will pass along the wire hence a given current rating. Likewise it is assumed that 3 core mains flex has a nominal 230vAC passing along it and is rated accordingly.

Also see http://electronics.howstuffworks.com/wfc1.htm

Regards
Iain

[ZR1cliff]

eliot said:

Should always make it as thick as possible/feasable. You will get a volt drop the longer it is. I used the thickest stuff from here: www.vehicle-wiring-products.co.uk/VWPweb2000/cables/cables.html#gen10 I think they sell battery boxes too.

Looks like a good supplier ,ive also found these new optima batteries,what do you guys think?,will it still need venting.

www.optimabattery.co.uk/english_optima/red_top_optima.htm

[dilbert]

Incorrigible said:

I understand the physics of it, but carrying the same current along a longer length means the same voltage drop/m = the same temperature rise as the shorter wire shirley OK so the voltage at the starter may now be 11.4V instead of 11.7V Or is the drop much greater (genuine question) That's why cables are rated by current, not a combination of current and length, unless I'm missing something obvious

I think it's to do with the current. A factor left out of the previous post. If your starter draws 80A, then a 5V drop would need an overall cable resistance of 0.06 Ohms.

If the cable is ten times longer than it was, then it's resistance is going to be ten times more.

If it was 1.82 milli ohm/meter and a meter long (10mm2), and you make it five meters long its going to be 9.1 milliohms.

If you use like for like cables the power dissipated by the cable is directly proportional to length that it is.

At 80 amps most things consume some ammount of power anyway.

Most manufacturers will specify a current rating, on the basis of an arbitary cable length. Question is, how long is a piece of cable?

[Incorrigible]

dilbert said:

Most manufacturers will specify a current rating, on the basis of an arbitary cable length. Question is, how long is a piece of cable?

Twice as long as the distance from the middle to one of the ends

So does anyone know the resistance/m of standard starter cable wire (or the voltage drop at the starter) so I can either admit I'm wrong and sulk or gloat triumphantly

>> BTW I'm not sure if you misread my post, I was saying the voltage drop therfore power dissipation in the cable would be the same PER METRE on any length of cable

So you'd only need a thicker cable if you had significant voltage drop

My car, for instance has had an extra 3 metres or so of mains cable put in it, as I have a battery isolator, standard cable used. Maybe I should just get the multimeter out

>> Edited by Incorrigible on Thursday 29th September 11:43

[denisb]

I think the Ultima boys now use Optima batteries. Might be worth getting their opinions.

[dilbert]

Incorrigible said:

dilbert said: Most manufacturers will specify a current rating, on the basis of an arbitary cable length. Question is, how long is a piece of cable? Twice as long as the distance from the middle to one of the ends So does anyone know the resistance/m of standard starter cable wire (or the voltage drop at the starter) so I can either admit I'm wrong and sulk or gloat triumphantly >> BTW I'm not sure if you misread my post, I was saying the voltage drop therfore power dissipation in the cable would be the same PER METRE on any length of cable So you'd only need a thicker cable if you had significant voltage drop My car, for instance has had an extra 3 metres or so of mains cable put in it, as I have a battery isolator, standard cable used. Maybe I should just get the multimeter out >> Edited by Incorrigible on Thursday 29th September 11:43

Sorry for not replying.

I'm not sure there is actually such a thing as standard starter cable.

If you can tell me the CSA or number and size of conductor strands, I can tell you the resistance per meter. Once you know that and the current carried, the temperature rise can be established.

Usually the rating for the cable is set on the basis of, but not usually at, the temperature that the insulating sheath can withstand.

This is why they use horrible kapton insulated cables in aeroplanes. They can put one hell of a current into a small cable, and tolerate the temperature rise that results, because the insulator can tolerate the temperature rise. Smaller wires are lighter.

The down side is that as an insulator it's brittle and will catch fire in quite a reactive way, when it finaly succumbs.

Now you know this, you know where the "cabin heat" comes from!

[ZR1cliff]

denisb said:

I think the Ultima boys now use Optima batteries. Might be worth getting their opinions.