You'll need to use the same fatness cable to connect your battery to the earth in the boot - the same current flows in both.

giggady..

Neil, i may call you as i have a wiring idea!!

Neil sorry for my ignorance, but you mean that the earth cable that you need to use is the same one?

Wherever the battery is in the car, all the current that flows *out* of it also has to flow back *into* it - conventionally we think of current as flowing from the positive to the negative terminal.

The significant current draw is during starting; this can be several hundred amps. If the cable is not rated for that current, it will get hot and potentially melt its sheath; that leads to a short circuit to a convenient earth (usually the chassis) and that leads to massive currents, massive heat, and fire - because the lead acid cell has a very low internal resistance.

So, the positive wire has to be fat enough to pass the necessary current without heating. The negative wire returns usually through the chassis, but has to connect to it somewhere - that wire has to carry the same current as the positive, so it should be as thick...



So, yes...

btw - the alternator can supply well over 100A on aircon models, so after a few seconds of very high current starting, there could be several minutes of high current going the other way to charge the battery. However, most high currents on the positive side - alternator to aircon, fan, and lights, circulate to the main fuse box which remains at the front of the car. It's really only starting and charging currents which are of concern here.

(A secondary effect is the voltage drop across the resistance of the wire. You want that as low as possible, hence as thick a wire as possible. On my kit I have to run both the positive and negative from the back to the front since I have no chassis to carry the ground; the fiat wiring shows many earths but they all expect to pick up an earth from the chassis - that is, they're providing an earth path *to* the chassis from the various lights and whathaveyous. The chassis is clamped very close to the battery on the coupe loom and can be considered to be a very low impedance path back to the battery.)

so then you actually don't recomend connecting battery negative to the boot? as you say "you have to run both the positive and negative from the back to the front since I have no chassis to carry the ground".
would this problem be soolved by getting a larger seccion cable like insted of 25mm a 35mm ?

I have only the pos.cable to the front, absolutely no problems at all. Saves weight too.

Nah - on the coupe, there's all that metal chassis convenient to hand - a big bolt (m6 at least) through a proper tag and with a star washer to cut through the paintwork. You'll be fine then and you only need a foot of 25mm cable to connect it.

I have to take the negative forward because my kit has a non-conductive load-bearing fibreglass bodyshell, no chassis.

cheer Per.
Forgot about that Neil.

Haven't had time to finish this mod as im needing also tu buy the fuse box. As I see, I should go for the 250amp fuse, but can i fit a 200amp fuse or is it very little? I have one in my garage and its to not buy another one.

Cheers

See if you can get the wire from a gtv breaker as they have it back there as standard and possibly workout cheaper then buying new and having to get new terminals etc etc.

im going to be doing the same to my coupe but i think the most advantage is the mount of space it frees up in the engine bay which im after more than anything

Be aware that if you move the battery to the rear of the car, you will get a larger voltage drop due to the internal resistance of the copper cable being over a longer run. In other words, copper cable will drop x volts per metre due to its internal resistance. The longer the cable the greater the voltage drop. I would suggest a thicker cable be used for your new cable run to the battery in the rear than you currently have in the front.
Have a look at this site www.stealth316.com/2-wire-resistance.htm to see what your V drop is for a given length and diameter cable.
All this means is that you will get the amps you need to turn your starter motor properly and the fatter cable will reduce the build up of heat in the cable and hence less risk of a melted sheath or worse still ... fire.

I am in the process of doing the exact same thing in my Stag

Cheers

you dont have to make calculations you put a cable from an alfa 164 that has the battery in the boot no problems or calculations its been done in alfas from factory

I tried starting my coupe with a battery in the boot running through an amp cable, wouldn't even kick, with a new battery. So leave the amp cables alone in my opinion

Forget amplifier power cables. 25sqmm works fine with five metres of cable on my kit with the 20vt engine.

The point I was making was that you will need larger diameter cable to the battery in the back than what you may already have to the battery in the front due to higher resistance in the copper cables.
What is an amplifier cable going to do unless it is a fatter cable?

Quite. You also need to consider the power dissipation in the cable, which is proportional to the resistance and the current flowing. An amplifier cable *looks* fat in comparison to other wires in the car, but is probably rated at no more than 40A peak, probably half that continuous rating, and the makers have been known to make them *look* bigger with thicker insulation...

This would also make it easy to run a seperate power to the fuel pump, like 2 birds with one stone this one! Well maybe 3 or 4 actualy..

Weight distribution
Engine bay access
Fuel pump mod
.......

Yepp!

just going through this thread i have done what is wanted here after lots of testing ... i use a red top battery which is a gel and sealed so it can be put in any position i know its not on a coupe that i have fitted the battery to but i know it works with no problems
1 get the thickest battery cable you can within reason if you have aircon the bigger the better
2 with a red top you could mount the battery on the bulkhead behind the back seats as high as you can or behind the side fittings (carpet)
3 this is the big one put in a earth cable to the body in the boot and you most run a earth cable from the battery to the engine. i dont care what people say the earth works .... i get better cranking with the earth every time
this is what i have done on my bro's celica st185 track car
http://i1129.photobucket.com/albums/m508/louieuk/st185/P1080003.jpg
http://i1129.photobucket.com/albums/m508/louieuk/st185/P1080005.jpg

and you can do the fuel pump mod

One other benefit is of course you can put the battery much lower than std position; in the sparewell.
So I disagree with putting the battery high up and the need of a thick heavy long earth cable.

the thing with a sealed gel battery is it can be put anywere in the car and it weigh's a lot less than a lead acid battery about 1/3 the weight
Per the reason i was saying put it high up in the boot is because the wheel well and boot can still be used...... as for the earth you dont need to put a large cable in as long as the battery is earthed in the boot as well i just found that having a earth running from back to front gave a lot better cranking over power

Hi guys, any follow up on this mods? Curious, what model of battery do you guys use? The stock in Malaysia is Din55, but some mechanic recommends Din66. do you use the same calling name? Thanks

I use twin gel MC batteries, 2 x 18Ah. Quite cheap around here (ca 75euro each).
Looks like below.

Also, putting an extra earthing strap between engine and body sorted out the last cranking issues. No need to run a full lenght earth cable!

http://sfk.ibk.se/forum/showthread.php?5311-Coupe-projekt-batteriflytt/page5