relocate battery

stranger12

hi guys

I want to relocate my battery to boot on my race car

what wire gauge and amp will i need?

if my cold cranking amp is 320amp or so and battery is only 50 amp then do I need a wire that can handle 50 amp ?

is the cold cranking momentarily load thus even though it is 6 times more than the capacity of cable, it won't burn it ?

molerat

You need a cable that will take whatever your starter motor demands, probably around 200+ amps for an average small car engine. Look at the cable that is currently used.

onomatopoeia99

stranger12 wrote: »

if my cold cranking amp is 320amp or so and battery is only 50 amp then do I need a wire that can handle 50 amp ?

No. If you mean your battery is 50 Ah (amp hours) that does not say anything about the maximum current it will supply, it means if you have it fully charged you can draw one amp for 50 hours before it will go flat (or 50 amps for one hour). You absolutely must not try to put 300 amps through a cable rated for 50, the insulation will melt in a few seconds. And yes, I have accidentally shorted such a cable to the bodywork and observed the effects at close hand.

Have a look at a set of jump leads. Not the bargain bucket £10 set from Halfords, but a decent set. They are only used for a few seconds as well. See how thick the cable is? That's what you're looking at.

Cable thickness is measured in millimetres squared in this context. As the resistance which causes the cable to heat up increases with the length of the cable, you will need even thicker than jump leads as you want to go the full length of the car. You will need something ticker than the cable to the starter at the moment as the cable run will be so much longer. So, look at 35mm2 or 40mm2 cable. If you get anything thinner, you may regret it. Not only will it get hot but the voltage will drop as well making the car less likely to start.

By the way, race scrutineers check battery installations. While they are more concerned that the battery is secure so can't come adrift and short on the bodywork and therefore don't generally look at the quality of the wiring, if it was really blatantly awful and could to set the vehicle on fire, one might say something, or even not give you the sticker that lets you out on the track. I'm talking MSA race meetings in this context though, rather than short oval or autograss, which I'm not familiar with other than as a spectator.

Strider590

Go same size as the existing battery cable or one gauge bigger to be safe, got to make sure the momentary current isn't going melt the cable insulation, it's also extremely important to route the cable so as to avoid chaffing the insulation and causing a short against any metalwork.

The ugliest thing is working out how to join the cables and this depends a lot on your particular setup, try to avoid using remote battery posts if you can.

onomatopoeia99

Strider590 wrote: »

The ugliest thing is working out how to join the cables and this depends a lot on your particular setup, try to avoid using remote battery posts if you can.

Just do one long run from the new battery location to the starter. No need for joins, all the connectors can be bought from kit car supplies places (or in many cases, even from Halfords)

stranger12

Many tbanks

Is it as simple as taking existing diameter or current cable?

Inwas thinking of getting a 90mm but that is overkill and belive my current one is hardly 30mm

Would you run a negative back to engine bay or just connect it to chassis in the boot

onomatopoeia99

When I relocated mine (to the front - it was a rear engined car) I used a 4m length of 40mm2 cable and it was fine. I made a new earth point near the new battery location.

joeblags

4 guage cable can vary with brands. cheap stuff is cheaper material and have thicker insulation. not just 4 guage but all brands, it would depend on the length of the cable aswell.


https://www.amazon.co.uk/KnuKonceptz-Complete-Gauge-installation-wiring/dp/B0050I6KII


look at the pictures on the left of different brands and the size of the copper % in each cable

knightstyle

The cable does not have to carry the high current for long so a smaller cable than normal for this current can be used.
The main problem with long runs is the volt drop which can mean the 12v from the battery will only be 8v at the starter, the bigger the cable the smaller the volt drop.
When I helped a friend renovate an old racing midget we used 50mm and ran two lengths, using one for the earth return, he never said he had any problems even in cold weather.

reeac

knightstyle wrote: »

When I helped a friend renovate an old racing midget we used 50mm and ran two lengths, using one for the earth return, he never said he had any problems even in cold weather.

I second the point about using an earth return wire instead of the chassis or bodywork. Did it recently on my MGA and it made a big difference to the operation of the starter motor. Assuming that you have access then run the return wire from one of the starter motor attachment bolts. The MGA batteries (originally two 6v ones) are housed near the rear axle. I just used the same gauge wire as the existing one and cable tied the two together.

forgotmyname

Larger than the current wire size. Running at extra 15 foot of cable will drop the voltage, so you need to go UP a wire thickness.

And be wary of cheap wire. I buy reels of wire for various projects and had some 11amp wire and 10amp connectors. Pulling a continuous 6 amps the connector started smoking and the cable is fairly warm.

You dont want them when trying to start your car.