Battery earth cable advice
number-1 - 7/3/24 at 05:34 PM
Can anyone advise me on what AGW size i need for a 300mm long battery earth cable? I have a 2AGW but it seems way too thick. Any suggestions on what i
need?
The battery is 12V 52Ah 520A and is a 4 cylinder 2L Turbo
scudderfish - 8/3/24 at 02:21 AM
I didn't think an earth cable could be too thick.
HowardB - 8/3/24 at 09:02 AM
the max current that can be delivered by the battery is the cold-crank-current
However the maximum that can be consumed could be much lower
I would spec the cable to the CCC as it will degrade over time and the limiting may not be today but after some time.
At 300mm even the thickest cable is not going to add a huge weight!
peter030371 - 8/3/24 at 11:00 AM
Honestly never seen a 300mm standard battery cable of 16mm2 (6awg) struggle when starting a normal car engine. 300mm is a normal length and people
like Car Builder Solutions do a range of pre-made battery cables and they are all 16mm2
Don't be confused by the CCA, that is a short burst at cold temperatures and nothing like that ever happens in the real world. It is just a
specification to allow batteries to be compared.
Most starter motors take a short large pulse when cold, this is in milliseconds. A 16mm2 cable will handle this pulse. They then keep turning over at
well under 100amps, exact amount depends on load, until you stop it. Last time I measured my 2l Duratec with a clamp meter it was 42 amps on a spring
day so 15-20degC with a cold engine, I didn't measure the peak as it is of no interest.
Mr Whippy - 8/3/24 at 01:02 PM
quote:
Originally posted by scudderfish
I didn't think an earth cable could be too thick.
ditto, and use brass bolts/washers
[Edited on 8/3/24 by Mr Whippy]
coyoteboy - 11/3/24 at 10:41 AM
I would personally spec to CCA x 2. There's literally no negative (no pun intended) to it other than cost.
peter030371 - 12/3/24 at 03:11 PM
quote:
Originally posted by coyoteboy
I would personally spec to CCA x 2. There's literally no negative (no pun intended) to it other than cost.
Sorry don't want to be picky but have you ever chosen a wire for this application?
A simple example, just put a new battery in the wifes mini. A bosch S5A08. The CCA is specified as 720 (EN test) or 800 (SAE test). So lets go with
the 720 and multiple that by 2. Now we need 1,440amp cable. Is that what you are saying? or have I missed something?
As I said before 16mm2 (6awg) is more than enough for a normal 2l engine which has a starter motor that is probably only about 1kw i.e. 83amps!
gremlin1234 - 12/3/24 at 04:54 PM
old school maths suggests any wire should be capable of it's circuit's max fault current, in this case the cca, else the wire becomes the
fuse.
peter030371 - 13/3/24 at 11:33 AM
quote:
Originally posted by gremlin1234
old school maths suggests any wire should be capable of it's circuit's max fault current, in this case the cca, else the wire becomes the
fuse.
When was the last time you have seen a fused negative lead on at 12V car system? The fuse(s) are on the positive side and so not needed on the
negative side. The fuse on the positive side, maybe 20, 30 or even 40amp, will pop long before a normal 16mm2 (110amp) negative wire even starts to
get warm.
If you want to base it on CCA then please can someone then find some '1,000amp' cable for the OP to use! If you are going this big then
forget wire and use a solid bus bar system just like every kitcar build you see.....not 
Back in the real world CCA has nothing to do with how much power a battery cable will need to handle safely in use on a car like ours.
Honestly thousands of kitcars have been built with 16mm2 battery cables and they are perfect for the job. Anything bigger is for special applications,
maybe high compression V8/10/12 engine beasts or long cable runs, but not a 2l turbo that the OP has.
Mr Whippy - 13/3/24 at 12:05 PM
Halfords sells standard bare wire earth cable for between the engine and the frame. They also sell 10mm coated battery cables which are more than
enough for normal cranking for cars.
If it's a really long run between the starter and the battery >1m, then I'd up the positive cable to 16mm to reduce voltage drop. But
always take the earth off the frame as close as possible to the battery, where a bare wire is best. Remember cranking only lasts a few seconds so
along as the connections are good (the cause of most heating) you will never have an issue, if you were cranking for hours, yes you'd need very
large cables indeed, but you have other big issues to deal with if that's your problem 