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Discussion Starter · #1 ·
Not very old battery started to struggle. Charged state not dead, but always a bit low. 12.2 volts but drops quickly and takes a long time to recharge.

checked voltage with engine running at food speed (2000rpm). Only just 13 v at battery

14+ at the alternator. So about a 1v drop between alternator and battery, this seems to high to me.

my plan is to simply run a new cable. I’m guessing the old one has issues, there seems to be a join, don’t know how good that join is, etc.

my question is do I need a fuse? It would need to be over 80amp or whatever my alternator is? Or do I just connect the alternator to battery with a single wire?
 

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just a good guy.
2009 gu ute
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Start by checking both terminals at each end a nice and clean , corrosion can be a killer. why is it joined is a question and if no fuse present why would new cable need one.
 

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Discussion Starter · #3 ·
thanks. I checked the terminals at each end they look OK. The join I don’t know, very hard to reach without taking a few things apart.

i don’t know if there is a fuse. the cable runs along with a lot of other wires, hard to see the entire length. it is definitely joined. I presume it is joined because they were stupid and didn’t have a long enough cable. The cable also looks a bit thin to me.

I couldn’t follow the cable the entire length

the car is re engined, and some of the wiring is dodgy. It was an amateur build.

why would it need a fuse? for all the normal reasons you you need a fuse. Why would it need one if it doesn’t have one now? I don’t know if it has one or not, and even if it doesn’t I don’t trust the people who did the original wiring.

so does it need a fuse or not?

is a 1v drop excessive? It’s not getting a good charging voltage at the battery.
 

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just a good guy.
2009 gu ute
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We have a resident battery expert on here somewhere he may have a suggestion, battery not very old could have dropped a cell maybe. @Yeti's Beast
 

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2018 Mazda BT50 Dual cab
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Did someone knock on my door ?

1 volt is way too much !!
It is best to check the alternator output when the battery is fully charged.
Back in my day when Holden’s still existed, the older ones simply had a wire between battery and alternator and I have done it myself without issues.
As days went on fusable link wire was added which is a strand of wire designed to blow like a fuse as a safety measure.

I say try a new piece of 6 mm wire and test the results


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GU 2007 Cab Chassis Leafie, 6.5L Chev diesel, manual, slide on Tommy Camper on back
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The Haynes Manual electrical diagrams for a GU (is that what you have @MartyS?) show a 100A (diesel) / 120A (petrol) fuse between the alternator B terminal and the battery.
If thats what Nissan thought was a good idea, then I'd follow suit.
Yes, you could do it without a fuse, but this would be some sort of survival situation, eg you are somewhere in the bush and dont have access to parts and tools - then otherwise, why wouldnt you do it properly? A short circuit in the alternator to battery cable could create a fire and burn your car to the ground.
 

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This is a good post showing photos of the fuse and how to remove it. Maybe you still have the fuse installed?
 

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The projecta fused battery terminal is a neat way to add a big fuse (and some more fuses for distribution). I use one on my secondary battery and completely happy with the product.


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I had a similar problem years back on my first Troll, it turned out to be the wire that ran from Alternator to Battery was knackered.

Over time I must of been drawing so much out of the battery or the regulator in the alternator failed that the alternator was outputting near max charge all the time, this over time made the wire running to the battery hot enough it turned the sheath of the cable hard and colored the wire black.

Replaced wire with 8mm (all I had at the time) all good again.
 

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Every dog has his day
2005 TD42 GU IV
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Good grief!

8mm is far too small let alone 6mm!

I would use nothing less than 16mm2, and on my own car I used 25mm2, and a 100A manual reset circuit breaker.

6mm auto (4.58mm2) is good for bugger all in this context, even the factory main feed is bigger than that. And yes from factory it certainly has a fuse, so I wouldn’t run it without one. Like Monkey said, could be a fire waiting to happen.

And as Yeti says, 1v is far too much volt loss in such a short run. The volt difference would reduce as the battery approached full charge but it should never get that much in any case.
 

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Good grief!

8mm is far too small let alone 6mm!

I would use nothing less than 16mm2, and on my own car I used 25mm2, and a 100A manual reset circuit breaker.

6mm auto (4.58mm2) is good for bugger all in this context, even the factory main feed is bigger than that. And yes from factory it certainly has a fuse, so I wouldn’t run it without one. Like Monkey said, could be a fire waiting to happen.

And as Yeti says, 1v is far too much volt loss in such a short run. The volt difference would reduce as the battery approached full charge but it should never get that much in any case.
Why 8mm Battery Cable is capable of 135amp, maybe I should of said the type of cable I used or quoted 4B&S. Problem is a lot of people wouldn't know what the hell is B&S, AWG or IOS Cable means.

battery.jpg
 

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Every dog has his day
2005 TD42 GU IV
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For a start, on an auto forum, if you don’t specify otherwise you’re assumed to be referring to auto cable. 8mm, in most brands, is usually about 7.1mm2, with some variation between brands.

By your own chart, the 7.71mm2 8B&S is ‘rated’ at 74 amps. Frankly, anyone who would happily use cable ‘rated’ at 74 amps for their 90 amp factory alternator, should not be unleashed on their own, let alone anyone else’s, electrical systems.

Secondly, there is no such thing as an ‘amp rating’ on auto cable, no matter how many shiny charts people wish to google. A cable’s suitability is determined by the volt drop in a given circuit, and this cannot be determined unless the length is known, so how can an amp rating be offered without other variables being known?

For example... narva ‘rate’ the 6mm (that’s auto, so obviously 4.58mm2) cable at 50 amps.

50 amps!!!!

Anyone who would push 50 amps through 6mm cable.... see above about being unleashed on electrical systems.

As you know, the volt loss formula is length x current x 0.017 (for copper) divided by area, so let’s say you have a big juicy air compressor down the back of the car, it pulls 40 amps. Let’s say you’re one of the many brain washed into thinking it has to be earthed back to the battery or else. (That one is under ’gullible’ in the dictionary, by the way, you’ve just doubled your volt loss!)

So your volt loss is 12m (battery to rear and back) x 40 x 0.017 / 4.58 = 1.8 volts

Holy crap! 1.8 volts! And that’s only at 80% of the cable ‘rating’, and I normally accept a maximum of .3 volts in a given circuit.

If we went by ‘ratings’, then we’d all use 3mm auto (1.13mm2, ‘10 amps rated’ to run two fridges down the back. But of course anyone who would run even one fridge off anything less than 6mm... see above re being unleashed etc etc.

Then in the engine bay, going back to your alternator feed... of course we have to add about 0.4% volt loss per one degree celcius. Now that wire goes across the engine bay, on top of the engine, behind the fan.... so if we conservatively add 30 degrees to ambient, and that’s generous, that’s another 12% volt loss on top of everything else.

So yes, 16mm2 is the BARE MINIMUM in my opinion that should be used for an alternator feed. The better the voltage the battery receives, the faster and deeper it charges and the longer its life.

So anyone who wants to use 8mm knock yaself out... but I’d respectfully suggest don‘t give those kind of ideas to folks seeking decent advice.
 
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So your volt loss is 12m (battery to rear and back) x 40 x 0.017 / 4.58 = 1.8 volts
Please read the post again I replaced the Alt to Battery cable with 4B&S and no I didn't run it via the rear of the car the length of cable was around 1 to 1.5 meters.

Nissan them selves use cable that is 5.5mm OD on the GQ and 6.5mm OD one the GU between the Alt and Battery so unless they have some magic cable making system most people will replace cable with the same size as what they are replacing, and yes they could stuff up due to the way some cheap ass manufacturers cheap out and use substandard copper and/or make the cable look thicker than it really is by increasing the sheath size.

Also don't toss everyone under the bus for their answer if you know the answer to the O.P question jump in and answer it.
 

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And I only said to TRY some 6 mm to TEST and see if it made a difference
But also he needs to make sure his battery is fully charged as amps could be up and volts down if the battery is either flat, sulphated or faulty.


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And I only said to TRY some 6 mm to TEST and see if it made a difference
But also he needs to make sure his battery is fully charged as amps could be up and volts down if the battery is either flat, sulphated or faulty.
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This... Just put in parallel and see what happens. It will prove out the cable and connections etc.
 

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The alternator should have a reference input as well as the high current output.
On my GQ these two were joined at the alternator, which is not ideal. I'd measure 14.2V at the alternator and 13.8V at the battery.
Following advice on this forum, it may even have been from @Leethal I separated the terminals at the alternator and ran the reference to battery (+) using a 6mm cable with a 10A fuse at the battery for safety.
Voila, 14.2V at the battery over a good load range. Happy Days.
 

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Have not read full story. Have had a similar problem as this ie 14.5v on alternator and 13.5v on battery.
Problem was the earth strap between chassis and motor. This will put the ground of the battery and the ground of the alternator at different levels. ie it dropped 1v across the earth strap.
 

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Maybe worthwhile for all to have a think about the confusion (and heat) that the different cable "size" references in the above thread have created.
Auto cable size references are very confusing. Its almost setup for people to fail.
We have the following getting referred to: AWG, B&S, Conductor diameter, Auto MM (or mm), cross sectional area (mm2) and Amps (capacity).
All of the above get bandied around. If someone is talking about one size reference and you misunderstand it you can get very different (dangerously different) results.

Of all the size references, two are pretty much unhelpful:
1) "Auto MM (mm)" - because this is based on the approx outside diameter of the cable - ie conductor + insulation/sheath.
Some suppliers could substitute plastic for copper to make the OD the same but have reduced the cable current capacity for an unwary buyer.
I only buy "Auto mm" cable where I can read the actual conductor cross sectional area as well.
2) Amps - because as @Leethal has said the amp carrying capacity of a cable involves working to a maximum level of volt drop (which is proportional to both length and temperature). A roll of cable labelled "15Amp" at SCA is misleading because it might not work carrying 15 amps for the situation you have (cable length, temperature and resulting volt drop, may make the connected load not operate).

I cobbled together this table trying to make sense of the different size references (borrowed from various web sources).
It doesnt include current capacity - because making simple current capacity charts ends up being misleading and dangerous if they are used wrongly.
To work out current capacity of a cable, or required cable size for a specific current, you should ask someone who knows how this works - or as a minimum use an online calculator that involves questions about: Total cable length, return current path (by cable or chassis), acceptable volt drop and temperature.

Bottom line: Whatever you do though, be conservative. Saving $20 on going to a smaller cable size is small comfort if that cable fails out the back of beyond and leaves you stuck, or worse causes a fire in your vehicle.

531527
 

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...Please read the post again I replaced the Alt to Battery cable with 4B&S and no I didn't run it via the rear of the car the length of cable was around 1 to 1.5 meters...
Ok sorry I’ll go back and re-read as you suggest...

...Replaced wire with 8mm (all I had at the time) all good again...
Oh, it seems you said you replaced it with 8mm, not 4B&S??

...Why 8mm Battery Cable is capable of 135amp..
No it isn’t, even by you own chart. Own goal much?

If you say, for example,’6mm’ to an electrician (domestic, commercial, industrial, mining etc), he will, without fail, assume you mean 6mm2 of actual conductor.

If you say, for example, ‘6mm’ to a auto electrician he will, without fail, assume you mean 6mm auto which’s 4.58mm2 of conductor. (Yes I agree the ‘auto’ system is ridiculous, but it’s what we generally use in Oz so we’re stuck with it).

So given we are referring to cars, in your post to which I responded, you most certainly did NOT say you used 4B&S, (contrary to your later assertion), in which case I would have wholeheartedly agreed with you. You clearly said 8mm, and as nobody in the known world (um, ok, one person...) refers to cable size by the diameter of the conductor exclusive of the insulation, and as I was not privy to this brand new standard, I mistakenly assumed you meant what you typed.

As for the size cables Nissan use... FFS if they got that right we wouldn’t need to bloody replace it would we??? By that logic we wouldn’t fit spotties or a suspension lift either, because Nissan got it right the first time!

And I’m well aware you don’t run an alt cable to the rear of the car and back... if you actually read my post you’d comprehend that I was making a case for the fact that there’s no such thing as an ‘amp rating’ for auto cable because it doesn’t take other critically important factors in to account.
 
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Discussion Starter · #20 ·
Thanks everyone. Some good info.

it’s a 1996 GQ. re engined With a td42. The cable doesn’t seem to have a fuse able link.

im an electrical engineer, so I’m good with understanding cable sizes. There seemed to be a bit of confusion above between diameter and cross Section. I’ll probably use a 6mm or 8mm diameter cable. (That’s diameter of the conductor, not auto diameter stuff) that I will buy from a place that sells cable, not a auto shop. So it will have a crossection of over 33mmsquared. For a 2m run at 12v and 100A that should be a voltage drop of 0.3V, assuming good connections. I’ll might just run it in parallel with whatever is there, especially if there is no fuse.

I will investigate a fuseable link Though.
 
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