Wire size and set up

a few questions:

1) what size cable does a 90 300tdi need from starting battery to starter?

2) I am putting in an issolator (to save me removing my seats). 100amp seems to be standard size or next is 250. For a standard set up (no winch) is 100amp ok?

3) I will put in an Anderson connector on the seat box to assist with jump starting (hopefully just others!). 170amp seems to be standard size, would this be right? Do I match the cable size to that of question 1?

4) I will eventually put in a second battery and split charge to run my new kit (heated seats, radio etc). Is there any benefit in having Anderson connector to both batteries? And do I need seperate battery issolator switches?

Thanks

Rorie said:

a few questions:

1) what size cable does a 90 300tdi need from starting battery to starter?

2) I am putting in an issolator (to save me removing my seats). 100amp seems to be standard size or next is 250. For a standard set up (no winch) is 100amp ok?

3) I will put in an Anderson connector on the seat box to assist with jump starting (hopefully just others!). 170amp seems to be standard size, would this be right? Do I match the cable size to that of question 1?

4) I will eventually put in a second battery and split charge to run my new kit (heated seats, radio etc). Is there any benefit in having Anderson connector to both batteries? And do I need seperate battery issolator switches?

Thanks

Click to expand...

1: 35mm is what is fitted as standard, but I have just upgraded to 70mm. Mine was starting to be slow at starting, and resistance through the wire wasn’t as low as I would like.

2: most people fit the red plastic handled ones. I think these are 100A.

3: I had one of each SB175 and SB50 on my old seatbox. The 175 was for jumpstarting, and the 50 for accessories. I used some 50mm cable to go from battery to Anderson. None of these are installed on the new seatbox yet.

4: this will depend on how you go about installing the batteries and connecting them together. If you go with a basic relay, like mine, then you could put 2 off SB175s in, one for each battery. Then if one battery goes flat, you could have a short cable with SB175s at each end, to use as jump starter. If you wire them up with a dedicated system, then most have the jump start facility built in.

I assume we're talking 35mm^2 and 70mm^2?
I've been looking at Simple Split Charge and also Vehicle Wiring Products but 60mm seems to be the highest size they all go to. 40mm^2 is the biggest a lot of places do. Is 70mm^2 maybe quite excessive? We must be up to about 500amp capacity there and with just a 100amp breaker, maybe i'd be doing it all wrong?

Thanks

Rorie said:

I assume we're talking 35mm^2 and 70mm^2?
I've been looking at Simple Split Charge and also Vehicle Wiring Products but 60mm seems to be the highest size they all go to. 40mm^2 is the biggest a lot of places do. Is 70mm^2 maybe quite excessive? We must be up to about 500amp capacity there and with just a 100amp breaker, maybe i'd be doing it all wrong?

Thanks

Click to expand...

Yes that’s CSA (cross sectional area, given in mm2). Yes 70mm is massively over excessive, but when you consider that resistance drops as CSA increases (inversely propertional), then why not fit the largest you can? If I were to rewire my starter battery to starter motor again, I would probably opt for 40-50mm cable. That’s still 300-340A, not 480A like my 70mm.

If you want a basic split charge, like mine, then you will struggle to get heavy duty wire to connect to a “normal” relay. My relay is 70A, and the cable is 10mm (I think, it may be 16mm), but it’s fused low anyway. I only have a 60A alternator, so didn’t see much point in making this circuit “heavy duty”. It doesn’t have the ability to jump start itself, so only for the camping fridge. If my main battery goes flat, I least have tools and it’s easy enough to swap the + lead between the two batteries.

If you want cheap, fast service, then I always go to altec automotive. If I’m after a larger choice of wire colours, then it’s pole volt.

The capacity of the wire and breaker are given as constant current. The transient currents are significantly higher, so the 100A breaker is ok for the start circuit, so long as your not holding it cranking for minutes on end.

Edit: don’t forget that a decent battery will quite happily push out 600-700-800A at short circuit. This is effectively what the batteries load is while cranking.

Thats great, thank you for the help!

I need a new cable from starter to battery, as current one is just too short to reach my battery, so i'll get a new cable for that as you've spec'd up.
I'll also get an Anderson connector for the seat box for this battery as a short term measure until i get my split charger wired up and take it from there.

Would you put an inline fuse on the anderson connector within the seat box? Or just directly onto each battery pole? I guess not really required for jump starting and would cause more issues than good?

I have 40mm (300A) from battery to starter sol. as its a non fuses connection I think you wont find an isolator that will match you cable carrying capacity and may introduce a resistance point / volt drop during starting
Cranking current can vary from around 300 to 600A dependant on engine and ambient temperatures.

What is the purpose of the isolator you will be fitting ?
1, is it an anti theft device (will need to lockable or removable key)
2, is to stop battery drain ?, Isolating the starter to battery lead will only stop drain from the engine bay fuse box fed systems
3, is it to facilitate two batteries so you can have a changeover system ? most changeover switches allow either battery or both to be connected to the output (to starter) and can be in an OFF position

something like this may be able to cope but is not lockable



Blue sea systems make some hefty battery isolators