Cable size for second 'leisure' battery

[sweetaswesty]

I'm going to fit a second 'leisure' battery (so called) in the empty behind seat battery box and connect them in parallel with the live out going from the first battery and the shared earth being made from the new battery. I'm reckoning on it requiring 1.7m lengths of each cable to do this. I'm actually going to buy two new identical batteries.
My question is what size cable do I need to do it? The biggest draw will be from the eberspacher. We also have a compressor fridge, which is the reason for getting a second battery. But the total current draw at its highest is only ever going to be around 30 amps and mostly, usually, always, less than that.
According to the calculator on 12v planet 4mm cable should be enough and 6mm will give me lots of spare capacity. 
But when you google connecting two leisure batteries together you get recommendations for using 20mm, 30mm cable, old jump start cables, welding cable and the like. Which seems a bit over kill really?
So is there any reason for using such monstrous cable sizes? I have no plans ever to use an inverter for instance.
Thanks!
 

[ZsZ]

For the equal draw from each battery and equal charge going into those batteries I would use a really thick (25-35mm2) cable.
With a thin wire the battery which have the loads connected will have to provide all the power.

[cobblers]

The issue is the charging current, not the current draw.
You have to make the assumption that all the charging current that the alternator can produce will go into one battery (it probably won't, but you have to assume it might).
Generally 25mm2 is reasonable, with a 80 or 100a fuse next to each positive terminal.

[Oldiebut goodie]

As above ^^^ - welding cable is good to use.

[sweetaswesty]

I'm not arguing - I'm happy to defer to your collective judgment but the cable from the alternator to the leisure battery only seems to be about 6mm when it emerges into the under seat battery box and the split charger relay is only rated for 30 amps?
I think a 35mm cable could carry up to 250 amps for the short length between the batteries?

[cobblers]

The traditional split charger wiring on these old vans is absolutely garbage. They use wire that is far too thin, and the voltage drop over it (because of high resistance) will usually limit the charge current to 30A or so, and it usually does. It results in randomly blowing charge fuses and very slow battery charging. I think it was done back when proper 100A+ relays were very expensive, so there was a significant cost saving to be had.
Nowadays the difference between wiring the van up in the crap old way and the proper way is about £100, and you end up with a more reliable, better performing charge system.

In this case case, you could safely do your wiring exactly the same with 6mm and 30A fusing. It would work, but it's not right and charging two leisure batteries up over a long 6mm cable would take about 10 hours of driving!

[sweetaswesty]

Thanks - that's all very helpful.
So to get the best set up I would need to minimally upgrade the cabling from the alternator to the leisure battery and fit a hd split charge relay plus the heftier cable to the second leisure battery?

 

[cobblers]

Yes, to do a nice job of things I would personally replace the split charge relay with a Victron Cyrix VSR and upgrade all the cabling to 25mm2. It'll charge much quicker, and when on mains hookup / solar once the leisure batteries are mostly charged, it'll automatically link in your starter battery to keep that topped up, too.

[AngeloEvs]

I used 40A 5mm for my leisure battery but doubled it with two cables per terminal. This makes routing a lot simpler because the two cables are run side by side giving it a lower profile. The leisure current draw is fairly low and probably totals a maximum of 6 amps when the water pump, TV, lights, compressor fridge are operating at the same time.

The maximum charge current from the ZIG is just 4 amps but even more modern chargers rarely exceed 8A but in built ZIG/chargers rarely need to give maximum output because the leisure battery will be fully charged and then trickle charge once on 240v hook up.

This leaves the charge current from the alternator but how likely is it that you will completely discharge the leisure battery? Also, there should be an in line fuse from the charge relay to the leisure battery to protect against possible short circuits and over current - the in line fuse on my van is 20A and never had to be replaced.

[sweetaswesty]

Thanks again, all very helpful and quite a bit to think about! I'm inclined to go with the 6mm option at the moment with the thought that I should upgrade the wiring from the alternator and change the relay later and at that point double up or replace the wiring between the leisure batteries. That will get me through this summer's travels hopefully.

[davidoft1]

30 amp cable is plenty , welding cable is probably 400 amps plus , look at the size of cable off the back of the alternator and the feed into the fuse box , only the starter cable is 25mm2 mostly because of the length to allow for current drop

[sweetaswesty]

Thanks, well that’s what I was thinking really and I think is going to be what I do for this summer’s travels when we just want a few more amps to run our fridge when we don’t have a hook up.

[stoneageman]

Hello, i was after some advice on Lesiure batteries. I last bought a leisure battery a couple of years ago and when i was driving with both the 'car' battery and the leisure one connected. The leisure battery sometime into the journey began to bellow out smoke - i pulled over and disconnected it. It was cooked and never worked again. My thought is that the split charger isn't working properly, it looks original and is very old. 
So, do i just buy a replacement split charger, i thought a Victron after reading up thread that these are good, i will also replace the wiring as this is old too. Does this sound the right thing to do or does the 'cooking' of the last leisure battery require some further exploration?

Thank you in advance