Redirecting Starting battery to rear of van

[bisonthecruiser]

Really want to free some space up around the 7.3 diesel and reroute one of the starting batteries to the back of the van where one of the old house batteries lived.

What thickness cable do I need to make this happen? It's probably a 30foot run if I had to guess.....

Last thing I need is to undersize the cable and create a BBQ. I can read a wire chart, but assume I don't need a 3/0 wire if not running constant. Will be going up to a 370A alternator so need to take that into account. I'd love to move the battery, but not if I need to go crazy on the wire in the near-term.

Adam

[Lilnuts2]

With that beefy 370 alternator you need beefier wires even if you don't move your battery. If your moving your battery 5' or less I would go up two sizes at least. When I did mine I bought welding cable and made my own cables, cheaper and exactly what you want. With 12v systems the voltage drop along the length of the wire is pretty significant. Don't be afraid to consult a voltage drop chart and go bigger.

[gahamby]

370 amp potential over a 30 foot run. 4/0 at least. Don't forget the over current protection.
Consult an ampacity chart.

[boywonder]

How many cold cranking amps does a cold 7.3 require?

[Twoxentrix]

Just pulled this off a search (I wasn’t doing anything else at the time):

In our opinion, diesel batteries should have a minimum of 800 cold-cranking amps (CCA) — and that's if there are two of them. A single battery should have at least 1,000 CCA and a lot of reserve capacity, something like a Group 31 battery.Nov 13, 2019
https://www.dieselworldmag.com › ...
ALTERNATORS, BATTERIES, AND CABLES - Diesel World

[bisonthecruiser]

Both batteries will end up being 850cc


https://www.oreillyauto.com/detail/c.../mot2/bxt65850

• Cold Cranking Amps (CCA): 850 CCA
• Cranking Amps (CA): 1000 CA

[boywonder]

Originally Posted by bisonthecruiser

Both batteries will end up being 850cc

I asked the question above to try to determine which of these use cases is worse:


Cranking a cold 7.3 that may not fire up instantly.....700 or so amps pulled even for 10-20 seconds will likely heat up even large cables. I'm guessing at how many amps a 7.3 pulls while starting; I'm pretty sure it's more than the gas engined vans....since the diesels have 2 starting batteries.

...or pumping 300A or so back to the batteries.....this assumes 70A or so of your 370A alternator is going to powering other stuff in the van....again a SWAG but I'm guessing I'm close.

If the batteries are lead acid chemistry I can't imagine they will be pulling that much.....LiFePo4 batteries different story.

[gahamby]

You have a lot less to lose oversizing the conductor than undersizing.

[Scalf77]

So your issue is that you the two different loads for your system, charging the starters and running the van while running, and the charge of the house batteries ( which I believe in your case is a sterling 120 amp DC to DC charger). So for the most part your actual load will be less than 200 amps. So now you have the two variable "Can my cable handle the worst case output of alternator 370 amps) and what size would I need to hold a reasonable voltage drop. I think you will find that even if you used 200 amps for a long run, you would have a difficult time making even a 3% voltage drop.

And that is just to get to the battery, you haven't even added in the length to get to the DC to DC. So I think in general it would be prudent not to move the starters to the rear of the van

-greg

[bisonthecruiser]

Thanks, Greg. Yeah, I think at this stage I'm just trying to figure out what I'd need cable wise to make it happen. Both starter batteries are lead acid and I was going to connect the DC to dc on a short run in the rear to that battery. A new battery is $200+ because it's a custom size and tough to source etc.

By replacing that small starter battery I figured that I would be better off in the long run from both a starting and battery lifetime perspective. I'll take a look again at fitting a battery on the frame rail

Edit: no room on the frame rails that make it terribly better anywhere farther back