Technical How to wire a battery to the trunk

Hi all! I trying to get started on mounting the battery to the trunk. Things have gotten a little tight up front and this Optima is pretty heavy too. I also don't want that solenoid on the inner fender.
Anyway, I'm committed to this route now and I have looked around on the inter web and have gotten myself pretty confused. I was thinking that I could run a #2 from the battery to a kill switch in the trunk, to a fuse or breaker, #2 to the hot on the starter and run my hots (alternator, and accessories)off the lug on the starter. I see that most still run the ford type relay in the trunk and run wire back to it, including a hot from the alternator to the battery ahead of the kill switch. Something similar to this.
I'd love to hear some opinions on what has worked for you. Please keep it as simple as you can. I want to do it right, I just don't work with this much!!

 

A couple of questions... How long will the cable be? How tough a load will your motor present (cubic inches, compression ratio)? I can say that #2 would be adequate if the battery is up front, but not in the rear if there's any distance involved. 'Typical' starter draw is about 250 amps, but big blocks and/or high compression can raise that number by as much as 100 amps.

 

That's pretty much as I'd do it although I would take the power feed to the alternator to the terminal block main post. rather than running it all the way back to the battery. They way you show it you still would have power to the back of the alternator when you turned off the disconnect switch and that may not be for the best if you need to have all power shut off. The shut off is either for theft deterrent or fire deterrent and if you have an underhood fire you want to be able to cut off all power to the engine compartment with that switch.
Side note: my BMW has the battery in the trunk on the back passenger side where the factory put it and load over the cable isn't an issue. I ran my 51 Merc with the battery in the trunk with a 1-0 cable without problems from that setup. It had an Olds 350R in it.
I don't see hassles if you do it right and have good clean grounds.

 

What car? How far? I've seen lots of Model A's on here in trunk battery setups & thinking I may do the same. Im also considering shutoff-kill switch under front edge of seat pan, by drivers heals.

 

Basically that is exactly what we do in race cars.

use a diode on the Alternator wire [up near the main disconnect switch], and put the fusible link near the alternator [to prevent the whole wire frying]
The diode prevents a dead short if you are tinkering with the alternator etc.

Another variant is to swap the main disconnect switch for a Ford solenoid near the battery.
Then run a separate fused wire from the battery to the ignition Key/ Power Feed [and another wire back from the key to trigger the solenoid]
You also bridge the main starter cable to the actual starter solenoid.

With this method all the heavy cables are dead until you hit the starter button.

 

As mentioned, good clean grounds are essential and be sure to use star washers on your connections. I prefer to run a new, fresh ground wire all the way up the frame rail instead of grounding to the frame in the rear. Then to a good clean ground on the frame near the engine and a short ground to the bell-housing. Another engine to frame ground is a good idea as well. I can't stress the importance of clean grounds too much. Depending on the environment in which you live, some folks recommend cleaning up your grounds every 5 or so years to avoid corrosion, rust, etc. Di-electric grease can help as well. Good grounds are your friends. They can help you avoid a lot of headaches.

 

I’ve had a trunk mounted battery in all my Rod’s, I like a kill switch that I can reach from the drivers seat, it works for a safety device as well as for security.


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Just as a side nite to this I'm using the positive cable from a 92-99 BMW that I parts out in my 34 coupe. Its pretty darn long and heavy good quality. It also has a secondary wire already attached to it if you needed constant power for something. You might consider grabbing one. Just a thought.

 

The cable will be about 15'. The motor is a 460 stroker over 500ci at about 11.5. Looks like I'm going to have to return the #2 for a larger size?

 

I like to use extra flexible copper welding cable and plated ends. If you can solder them all the better but multi-crimps are good too. Always cover the connection with heat shrink. Some available at electric suppliers have a "glue" in them and make an excellent seal.
Remember to have the wiring to the alternator large enough also and the #8 you've show should be sufficient. Additionally I like that wire to be extra flex also.

 

Ditto...except I us a remote FOB activated solenoid on my key chain, it kills all power, both battery and solenoid are located in the locked trunk/box.
Someone may very well get my car/truck, but it will take way more time than they figure or a tow truck

 

What about the wire from the alt back to the battery. Does it need to go all the way back or can it go to the power block and be on the switched side?

 

I run mine to the 12volt main power lug from the battery on the starter solenoid.

 

I'd go with 1/0 wire minimum to keep voltage drop to a minimum, assuming you're using the chassis as the ground path. I'd run the 1/0 from the battery to the terminal block, then you can use a short #2 cable from there to the starter. Also connect the alternator to the terminal block. If this is a race car where it will be run for short periods only, you could use a Ford solenoid as a disconnect back at the battery (if rules allow that). If street use is planned, don't use the solenoid as those aren't rated for continuous duty and the solenoid coil will fail sooner or later. Use a manual switch rated for at least 500 amps, and most race rules require a manual switch.

If you decide to run a full ground cable (grounded at the front only), bump the size up to at least 2/0, but 4/0 would be better.

 

I think that's where I headed with that. I wasn't sure why the alternator power had to be hot all the time, even with the battery switch in the off position

 

Run 1 cable from the battery to the starter then run your powers off the starter lug ( alternator etc.)

I would run 1 aught welding cable.

Ground from the battery to trunk floor
Battery to frame
Then engine block to frame
Trans to frame
All good solid clean grounds with stat washers and after installed painted or dielectric grease on top to prevent corrosion.

Can never have to much ground !

 

Put your starter solenoid in the trunk. Then the big cable is only hot during cranking. You can use a 6ga wire to the fuze panel.

 

Depends on requirements [or rules if applicable]

If you want the Alternator to be dead when the main disconnect switch is off then anywhere after the switch is OK [this can prevent draining the battery if faulty]

In most motorsports they require the main disconnect switch to kill the engine when running. So the Alt wire has to be connected between the battery and the main disconnect switch to prevent the charge looping back through the ignition system.
That is why we now use a diode on this wire, The wire is dead if it shorts out, but charge can still be fed back to the battery [this is the best of both situations]

 

Here is a simple schematic of how my last race car was wired.
It didn't use one of those battery disconnect switches but a simple Ford firewall mounted solenoid mounted in the trunk.
The main disconnect switch was a toggle switch mounted in the fender.
On a street car this switch could be substituted for "Key switch" [to prevent theft]

As long as the main disconnect switch is capable of handling the loads [eg: lights/horn /ign etc]
This method keeps all the H/Duty cables dead when not in use [safer in a race car incident]

 

Reading back through, I did not mention this is a street car so no track rules but I do want to be reasonably safe and not burn it up! I do the master disconnect because it will sit idle sometimes and I don't want any battery drain or fire worries.
I got the 2 gauge idea from a trucker friend who said his road tractor used #2 from the batteries and I figured that if it was good enough to start that big diesel that it was enough for the car.

 

For a street car put the battery disconnect on the ground side. That way if the switch shorts it’s not a dead short.

 

The last semi I worked on (2015 Volvo) had battery cables that were bigger than my thumb, my buddy thought they were heater hoses running down there to warm the batteries. There was a small #2 lead off the batteries for a power inverter in the cab.

 

Well I emailed Flaming River to see if their one gauge was solid copper. They responded that it was solid copper. It arrived today...4,700 strands of...aluminum, copper covered strands.
So, is this worth keeping and using?

 

Depends on how much load you think the engine needs to start. I wouldn't use anything smaller then 00 if going any distance or if the engine has any heat starting issues. My 250 starts just fine with remote battery and 0 gauge wire, but then it has little cranking compression and the cable is only 5 feet long.

 

Good ole welding cable is the best battery cable made, especially long runs. It's all I ever use.

 

This is what I used...................

https://www.ebay.com/itm/Battery-Ca...-J1127-/302318840814?var=&hash=item46639b6bee

https://www.ebay.com/itm/Battery-Ca...ellets-/131718430714?var=&hash=item1eab07bffa

https://www.ebay.com/itm/Black-Junc...8-Stainless-/152019084252?hash=item23650b23dc

 

That's marginal.... With a 250 amp load, it will drop .8 volt (a bit over 5%) at a 15' length. With your big stroker motor and higher draw, you'll see a bigger drop, maybe enough that under the 'right' conditions you'll have issues...

 

x2 on the welding cable

 

Gotta love pure aluminum coated with copper. Haha I'd return it to Flaming River just on principal. This is why if I by some miracle get good service or info I let everyone know about it. It's harder to find than Bigfoot

 

I have been using Welding Cable in My Merc for about 35 Years
with no Problem!
And I even made a set of Jumper Cables out some of the Welding Cable
that I used when I had my Shop.

Just my 3.5 cents

Live Learn & Die a Fool