Battery in the trunk....please school me!

Just remember it's a circuit and every piece of that circuit must be able to carry the current that the starter will draw when you try to start it in the 7-11 parking lot after 4 hrs on the road in 96* weather. That's when you need the heavy cable. I have the welding supply house make my cables to my specs.

You can run the ground cable to the frame but if you do, run a same size cable jumper from the frame to the engine up front. Without that jumper the current will try to find any way it can to complete the circuit usually with poor results especially in the heat of summer when the resistance is increased.

 

Tjet beat me to it! I run a Ford starter relay in the trunk so the big cable is only "Hot" when the starter is engaged. Much less chance of a super short circuit and conflagration! Ask me how I know, go ahead, just ask me!

 

X3 good idea so you do not have a 10 FOOT hot wire...

 

Ok, I'm reading ya'll on all of these points..and THANKS for all of the info! I do have a question about this cut-off switch, what's it's purpose in the grand scheme? Does this have to do with dangers of running the current from front to back, explosive gases, not having a hot wire always running through the length of the car? I would never have thought of it had no one mentioned it, because I figured it would be the same as having it under the hood, just over a longer distance.

Does this function as the cut-off like the Ford solenoid mentioned above? I'm looking at the Ford option diagram and wondering if there's a simpler in-line option where I don't have to mess around with my current wiring setup more than I have to.

 

Optima says different...

Trunk/Interior Mounting – If your battery is mounted inside the passenger compartment or trunk (common in many European makes), the battery must be vented to the outside of the vehicle. OPTIMA group 27, 51, 78, 34C, and 31 batteries all have ports for connecting a vent hose.

 

x2! When I first fired my latest project, I noticed my clutch linkage was smoking as I cranked the engine. Turns out I forgot to tighten my block to frame cable and electrons were trying to go through the clutch linkage to get back to ground!

I remember reading somewhere that new Kcars had a problem wearing out CV shafts and wheel bearings in a very short period of time during the 80's. Technicians and engineers were puzzled. Turns out that they came out of the factory with no engine to ground cable. The only way for the ignition spark to get back to ground was to jump through the trans to the CV shaft, across the bearings and to the suspension back to the chassis. Each spark would take a little bit of material with it on it's way, deteriorating the bearings and shafts. Quite literally, welding up the surfaces! Pretty amazing.

On that note, there's nothing wrong with using the frame as a ground path as long as your connections are good, tight, and clean.

Also, stay away from star washers, they may seem like a good idea, but are prone to corrosion and actuallly decrease the contact area of the connection quite a bit. A good, clean, flat, bare metal contact area slathered with dielectric grease works great.

 

Seen a cool set up at a car show the other week.
The guy cut the back out of an Igloo type plastic cooler and just sat it over his battery box.
No ugly naked battery in his trunk.

 

Forgive my inability to read wiring diagrams, but I'm not clear on the Ford starter relay idea? Are you saying that the long, big cable to the starter would only be hot when starting? If so, where does the car get power when it's running??

And on a related note, with this setup it seems like the trunk might not be a bad place for the fuse box, too. Yes? No?

 

The pink wire at the top of the diagram also runs to the front of the car and is used to supply power to the accessories and charge the battery. It would typically be fused and since it doesn't have to carry the load of starting the car can be fused much lower than it could be without the solenoid. I'm on the fence on how I'm going to do my car when I get it to that point.

 

My last set was 00 welding cable. It was overkill but I bought it for $.50 a ft.

Make sure thet you have a good ground and run your ground cable as short as possible. Like Z said make sure you vent your box to the outside.

The cables I have now for my new car I bought from summit for about 15 bucks, I think list on them is about 35.00. There is a lot to be said for making your own but if you don't already have the cable its more often than not cheaper to buy it already made up. Then you can spend the extra cash on a fuel pump or a camshaft.

 

Man, I wish I knew where you got your cables! Up here in North Jersey, they want $3.75 a foot!!!! w....t...H?!?!? Greedy foolz.

 

Okay, that makes sense to me so far. What about the "S" wire? What is that and where does it go?
(Forgive me, but it has been at least 25 years since I owned a Ford or Merc with that solenoid)

 

Good info above. I reemphasize the importance of good, high-capacity paths to ground. Make sure they are robust, and you won't end up at the side of the road wondering what the hell went wrong.

 

Dave, you are correct. The only time the big wire gets power downstream of the the ford realy is when you crank the starter. If you look at the diagram, the little brown wire going to the "s" on the ford solenoid was originally going to the "s" on the starter solenoid. (note: the ford solenoid is actual a relay, not a solenoid).

The small brown "jumper" at the starter is to extend the starter solenoid at the exact time the big wire gets power.

You will not be able to tap into your starter wire for pwr. The power source for the rest of the car comes from the battery.

An additional benefit is the "hold in" power requirement is much less on the ford relay than the chevy solenoid. This also takes the load off the ignition switch.

As far as safety, relays are always good to use.

 

No, this is just a remote power standoff if you need to jump start a BMW. Also, I agree with tman - run a vent even with an optima.

 

I was just in the right place at the right time. There's a local fella here you may have met him at a swap meet up your way big yellow camper/parts hauler. Anyway he snagged a spool cheap, so he let me in cheap.

 

Okay, I am slow but I am getting the point: run a short wire from the "Bat" to the "S" on the starter so they both get the juice at the same time. Use the pink wire to power everything in the car except the starter. What I still don't quite grasp is where that brown "S" wire from the relay goes. Alternator?? Ignition switch??

I may be too dumb for this setup and have to stick with the old master disconnect switch and call it good!

 

SCORE......20 bucks at the local pick-a-part. Only one in the area that had a 7 series Beamer. Took me a bit to find the battery, it's not in the trunk but rather under the seat on the passenger side. Pry the seat off and you'll find it. The cable runs up the bottom right side of the floor board, under a plastic tunnel, held in with plastic pop-rivets every 6 inches. It runs up to the center of the passenger toe board below the glove compartment. Before you remove the little battery box, remove the battery cables, as the nuts on this one were frozen at first...you'll need the leverage while it's still bolted to the firewall.

We'll see how this works....now for the Optima battery.

 

Yeah, I'm a little confused on how to integrate this correctly into my current GM setup. The Ford solenoid mounts on the firewall area, correct? Does this serve as the cut-off switch?

 

It mounts to the inner fender on Fords & Mercuries from the 1970s, but I think they are talking about using one in the trunk, close to the battery, as a way of giving voltage to the starter only when starting and the rest of the time juice would be going from that pink wire to the fuse box and then to the rest of the car. I'm just not quite clear on how the wires are accomplished so that turning the key starts the car and the battery charges when the car is running?

 

I am a a loss as to why guys insist on using the frame for running the (negative) from the battery. You would never use a raceway in your home in place of a neutral wire now would you? So why do you guys not do it right and run negative conductor were it must go to complete a safe and solid circuit? If you insist on using the frame for grounding? things, use it for your smaller load conductors and not the main negative that must carry a good 100 amp load! Bond the chassis to the negative and run all your other negatives to the chassis. Man, don't use your chassis for a 100 amp load carrying conductor!!
Dom

 

The Beamer had the negative grounded to the part of the floorboard directly under the back seat. If it's good enough for them, shouldn't it work here?

I've always thought of the frame as being part of the chassis. "In the case of vehicles, the term chassis means the frame plus the "running gear" like engine, transmission, driveshaft, differential, and suspension." Definition I dragged of the interwebs :/

 

Hey Ross, it's just me being an electrician for a living and i just can't my arms around the idea of using something other than a wire for a wire! Wouldn't the negative from a battery in the engine compartment be attached to the engine? Or is it to the chassis?

 

Try this link

http://www.s10racer.com/board/showthread.php?t=282

The idea is not to run 15 feet of 1 gauge that is always hot

 

Only thing I can add to this thread is something that I didn't think of in time to use on my roadster build unless I wanta crawl under the car, grind off several coats of paint, do what I'm about to suggest, then repaint.
The suggestion is on a full frame off build that in several strategic spots, weld a SS bolt to the frame, welding the hex to frame with the bolt sticking out in a place of non interference. Then when you start wiring you've got some good solid ground posts available.
Dave

 

I agree with 52 Hardtop. Also an electrician for too many years. You want to have as little resistance in a circuit as is practical. Resistance = heat, heat increases resistance. Copper is used to make wire because copper is an excellent conductor of electricity. Steel doesn't conduct nearly as well as copper. In a DC circuit, the negative post of your battery is "ground" and must be connected to complete the circuit. Introducing 12' of steel to the circuit increases resistance, not to mention passing current through
possibly rusty riveted joints at crossmembers, etc.
IMHO, cheap insurance for trouble free starting and ignition operation is to run a neg cable directly to the engine using a bell housing or starter bolt, from there to the frame and body. This always works. I use Kopr-Shield compound on all my battery and ground connections, even on some of the larger circuit terminations. Also a good idea to run a ground to the dash area. I don't ever have electrical problems due to wiring or grounding.

 

No, in a BMW it is a terminal post where the battery, starter ,alternator, and main feed all meet together.
There is also a post to hook jumper cables to.

I've got a BMW for circuit racing, the rules require a battery cut-off that instantly shuts the engine.
We simply fit a cut-off switch in place of this terminal box [a bolt in ], the only thing to remember is to connect the alternator wire to the battery side of the switch [ so the alternator doesn't loop back and keep the engine running ]

My suggestion about cannibalising a BMW for the cable should seriously be considered. the cables are quite heavy and suitable for the job.
The local wrecker in NZ sells me the cables for $10 [ they already have the ends fitted from factory ]

 

HMMM-----Thats strange!---Been running mine that way since converting in 1996, now reading 75,780 miles---NO VENT!!!!!!!!---------Don

 

I use the frame of the car for the ground as well...never a problem in 30 years of having my customs with a battery in the trunk! As for comparison to a house, it doesn't jive. Your whole house is not a ground circuit. Your car is metal, and IS. To do it correctly, though, I also run heavy groundstraps from body to frame, frame to engine, and engine to body. The frame to engine, I usually do right at the bellhousing, and use a #4 Battery cable.
As for how the Ford solenoid works, you use that small wire from the battery center post of the starter, and jump it to the starter terminal. Now the starter will run all the time that there is power in the battery cable.
The Ford solenoid interrupts power to the starter, using the battery cable, so it is "dead" all of the time...that is, until you hit the starter button. Then it energizes the cable up to the starter, which immediately starts cranking. When the starter button is released, power to the main cable is stopped, also stopping the starter from turning.
Only drawback on some cars, is that you need to reroute the main feed to the fuse box, as it comes from the battery post of the starter.

 

Already been mentioned, but, make sure that you vent the inside of the Battery Box to outside the car, with a sealed tubing of some sort. Not just into the trunk. you need the gases to escape into the atmosphere, not into the car.