Hot Rods Battery in the trunk

I agree welding cable sizes can be very confusing. I just found this nice size chart on the internet this morning. Maybe this will help some of us confused people. I use wedding cable on my trunk mounted 39 battery. Running a ground wire all of the way the engine block and making sure to have clean non painted connections cured my hot start problems.

 

Wire sizes can be confusing but once you start using it there are only a few you will ever need. 8,10,12, and 14 (8 being larger than 14) gauge are what you will use to wire your car. The lower the number, the more amps the wire will take. The larger gauges 1/0, 2/0 are used for battery feeds.Most commercial battery wire from auto supply is #1. Many people confuse #1 with 1/0 but they are very different. Commercial wire runs from the largest 7/0 (which I have never seen and can't imagine) , to the smallest number 46 (like a human hair). It goes numerically from the smallest number (7/0), being the biggest. ModelT1 yes, 2/0 is larger than 1/0. Welding cable is ideal for battery runs if you are willing to spend the extra money for it. I think you are really making too much over wire size, you will never need anything other than what I originally stated. There are charts available as to how many amps each wire size can take and that should be your guide, when wiring your car. Also remember that a direct feed from your positive side of your battery to the starter is not fused so you never want to under size it. If your feed is hot to the touch your wire may be too small.

 

39guy thanks for the chart. Pictures help us confused people. Also thanks to everyone who commented on this. It seemed so simple but wiring is never easy. I've wired a lot of cars, houses, boats, and other things. Yet these questions come up. Just looking at the weights on the chart shows what is larger.
What's amazing is most of us with a trunk mounted battery have them in a 39 or 40 Ford. I know others have it that way but for some reason we are the most confusicated or at least admit it.

 

Thanks Squirrel, I'm not losing my mind, not yet anyway!

Pat

 

[QUOTE="R Pope, I put a Ford starter relay in the trunk so the big cable was "hot" only when the starter was engaged. /QUOTE]

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That's a Great idea.

I'd still probe around with a voltmeter while cranking before buying new cable, and after, if voltage drop measurements were too high indicated a new cable should be installed.

 

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http://www.usawire-cable.com/pdfs/nec ampacities.pdf
The notes for the chart say The "ampacity" ( safe amperage rating) is based on temperature rise, not the voltage drop.

http://www.affordable-solar.com/Learning-Center/Solar-Tools/wire-sizing

 

Wire sizing for use in buildings isn't quite the same as wire sizing in cars.

 

So what is the best way to wire the battery if it is placed in the trunk? Is there only one way to do it? No. Aircraft use the frame for the return ground (negative) to the battery. There are a couple of reasons for this. First on an aircraft the reason is weight savings. Second, with the frame being the return ground to the battery, only one wire is required to hook up to the negative side of the battery. This goes to show that running a long cable from the battery to the starter and a short cable from the frame/body to the battery is a very efficient way of wiring a vehicle.

 

That idea may or may not have originated by Mark Hamilton of M.A.D. Enterprises, but I first saw an article by Mark explaining/showing his design many years ago, and is how my '32 is and '40 will be wired. The batteries are behind the seats (not really in the trunk) along with a Ford starter solenoid. The long starter cable is hot only during starting; then a large gauge wire feeds the entire car from a master disconnect and through a 100 amp fuse to power distribution.
Mark's design was intended for drag racing, but works very well with any rear mounted battery, IMO.

 

Actually, it is.... Ampacity is ampacity, no matter the use. The major difference between vehicle use and building wiring is factoring in voltage drop. Voltage drop is the real elephant in the room when designing a vehicle system. You'll have exactly the same voltage drop in either place under the same amp load with the same wire size, the big difference is if you lose 1-2 volts in a 120 or 240 volt circuit, the percentage drop is under 3%, in a 12V system you're talking about at least 10%, maybe as much as 15%. That is definitely enough to affect circuit operation. And that welding chart is worthless for sizing wire; that's for welding, not for anything else. Use that chart at your own peril....

The 'typical' 12V automotive starter draws about 250 amps when cranking. But this could vary from between as little as 150 amps for a low compression 4 banger to as much as 350 amps for a high-compression big block. Let's do some examples....

A 150 amp draw with 20' of number 4 cable will result in a .75V drop for 6.25%. Increase the draw to 250, and now the drop is up to 1.25V and 10.3%. Go to a 300 amp draw and now you've lost 1.5V for over 12.4%.

Bump the wire size to 1/0 with all else being the same, and now the numbers are: 150 amp draw 2.4%, 250 amps 4%, 300 amps 5%, 350 will be 5.57%. Much better, but you generally want to limit voltage drop to a maximum of 5%, particularly with motor loads.

Go to 4/0, and now you're at 150 amps 1.24%, 250 2%, 300 2.4%, 350 2.8%.

One very good reason for limiting voltage drop is it will reduce the current needed. Any motor is producing power, i.e. horsepower and a certain amount is needed to do the work. A starter that draws 250 amps at 12V will need 272 amps to produce the same power at 11 volts. Note that all these numbers are ***uming an ambient temp of 86F; increase the temp, and both voltage drop and current draw go up. If your battery cables are noticeably getting warm when operating the starter, they're too small....

 

The larger sizes go up to 4/0, then it switches to 'MCM' (thousand circular mils). Sizes start at 250 MCM and go up in increments of 50 MCM until it hits 800, then jumps up by 100 to 1000 MCM, then jump by 250 MCM to 2000 MCM (the very largest size made). 'Common' sizes are 250, 300, 350, 400, 500, and 750. 500 MCM is the largest size you'll generally see in copper (3/4" diameter), 750 in aluminum (1"). 2000 is 1.75" in diameter! The largest I ever used was 1000 MCM copper (for a 'cosmic ground' in a telephone building), the stuff was a ***** to work with....

 

Okay, I understand the requirement for a big cable going to the starter. Should the battery's ground cable be sized accordingly ?

 

Buckster; Yes, the ground cable should be the same size as the positive cable. The amperage has to go out, and come back, to the battery. The size of the cables are that large to accomodate the starting circuit. I fit weren't for the starter, we could get by with a much smaller cable to power our cars.

Roger

 

One problem I ran into in my last car, when I mounted the battery in the trunk, was on long trips, the heat from the exhaust and muffler, heated the battery to such an extent that it boiled the electrolyte out of the battery.
On my current build I mounted the battery 6 " off of the floor of the trunk, and no more heat problems for the battery.

 

Forget all the "chest t******* here" and go to your local U-Pull and buy all the cables from a BMW E36
E36's have a trunk mounted battery [and the German Engineers have done all the hard work for you]
While you're at it , pull the battery jumper box that fits on the inner fender [ this is where you hook up a jumper cable ]

 

#8 wire is only rated to 50 amps so the wire is likely to get very hot at 100 amps let alone 150. If the system gets a total short, the wire might handle the current for long enough for the breaker to trip.

http://www.cerrowire.com/ampacity-charts

 

Yes, you find lots of BMWs in American U-Pull facilities LOL. It takes a lot more power to crank a 350 cubic inch Chevy than the 3.2 liter which was the biggest BMW engine.

 

Been using welding cable for years but I always solder the connections not just crimp em and make sure the solder fills the cup never had any problems run ground to frame closest to batt and heavy connection to engine from frame most problems come from corroded or poor connections

 

What, you don't get 20 year old Beemers in junkyards over there?
We're stuck on a "poverty stricken rock in the south pacific" , and they show up in junkyards over here

BMW 750i's also have trunk mounted battery but the E36 cables are shorter .
It is the sort of part that nobody wants so it is cheap.

I used Beemer E36 cables on my Road-Race Corvette [ .120 over 454 with 12.5:1 compression ] I even loaded it on the trailer using the starter once.

The beauty of Beemer Wiring [in my case] was I needed a Battery disconnect by the lower RH windshield because of the rules.
I simply unbolted the "Jumper Block" and replaced it with a Battery disconnect switch. No crimped terminals are needed.

 

Seems like we've had this conversation before, but this is a great thread (for those of us who want to mount battery in trunk). Please allow me to summarize, and correct me if I am wrong:

OK, so it seems that as long as the correct wire size is used, and stud bolts are properly welded to frame, paint is removed, connections are properly made (per above), etc., then it really does not matter the ground path from battery. Battery to frame first, or battery to block first, does not matter, correct? As long as good connections are made with correct wire size between frame, block, body, starter, and battery.

If so, then once the proper grounding is accomplished, then voltage drop does seem like the elephant in the room:

Wire size, wire size, wire size . . . connections, connections, connections.

Oh yea, it helps. Great set up.

 

As long as the battery is connected properly it will work. There must be a complete ground path back to the battery, the ground is as important as the power. By using the frame, body, or other metal object, you save running a new wire that's all.

The ground may be fastened by a screw or bolt as long as it makes a good connection. It does not have to be welded.

 

My old roadster had the battery in the truck,I used a boat battery storage box to keep it tidy. HRP

 

82 of 'em out here at the Pick N' Pull in Yuppieville. The roach coach out front has wine and oysters on the half shell.

 

I am not going to argue anything but I'll tell you what I have been doing for the last 40+ years that works for me. I run the ground out of heavier cable then the hot and run it as short as possible. I have been known to run my hot with 00 welding cable. I prefer finer stranded wire but it is just my own personal taste I suppose. Get the place that I land the ground as clean as possible, run good grounds straps from my motor to the body and ch***is and run with it.

I have studied electricity pretty close. Close enough that the company I worked for in the '90s sent me to an electrical course as a wringer to evaluate before they spent money sending electricians. We could talk theory until we are blue in the face but I prefer real world experience to book knowledge. Don't you?

 

So you read this in old school rods and it's gospel!,really?

It appears to me you just haven't got the answer you want to hear.

You ask your question here and people I respect have given you thoughtful and correct answers relying on years of first hand knowledge,you can opt to use the information offered here or a article written in a magazine by one guy suggesting he has the only correct way of doing this task. HRP

 

That would be a millennial with the only correct way to do it. The reason that rag is so far off base as a rule is it is written by people who were not there and don't know anyone who was.

That said it is becoming a common thing for someone on the HAMB to have read something and think it is gospel even if someone else knows that it isn't. Doesn't matter knowing a different way sometimes gives this dumb old **** an edge even if he went about it the wrong way.

 

Pretty sure the engine in the 850 is bigger.

You know, it being a 5.6l V12.

Sure, it's no 5.7l V8....

 

Wow, much ado over nothing. I'm working on 2 very well-healed cars from long ago. One's a J Duesy, the other a Packard. Both are grounded right to the frame at the batteries (one on the side of the frame under a cover, one under the seat) and then off to the starter relays/switches and main power areas from that. Grounds are sometimes within the harness and others are in the open, woven cables being used to "bond" the ground to things that are insulated for noise or vibration. Come to think of it, where's the damn ground on a Model A? They've been running that way just fine since 1927 (late 27 for the 28 model year so no correction needed). All this double wire-to the starter-extra solenoid-relay everything-blah-blah-blah, why? Do what the **** they did back then because it works awesome today just like back then. Best advice on this topic, use good cable of proper size and keep the connections clean. Star washers were mentioned, hell yes use em, but under the bolt ON TOP of the connector so it can't move around ON THE FRAME OR BODY where it's clean and sealed with grease for corrosion resistance.

I'll review too. 1 big wire from the trunk to the power lead, nice grounds from the battery to the frame, or body if it's unitized construction, ground everything else to it. Magazines? Most are just ad flyers with car features, or like TV, entertainment. I will say that much like a blind squirrel (not you brutha) there's an occasional nut found but it sure ain't gospel. Those are TJJ #2 and #3.

 

sometimes a bad ground is as simple as paint....last year I could not get the lights to work on my car trailer....the culprit? I painted the hitch and the portion that slides in....I only did this for aesthetics.....but what a PITA.....some alligator clips helped diagnose the issue and I found myself sanding down the hitch when I got home.....didn't notice when I left in the morning, but driving back home that night would have been very dangerous with no trailer lights....

 

Lots of old 6V cars used a woven ground and they were grounded to the frame. I have driven old fords that had firewall mount batteries and were grounded to the body. The operative word here is driven. One may note that 6V cars are known for the starter being a little anemic, that is because they pull a lot of amps. You don't drive a car that won't start.