Technical How to wire a battery to the trunk

Lothian's ground wire strategy is good. The Vdrop thru the frame may surprise, as more than if copper was run fwd.

 

Not to be arguementive (spelling minus 5), but the mass/volume/electron path of the entire frame and body are no problem at all in providing plenty of "ground" for everything on the car. But that's just me........
https://49fordcoupe.smugmug.com/Wiring-the-Car/

 

....................You are probably correct if the frame is fresh and new, but if it is 50-60- 70 year old metal or older, like what we often deal with, metallurgical changes at the molecular level would predicate that a fresh, isolated ground line run from the battery to the front of the frame rail and subsequently to the bellhousing might be beneficial. I completely understand where you're coming from and I've done it both ways. I just feel like it can avoid possible issues with conductivity by doing it as I described.

 

The single 2-month long self-made misbehavior on my previous build? I assumed the trunk Neg line would save weight/copper if I went direct to the frame. AND picked up forward near the firewall to ground the electrics on the engine side of the firewall. Bad move! The voltage (forgive my less-than-Trad. gear used) when Starter engaged dropped to 8.8V...! My ignition system would promptly take the day OFF.

 

Good news, putting a battery in the trunk is now officially obsolete.
Here's why. You can go on amazon.com and get an anti-gravity ATX 20 lithium ion battery for 370 bucks. it is 7 inch by 7 inch by 3 & a half inch and weighs 4 1/2 lb. It will charge off of your regular alternator and will spin that big motor over like the spark plugs are out of it. You can mount it down on the frame rail under the floor as close to the starter as possible then you can get by with your number two cable. You can put a disconnect switch right up through the floor so you can reach it from the driver's seat. This battery weighs less than the cabling that you would run to the trunk and is cost-effective compared to a lead acid battery and all of the cables to run to the trunk. because they last 10 years.

 

I'm still working on this. THANK YOU all for the information!
I think that I will use a switchable breaker for the main power line from the battery. That way I'll have a switch and be able to reset if I somehow dead short instead of dealing with a blown fuse. My question is what amp breaker to use?? No big stereo just that gear drive starter and fairly high compression.
https://www.delcity.net/store/EATON...(Switchable),-Surface-Mount/p_823674.h_821984

 

It's not really relevant, but our 8.9L Cummins routinely see 450 to 500 amp starter draws when tested. Normal shop trucks see 200 or more with a little age on them. I wouldn't go less then 200 amp if you are installing between starter and battery.

 

x3 on welding cable and on Lothian suggestion of not using frame for battery return path. Use 2 single 0 welding cables and I put the disconnect switch in the passenger compartment on the ground side as was recommended by the wiring harness manufacturer, after thinking about it for awhile.

 

Remember electricity travels on the surface not in the wire. That's why those cheap jumper cables do not work well. I use 00 welding cables.

 

You're talking about skin effect, and that's not an issue with DC current. It's not even that much of an effect in AC unless the frequency and current get up there. This term was made popular by some elements of the audio crowd (referring to speaker cable where frequencies can reach 22 KHZ) but even there it's effect is so small as to be not measurable except by sensitive laboratory equipment (it certainly won't be audible). If you're transmitting megawatts, then it becomes an issue....

 

What size diode do you use? Should it be rated to max output of the alternator? ie 90amp

 

One point that needs to be addressed battery box or battery vented to the outside. Fumes from a battery are corrosive and explosive.

 

If you're running the late model F250 PMGR starter welding cable won't be necessary. I'm wiring my 545 with a PMGR with #2 and battery in the trunk.

 

While I agree that good grounds are important, we in the fire service have learned the star washers are trouble down the road, especially if water is applied regularly, such as on the frame where rain gets to it, or if in our case washed a lot. The problem is the star washers allow moisture to get in between the connector and the metal and will set up a situation for corrosion, leading to an open connection. Emergency vehicle technicians were warned about this several years ago through our seminars. I have experienced the problem personally. Just my .02, Bones

 

I could see how that could happen.

 

I was told once that you should clean up and re-establish all of your ground locations at least every 5 years. Sadly, most of us don't do that in the real world. We usually don't fool with them until there is a problem.

 

An old habit from building boats and boat trailers is using a silicone dielectric grease on ground connections to slow down corrosion of connections and grounds.

 

Hi guys, I noticed on page one that the installation of the cut off switch was recommended as being placed on the negative battery cable, however at tech inspection the race tracks require the kill switch (for a trunk mounted battery) to kill the running motor in the event of an accident, if the switch kills the ground wont the alternator continue to power the motor since it is still hooked up? My understanding is that this wont pass tech inspection or are you not worried about that? Just curious.

 

That is correct!
You don't actually need the diode at all [in my schematic drawing] It is only a safety measure for when we work on the engine at the track, accidentally shorting wires with a screwdriver etc.
A fusible link as shown is enough for safety .

The alternator should be wired so it always "sees the battery" while running. If the isolator switch is thrown [at RPM] it doesn't toast the Alternator.
In the unfortunate situation like an accident where the Alternator wire shorts out, the fusible link will blow.

 

Run a separate wire from positive side kill of the switch to the hot side of the alternator. I had to do that as my car would keep running. The tech guys would ask to rev your engine to around 2000 rpm and then hit the kill switch to make sure it shut off.

 

welding cable- get lugs at any auto parts store
I usually put 2 or 3 heavy grounds on with cable cut offs, then make sure your lights are individually grounded. Can't have too many.

 

See the last paragraph above yours. Wire the Alt charge wire to the Battery side of the Isolator switch.
This prevents damage to the Alt at RPM , AND the charge cannot loop down the ignition wire when the Isolator is switched off.
The only downside is this wire is Live, so be careful when playing in the engine bay. Or a faulty diode in the Alt can drain the battery.
For safety add a fusible link....... & to prevent battery draining AND safety when working on the car ,Add an inline Diode.

 

There is no reason not to use the solder on battery terminals, they are easy to install before the cable goes on the car and will give you the best, most permanent connection.

I read with interest the views on vintage frames and negative cables. I am another who keeps the battery in the rear and have always run the negative cable to the nearest possible frame location. I also run the shortest negative cable possible from the frame to the front of the engine. Neither of these cables has ever been longer than 10 inches. Never had an issue of any kind.

But then I'm often a step behind, thanks for the tip.

 

Solder battery cables??? Ohhh, Crazy Steve’s going to get you.......

 

And, it's usually cheaper than battery cable and a lot more flexible.

 

As a longtime electrician and electrical inspector I would use the NEC method for wire sizing at 125% of the amperage draw when the starter is engaged. I do not know if that applies to auto wiring but it is what I would do and I know I would be on the safe side. The wire sizing to the battery would really not be much of a voltage drop in a matter of 5 or 6 feet but the draw on the starter being engaged is what I would consider.

 

If you are worried about grounding to an old frame with the battery in the trunk you could easily run an additional ground to the front of the frame.

 

I never figure all that fancy stuff, for battery cables it’s alway 00 welding cable for starters and winches as long as it’s under 15 ft. Not much bigger or harder to run and Your sure it’s enough. Sometimes I use the frame, sometimes I run a designated ground, depends on the job. Always solder and crimp the cables and use heat shrink. Just my way, Bones

 

Exactly my point! A kill switch is wired on the positive cable. A disconnect switch may be wired on the negative cable but wont pass tech.

 

Always mount your battery width ways in a car, not length ways. It lasts longer. That's the way OEM`s do it. Or used to do it. Or is this a Myth.