[DeTomaso] Thoughts on grounding, remote-located batteries etc.

[MikeLDrew at aol.com]

Hi guys,

Below is a well thought-out post regarding the mechanics of relocating a 
Shelby Mustang battery to the rear of the car.   The issues raised in doing 
so, are basically the same as the Pantera has from the factory, with the 
battery up front and the engine etc. in the rear, and resultant long cables.

The author is a real tech wonk, and is not prone to pulling ideas out of 
his anal orifice.   His words below probably should be considered well by 
Pantera guys.

Cheers!

Mike

====

Don writes:
 
To understand this, you first need to understand where “Ground” is. Where 
this seems like a simple subject, but in my professional career as an 
Electronic Engineer, I have seen more mistakes made on “grounding” than almost 
anything else I can think of. Probably (as a group) one of the few set of 
folks who understand this best are Electrical Engineers who spent time at the 
working level of PG&E, Southern Calif Edison, or some outfit like those. Now, 
the basic definition of “Ground” is just that … the earth. Which is what 
the British call it, “Earthing.” For cars, of course, from a practical 
viewpoint, this doesn’t work because cars aren’t hooked to the earth very well.
 
So, for a mobile device, like a vehicle, then the CHASSIS is GROUND. 
Period. End of statement. The negative terminal on the battery IS NOT GROUND, it 
is merely supposed to be hooked to ground with the shortest wire practical 
(that keeps the negative battery terminal at least close to ground). For body 
on frame vehicles, the frame is ground, and the body needs to have a short, 
fat wire going from the body to the frame. For unit body cars, it’s the unit 
body.
 
The rub on understanding automotive grounds is muddied up by that 
convenient thing called a starter. The inrush load (the max current the starter draws 
is when it is first engaged) the starter puts on the battery runs roughly 
from 300 amps to 600 amps (like on your high compression 427). This is the 
maximum current required of the battery but it’s only required on a temporary 
basis, like when you start the car. This requirement makes it practical to 
put the battery close to the engine and run the negative battery terminal 
directly to the engine block with a short fat wire. The important thing for the 
starter, is that you want the electrical resistance of the + and – wires to 
be real low. At 600 amps, 10 milli-ohms ( 0.010 ohms) of resistance will 
result in 6 volts of drop. Then you often find a wimpy “g round wire” from 
the engine block to the frame or chassis … and now you get confused about 
where ground is. And then you forget to hook up that wimpy ground wire from the 
engine to the chassis when you R&R your engine and then wonder why you 
lights are dim and variable as you drive down the road in the evening. You 
thought the engine block was ground; well, in fact it was for you, but the CHASSIS 
SHOULD BE ground. That means that the wimpy wire from the engine block to 
the chassis SHOULD BE a short fat wire.
 
This grounding thing wasn’t terribly important on old cars (woops, I mean “
Vintage” cars) like early Mustangs and such. But on more modern cars with 
electronic fuel injection and high-zoot elect ronic ignitions that you can 
program with your laptop, this grounding thing becomes important. Why? Because 
these devices are grounded from their chassis to the car’s chassis wherever 
they are screwed together plus (usually, hopefully) a short wire from the 
box to the car’s chassis ground.
 
A good friend of mine, who is an excellent Mechanical Engineer, race car 
constructor and who was once an SCCA National Champion in his class, had a car 
with a new electronic ignition (the kind with a bunch of screw-driver 
settings for base timing, beginning advance RPM, advance rate and total high RPM 
advance) and he was complaining that this ignition was a piece of crap 
because the engine popped and farted when he put his foot to the metal. I asked 
him, gee, where is ground on your car? He showed me this fairly long 16 gage 
wire going from the ignition box to the chassis. So, I said, you mean the 
Chassis is ground? Yup, sez he. OK, sez I, where does the battery – terminal 
go? He points to the fat wire going from the battery to the engine block. So 
I query, where is the wire that goes from the chassis to the battery or the 
engine? Ummm, well, it goes from the drive train to the chassis sez he. 
Ummm, just where? Sez I. Finally, he points to a 16 gage wire going from the 
engine to the chassis. So, I said, do 2 things: run a 4 gage or larger wire 
from the engine to the chassis, and run the shortest ground wire you can from 
the ignition module to the chassis ground. He did and the ignition was just 
fine after that. The circuitry in that device just could not tolerate the 
voltage spikes in the ground wiring that cause IR drop as well as the 
inductance in the wiring (v=L*di/dt) caused by the primary ignition coil current.
 
So, bunky, ya wanna move yer battery from the engine compartment to the 
trunk, eh? If you look at wire sizes, your basic 16 gauge AWG (American Wire 
Gauge) wire is about 50 mils in diameter, 4 ohms per 1000 feet ( or 4 milli 
ohms per foot) and good for 10 amps max. A 2 gauge wire is just over a quarter 
inch in diameter, is rated at .16 ohms per 1000 feet (or 0.16 milli ohms 
per foot) and is good for 125 amps (continuous current). So for 10 feet of 2 
gauge wire from you trunk to your starter, that will be about 1.6 milli ohms 
total. That will give you 0.8 volts of drop for a 500 amp surge current from 
your starter. If you ALSO run 10 feet of 2 gauge ground wire, then you have 
1.6 volts of drop. You can see from these numbers that maybe the 
connections might be important too. Now, for the chassis ground, compared to 1/4” dia. 
of the 2 gauge copper wire, you have this (relatively) enormous iron cross 
section of the chassis. To give resistivity comparisons, copper is about 
1.77 u ohm-cm, iron is about 10 u ohm-cm (or about 5.6 times as resistive as 
Cu). Aluminum is 2.83 u ohm-cm.
 
So, I recommend that for a trunk mounted battery, that you run from the – 
battery terminal to the chassis a short 2 gauge wire AND that you prepare the 
chassis surface of the mild steel sheet metal with a washer at least 1” 
diameter that is copper or brass soldered to the chassis … or a steel washer 
brazed (preferable) or weld ed to the chassis. Obviously, the paint in this 
region needs removing down to bare metal prior to soldering, brazing or 
welding. This washer is intended for two reasons: 1) as a resistance or current 
spreader into the relatively thin sheet metal, 2) a good low ohmic surface for 
the terminal on the 2 gauge wire. A bolt ( at least 3/8”) with a similar 
size washer on the other side of the sheet metal to mount the end terminal to 
the fat wire to the chassis with a nut to provide compression force. Now, up 
front, you need a similar connection from the chassis (ground) to the 
engine block via a 2 gauge wire. Then you need a 2 gauge wire from the + battery 
terminal to the starter. This wire should (in addition to its normal 
insulation) be inside of one of those plastic wire routing tubes and should be 
attached in several places with adel clamps so it can’t flop around. In 
addition, for any pass-throughs on sheet metal bulkheads, there should also be a 
rubber or nylon pa ss-through chafing ring to prevent the 12 volt wire from 
ever shorting to the chassis. For the chassis +12 volt connection, I recommend 
running a separate wire from the battery to the original chassis connection 
up front where the battery used to be. Why? Because there is substantial 
current running to the starter when this circuit is interrupted (the engine 
fires and you disengage the starter; you get v=L di/dt voltage and this can 
result in voltage spikes up to 24 volts … which can fry electronical goodies. 
So, please don’t attach chassis +12 volt connections to the starter relay 
terminal when the battery is in the trunk. Run a separate wire that is 8 gauge 
or bigger from the battery to the +12 volt chassis connection up front. Any 
place you cut the 2 gauge wire, the end should be stripped and the connector 
soldered on … and let the solder run through all strands of the end of the 
wire; this will keep the connector resistance as low as possi ble. If you 
want to do a nice job of this you can also get “shrink tubing” from an 
electronics store. Put the shrink tubing on the wire before soldering the 
connection, and after soldering, slide the shrink tubing over the connection and 
apply heat with a “hair dryer” to shrink the tubing around the connection. An 
actual hair dryer will work although there are industrial heat guns that are 
called “hair dryers” (don’t try drying your hair with one of these 
babies!).
 
If you want a “Kill Switch” it will need to be placed along the 2 gauge 
+12 volt starter line and should be accessible from outside the car (for 
corner workers access) and if convenient also accessible by the driver. F or 
alternator equipped cars, if already running, the kill switch will not result in 
the engine quitting as the electrical load will now be supplied solely from 
the alternator. So, your Kill Switch must also have a terminal to short the 
alternator field connection to disable the alternator, then the engine will 
also quit. Some folks recommend the addition of a resistor on the switched 
line to shunt any residual current generated by the alternator to ground. In 
addition to the starter, the chassis +12 volts needs to be on the “switched”
 side of the Kill Switch. For this reason, the Kill Switch should be fairly 
close to the battery (even if the driver cannot get to it). In case you are 
wondering why the Kill Switch should switch off the starter, if you have an 
engine fire, sometimes the starter will engage and it will not stop with 
the ignition switch, so the Kill Switch would be the only hope left. And if 
you can’t stop the starter, then the battery, or wirin g may initiate another 
fire.
 
Since I’m a bit of a cheap skate, one good source of 2 gauge wire is jumper 
cables, get them long enough for your installation and you will find the 
cost hard to beat. This way I run one wire for the starter, and the other one 
for the chassis + 12 volt connection. They route in parallel most of the 
way.
 
One other thing, if the battery is in the trunk or in the driver’s 
compartment, then the battery needs to be inside of a box. Note that complaints we 
re received on ’65 Shelby GT350s that had the battery in the trunk, 
complaints were the smell of battery acid by the driver. As a result ’66 GT350s left 
the battery up front. Also, you get hydrogen emanating from lead-acid 
batteries, which is a fire/explosion hazard. So, the battery should be in a “sealed
” box, preferably plastic so the battery fumes won’t cause corrosion of 
the box. Well, almost sealed. There should be a drain tube going outside the 
car. I also recommend having another tube running to inside the box where the 
other end of the tubing is placed in a positive pressure area of the car (I 
use the cowl plenum); what that does when the car is moving you get a 
continuous circulation of new air into the battery box.
 
Something real simple like moving your battery to the trunk gets kinda 
complicated.
 
That’s all I can think of at this sitting.
 
Regards
Don Wollesen