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

Tue Sep 6 12:15:47 EDT 2011

The #4 wires I have running from my Pantera battery to ground and alternator
and starter are all of 1' to 2.5' long.
Been that way since 1985.
:-)
Yes, my battery is in the rear right in front of the right rear tire.
All upside except for the fact that weight has been moved from front to
back.
Oh well....

FWIW,
Kirby

On Tue, Sep 6, 2011 at 10:16, <MikeLDrew at aol.com> wrote:

> 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
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