Alan

Dr Bob - yes you could intercept the feed at the jump post. There is a feeder to the alternator and 2 feeds to the CE panel top.

The alternator is connected directly to the starter.

Some issues:

ABS hydraulic unit (pump & valves) connects to the starter
ABS control unit connects at the battery with a seperate feed
LH ECU connects at the battery with a seperate feed
EZK ECU connects at the battery with a seperate feed
Cooling Fan 1 connects at the battery with a seperate feed
Cooling Fan 2 connects at the battery with a seperate feed
PSD pump connects at the battery with a seperate feed

You see why I don't like the way Porsche supplies power.. BTW several of these are not fused at all - most that are get fused remotely from the feed (e.g. @ CE for Fan1/2 & ABS Control), NOTHING protects the feeder (fusing 101 obviously got lost in the mail...).

An advantage of your proposed location is that it will safely cut off the engine (but it basically does this as the ignition switch would) - a single rear mount switch cannot do this safely. I don't think it would meet most racing battery cut-off requirements though...

It also leaves a quite large list of possible current consumers still connected. However you can always test if it will work for you with a temporary switch install - if no good - put back to stock (All - careful of those jump post wires though - you can start fires & do impressive welding with them...)

So it really depends on the purpose.

For most you'd wan't to shut everything off to be sure to be isolated so you can work on the car - or so there are is no leakage. So best to turn off everything @ battery ground (or +ve but Gnd is just more convenient).

For racers you need a way to externally kill the engine & shut down all electricity feeds to avoid sparking - and most especially the fuel pumps. The best way I can see to do this on a stock configuration (so the alternator does not continue to run the car) is to use a dual battery switch at the rear that cuts power from the battery to the ECU feeders (also shuts doen the fuel pump), and that also disconnects the battery from all the other feeds. The Alternator will continue to generate power until the engine stops and could possibly damage things with no battery connected... but its only for emergencies...

I see no way to immediately stop all power short of also 'crowbar'-ing the alternator and I don't think any racing orgs would require that. It would almost certainly destroy the alternator if you did.

Alan

Ed Scherer

Carl, you've got Gustav Kirchhoff spinning in his grave...

Daniel Dudley

Carl is correct about connecting to the negative. Think resistor.

docmirror

<edit> Ectually, as mentioned way back, the negative is the right place to use it, but not for the reason given.

Jim bailey - 928 International

The better cut off switches use a resistor grounded to the chassis to drain the alternator charge when the battery is disconnected..."F.I.A. Battery Master Switch

The only one to use if you are going to compete in your car!
Has facility to 'dump' alternator load when isolator is operated"

Alan

I'd assume it's connected to the field exciter circuit (61) which is a reasonable way to try to gently ramp down the alternator output - though how the alternator handles this loading is dependant on design - it could possibly blow the regulator diodes.

I'm pretty sure this switch cannot sink more than perhaps 20A though its resistor - so their comment "dump the alternator load" is misleading at best... probably closer to say "shutdown alternator voltage generation".

An AFD switch integrated into the battery switch is a much better way to do it - but requires an external regulator with a rotor field circuit you can intercept (...& of course we don't have one).

On a real racer you'd just reconfigure the whole wiring to make it easier - rather tough to do on a street car...

Alan

Carl Fausett

Again - consider cost and application. This switch is not designed to be or to replace a master cut off switch for racing. You really should not compare the two, their purposes and hence their design is different. If you install this switch in the negative battery cable, and use it occasioanaly, and turn it fully in and out when you do use it, it works fine. I have many of them out there with no returns!

But - if you are going racing - get a instant-on, instant-off water tight fender mounted kill switch like your sanctioning body requires. But again, thats a different application.

Sorry to disagree with the 'electricians" in the group, but the draw after the load is less than before the load. The load ( a fan, a motor, a pump, whatever) converts some of the electrical energy to "work" (heat and friction) and the current after the load will be less as a result. I stand by what I said.

John Veninger

Carl,
The current draw is the current draw, your confusing voltage drop.


Not an electrician in the group, but do have a BS in Electrical Engineering (not that I use it much)

Alan

Carl - enough of this - get a meter and measure it - it will be EXACTLY the same - or maybe try replacing your battery ground strap with a 16AWG wire - after all you think the starter is consuming all the current...?

Just test it and stop spewing this crap - do you really think we are either stupid and don't know what we are talking about OR just like baiting you?

You just don't get the fact that current does not get consumed in transferring energy (how would the electrons disappear? & what would you be left with?).

Here is the possibly helpful water analogy:

If water running downhill turns a water wheel - energy is transferred - but the same amount of water is still in the channel after the wheel - none of it "disappeared".

Just read some real electrical theory: esp. Kirchoff's Current Law, understand the difference between voltage, current, power, charge(coulombs) & energy(joules)... it may eventually start to make sense...

And really enough - I won't argue with you on areas of your expertise - if I say something and you tell me I'm wrong - I will just accept it and STFU.

Alan

dr bob

Carl, some of us "'electricians' in the group" ar EEs, and a few are even qualified to teach. Remember if you will, the design of a single-element circuit and the current flowing in a loop. You have a source of current (battery in this case) and a flow of electrons between the battery and the load. Electrons flow from the negative battery terminal, through the load at a rate determined by the EMF (measured as voltage) at the battery, and the resistance to current flow (measured as resistance to current flow) in the -total- load circuit. That circuit includes your target load device and all cunductors in between. Every electron that leaves the battery at the negative terminal comes back to it at the positive terminal. At any point in the circuit, the flow of electrons is the same as any other point. If you choose to use a smaller conductor in one section, its resistance to current flow will be higher, causing heat. Since less current flow through the circuit thanks to that resistance, the current in the whole circuit is reduced.

As an experiment, wire a light bulb, a common auto light bulb, through your ammeter, and connect the circuit to a battery. Try it first with the meter on the positive side, then reverse the battery connections so the meter is between the battery negative and the bulb. See any difference in ammeter readings? I'll go out on a limb here and suggest that the meter readings will be exactly the same.

Since wire ratings are normally determined by current flow and total resistance allowable in a circuit, you really want to have adequate current-carrying capability on both the positive and negative wires. There may be some validity to the idea that the total resistance in a negative wire should be the same as the total resistance in the positive wire. Using that philosophy in a car where the negative wire goes to a local frame ground rather than all the way back to the battery, you might be able to justify using a smaller gauge wire for the shorter lead. Where the fun starts though is in conductor heating, and the ability of insulation to survive the extra heat generated. Ground wires in the car are typically out in the open rather than in bundles/harnesses/looms, so insulation ratings are different. It can get rather complicated, as you can see.

But in the end, the current flowing through the ground strap at the battery is EXACTLY the same as the current flowing through the positive cables to that same battery. Same as the current flowing through all your loads, including the wiring and other conductors like the car body.

HTH!

docmirror

I understand where Carl is coming from, it's a hard thing to equate states of energy, with the physical constants of the atomic model. For a very brief explaination, think of the electrons in a battery as ants. They are happy to lay in the battery until something upsets the antpile. Then, they 'get up' which would be equivalent to the electrons changing their valence state. Now, these 'excited' ants/electrons start marching out the exit(negative terminal) and march down the wire to where they are needed. The electons have to stay in line along the various conductors, but they remain excited(higher atomic shell) so they are easily moved from one atom(really, molecule) to the next.

When they get to the motor, or whatever, the electons all work real hard together to pull the armature around. Then, they get to take a break, reduce their excitement, go back in their shell(lower valence) and return to the antpile(battery). Now, the ants/electrons don't die, or disappear when they pull the armature around, but they do get tired. This changing of excitement is where the law of conservation of energy comes in. It takes energy to move the electron from it's resting shell, up to a higher shell, and yet more energy to get it off the atom, and moving to another adjacent atom. However, this energy is quantifyable in terms of motion and heat. Heat being the byproduct in the case.

So, each ant/electron that leaves the negative terminal gets to go back to the antpile/battery after his exciting trip out into the world. All electrons are accounted for, or at least I would say prolly twenty 9s worth. Maybe a few electrons get deposited on the stator, but for the most part all of them get home.

while E = MC^2, we're not exciting the matter to the speed of light squared, all we're doing is moving it down a wire, in a slightly excited state. Please don't confuse electron movement and excitation with electron consuption.

Now, ask me how a radio station transmitter works.

Carl Fausett

I've said it before - the "Shark" is not the car, its the Owners.

worf928

I think you forgot to work into your analogy that these ants are kind-of glow-in-the-dark ants. (Or some other suitable mechanism to account for the photons...)

dr bob

I need a box of five-ant fuses, please...

worf928

Arg! That one's going to bug me all day.

Good luck finding seven and a half-ant fuses...