why battery cut off goes on negative terminal instead of positive?
 

I bought a battery cut off switch, primarily as protection against electrical fires.

I'm surprised to read that it is supposed to be installed on the neg terminal instead of the positive. I guess in the event of a short and things starting to melt, disconnecting the negative terminal will have the same end result as disconnecting the positive, but can someone explain the pros and cons of either approach?

Don't know the answer for sure but since our chassis are negatively grounded and the disconnects usually pass threw sheet metal, you don't want the disconnect insulation to fail and cause a direct short to ground.

When you remove a battery from a car you should ALWAYS disconnect the negative terminal first and then the positive.
The reason for this is because as soon as you disconnect the negative terminal there is no way you can accidentally short the battery when removing the positive terminal.

As far as kill switches go it really doesn't matter because either terminal will kill power from the battery.

BUT.... If you put it on the positive terminal the battery negative terminal is still connected to the chassis of the car so if anything conductive comes in contact with the positive terminal and the chassis it will short the battery.

A lot of racing cars have a kill switch that can be activated from outside of the car... The idea is if you pull the switch, the electrical system in the car immediately dies.
In a crash the body of the car can become deformed enough to contact the positive terminal or cable clamp. Or the positive cable can be crushed inside the body somewhere.

There's a more practical reason, the (-) negative post is more accessible and it doesn't have the extra hot wires going to the (+) positive battery clamp to deal with. So you're less likely to have the "shower of sparks" when disconnecting the switch since you're further away from the fender well sheet metal.

+1 to Peter, he's right on.

I have my battery cut off switch hidden on my car. I measured the distance to each (-) terminal and had custom cables made to reach the switch. I use this switch to disable the car when it's parked too.

Steve

73 911 T MFI Coupe, Aubergine

it goes on the negative BECAUSE it does ....

If it works ... don't fix it ... don't ask why an orange is orange or would they taste different if it was called blue .... they are oranges in the end of they day ..

Have a look at this. It works great for me
http://forums.pelicanparts.com/porsche-911-technical-forum/523892-inexpensive-very-easy-kill-switch.html
Steve

I disagree with this.
If the charging system is properly grounded, and the ground of the battery is disconnected, and the engine is running, then the alternator is still supporting the ignition.
The end result could be terrible...the engine continues to run...fuel is still being pumped...oil pressure is maintained (and possibly leaking or spraying all over....and the wheels might still be driven.
The only safe system I can think of is to remove the positive in such a way as to kill the ignition.
It would require a 2 section switch....one to the main bus...and the other to the ignition circuit.
Bob

The current flows from the negative terminal to the positive, so it follows that you should disconnect the outflowing terminal first to deenergize the system.

Most race car cut offs are on the positive side. For example:

Master Battery Cutoff Switch

Look at the wiring diagram.

Scott

Bob is ‘right on’ in the result.

Our 911 application is somewhat unique.
The voltage regulator needs an attached battery as a reference.
If you read the ‘fine print’ in our Owner’s Manuals, Porsche states that you must not disconnect the battery while the engine is running.
In practical terms, if you disconnect the battery while running, the voltage will rise high enough to cause failures in many electrical devices.
Common are the alternator and the CDI ($$$).

For track applications there is a 2-circuit cut-off switch – one circuit for the battery and the other for the ignition & alternator.
This allows you to safely turn off a running engine and isolate (disconnect) the battery.

A single circuit cut-off switch (safest in the ground side of the battery) does just fine for working on the car and preventing starting – just don’t use it to turn off a running engine or …. :mad: :(

Of course, disconnecting the ground wire does the same thing for no $ and nothing new to fail.


This also brings up an important maintenance point.
You must keep the battery terminals tight and the wires properly connected.
I suspect some (many?) alternator failures may be traced to an intermittent connection at the battery.

Best,
Grady

Electrons are negatively charged (-) and are pulled from the ground source. These electrons flow to an area that is deficient in electrons to even things out. So if you remove the source of electrons, you have no flow, ie...no juice. At least thats my understanding of it.

with that in mind, my battery cutoff switch is connected at the positive terminal and has a small resistor of sorts to drain off any excess in the event the cable is pulled. I know I shouldnt have done it, but I pulled it one day with the engine idling. I didnt notice any problems afterwards.

You were lucky and the engine was at idle.

Best,
Grady

there are two different kinds of switches being discussed.

one is commonly used with engine already shut off to prevent the battery draining when not in use or when you are working on electrical system. They are cheap and sold at any auto parts store. Those should be put on negative for the reasons stated (less risk of shorting battery accidentally).

The other kind is a switch used to shut off engine in an emergency (like wreck during a race). Those should be wired per the manufacturer's instructions, and may include something like a diode and additional wiring used to prevent alternator damage.

Definitely have to make sure your switch is wired to stall a running engine, without damaging any electrical components. I don't say this just from an additional cost of repair standpoint, after you've gone off track and balled up your car. I say this from a tech inspection standpoint too. Many racing organizations will require you to demonstrate the switch's proper operation as part of a pre-race or annual tech inspect requirement. Point being is it is a switch that is used more often than in the event of a crash. I use mine all the time to kill the power when working on the electrical system. Which seems to be quite often- tidying up routing, installing new stuff (data logger/lap timer, cool suit, new tach...) or eliminating street car wiring that no longer has a purpose in the dedicated race car.

Another good idea is an Inertia Switch to cut power mainly to the fuel pump in case of a hard hit. If for some reason you are dazed the switch knocks out the power. Most new cars have them and can find them in any bone yard.

Yep those are a good safety measure and cheap, even if bought new

http://www.smartracingproducts.com/pdfcat/srp2009_bodyelectrical.pdf

See page 3 for the fuel cutoff and a picture of the 2 circuit battery cutoff switch is on page 4

Here's their suggestion on how to use both

http://www.smartracingproducts.com/pdfdocs/935100_fuelshutoff.pdf

http://www.smartracingproducts.com/pdfdocs/936100_battery_switch.pdf

Not trying to endorse SmartRacing necessarily, or say their way is the only way those switches should be installed. They were simply the first source that came to mind with fancy printed documentation/description information to share.

ow!!!

[QUOTE=schumicat;6042907]there are two different kinds of switches being discussed.

one is commonly used with engine already shut off to prevent the battery draining when not in use or when you are working on electrical system. They are cheap and sold at any auto parts store. Those should be put on negative for the reasons stated (less risk of shorting battery accidentally).

QUOTE]

Thats what I'm up to. When something starts having a melt down I want to be able to cut the power quick. It has happened in the past on a VW.

No one is trying to re-invent the wheel. Just trying to learn.

I figured the fewer big hot 12v cable I had running around the trunk area that could get shorted out the better.

^^

That was my thinking cgarr, but I guess it makes sense not to have a big lump of copper bolted to the pos teminal just waiting to make contact with the body in the event of a bump or shunt, (or even just a loose battery).

The end result is the same putting it on the neg. It still allows you to quickly disconnect the power, just must take on board the point of killing the ignition first.

Battery kill switch
 

Not to hijack, I appologize in advance, but could the kill switch as outlined on the negative post work on a 2004 Boxster? Lynn

The simplest reason I've heard is your swinging a wrench to disconnect and when undoing the ground your less likely to hit anything hot with the wrench. But if you undo the hot your likely to hit a body panel which is grounded. On old British trash like Aston Martins with + grounds you undid the + ground first.

FD cuts negative on a car job 1st

just because you disconnect positive 1st doesn't mean there isn't juce still available from some electricals somewhere + the tools to ground safety issues involved of going after the positive 1st

Exterior Kill Switch Location
 

On a different note, what is the philosophy behind putting the exterior kill switch here? Is this the most ideal location? Why not by the right side? Sorry for borrowing the pic.

Thanks

http://forums.pelicanparts.com/uploa...1306383019.jpg

its close to you when you get out the door ????

on the other side you would have to crawl over the hood or run around ....

plus you can shut it off on your way to the front to open the hood ........

Steve

Also, an emergency worker, given the choice, will approach the drivers side first. If they then need to disconnect power the switch is close by.

Glad I found this thread.
I'm helping with a race car and they are using a neg. kill switch. The motor is new so we need to check that the alternator or other component doesn't keep something alive that you want kilt.

Change it so it breaks the positive line and removes power from the ignition system.

Scott

If I were to disconnect the positive battery lead (i.e. switch) while the engine is running, how could this damage the alternator? All the electrical equipment will be cut off at the same time: Ignition, fuel pump, alternator, lights, etc.

Electrons flow from negative to positive outside the battery. That is physics. Electrons flow to negative inside the battery due to the chemical reaction. That is chemistry. So, the path for the electrons is from negative to positive.

That being said, put the kill switch anywhere you want.

That's my understanding too. My EE coworker says that's what occurs from an electron physics standpoint.

It's baffling to most when you tell them this.

Battery also acts as sort of an electrical "damper", and absorbs the transients and spikes the alternator can produce. So, you risk frying the diodes, and doing bad stuff to the electronics you may have. There is a lot of info on this in the internet.

When properly set up, the battery acts like a large capacitor. It determines a reference voltage for the alternator through the regulator. When you suddenly remove the battery from the circuit, the regulator responds by trying to compensate and without the battery to set a reference voltage, a large current spike will flow through every active circuit. The higher the rpm of the alternator, the more massive the current spike. Race car cutoff switches usually have at least two poles (battery and alternator) and should be installed on the positive side disconnecting and isolating both the battery and alternator simultaneously. For us hobbiests, we use a cutoff switch more as an electrical system disabler allowing us to disconnect the battery when the engine is not running in order to work on our machines or as a theft deterrent. For this purpose, installing the electrical disconnect switch on the negative side is a more appropriate solution as you are shutting off the source of electrical current flow through the single line without worry that there is some single circuit (probably cobbled in by a po) still connected or available to create a light show. So:

Cutoff switch: get the right one, install it in the positive circuit including the alternator to stop the engine when it is running (btw, some racing bodies require the external switch to be installed on the back of the car).

Disconnect switch: single pole, install it between the - battery terminal and chassis to isolate the battery when the engine is not running.

In fact, I'd like to get rid of the original ignition key and install a cutoff switch: If I were to simply replace the ignition key with a rotary on-off and additional switches for, say fuel pump, ignition and push button for starter, which would replicate the original ignition key functions, I guess this wouldn't affect the alternator since it doesn't actually cut it from the battery circuit, right? But this type of setup wouldn't be approved by any racing organization... ?

And for those that don't know, on all newer cars, it's 'a must' to take the ground off first, and to reverse the procedure when putting the terminals back on...+ then - ...The same goes when jump starting.

By taking the ground off first, then the positive, you will not damage the computor(s)

A lot of these replies are not really accurate, or not truly helpful. The fact is, if either terminal is disconnected, the battery is no longer involved. If the engine is able to be run (how?) without the battery, then damage may result, of course. But the choice for disconnecting either terminal with an isolation switch is (ideally) of no significance. In our non-ideal world however, safety in the event of battery shifting, etc. probably makes negative switch preferred. But this will always also stop all accessories, whereas with a positive switch you can have a low-value fuse by-pass directly to clock, alarm, etc., which might be convenient (as they would not stop). Hope this helps.

FIA kill switch
 

X2 on the guys above who actually understand this issue from a track-car perspective

- our racing clubs will tell you that an "FIA" kill switch is required & they will test your switch in Tech to see if the engine stops running when the kill switch is theoretically switched off -

secondly, as pointed out by a few above - if you don't have the correct FIA switch with alternator protection - you will kill your alternator - maybe worth the $$ risk in an old chevy (a $35 item), but not for my Porsche, nor even for my alternator equipped Manx buggy

i don't fathom why Porsche guys with expensive cars & expensive parts want to save a couple $$ by using a cheap HF or Walmart switch???????

you can find the correct wiring instructions HERE - SmileWavy - https://www.pegasusautoracing.com/group.asp?GroupID=SWITCHBATT

or just google it -
https://www.google.com/search?q=FIA+kill+switch&ie=utf-8&oe=utf-8