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PostPosted: January 4, 2016, 12:21 am 
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Seems there’s differing opinions on whether the master-shutoff (“kill”) switch should be hooked up to disconnect the battery’s positive or negative side. IIRC, NHRA rules mandate a positive-side hookup but most stock car sanctioning bodies want the kill switch on the negative side. Not sure what SCCA requires.

As the argument goes, on a switch connected through the positive side, the exposed terminals on the back of the kill switch or a broken positive cable could contact sheet metal or the frame during a crash, creating a short to ground and safety hazard. As the negative cable connects to the chassis ground already, there’s no additional danger if it rubs-- the same principle as why the battery’s negative side should always be disconnected first when performing electrical service.

I had always been of a mind to put the master disconnect on the positive side, but never liked the idea of routing that big +12v cable around the chassis to the switch. So now I’m confused. :?

Interested in your thoughts…

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PostPosted: January 4, 2016, 12:32 am 
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To properly shut down an electrical system a two pole (DPST) switch is needed in the positive feed.

The alternator charge line must also be lifted from both the battery and the car.

A running motor will provide an alternator output that the car will still run on even with the battery removed. This can have disastrous results ruining on board electronics.

Breaking the negative battery cable will not stop the alternator from continuing to supply power to the car.

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PostPosted: January 4, 2016, 9:32 am 
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What is your purpose for the disconnect switch? Do you need to meet racing requirements? Or do you just want to stop battery drain while being dormant? The simple Harbor Freight 2-terminal version works well for the latter use. Either positive or negative terminal works here. I have one in my car and it is on the negative side of the battery. One terminal actually bolts directly to the frame and doesn't use a lead wire.

If you are using it to meet racing requirements, read their rules.

The problem with many switches is that they rate only the current carrying capacity. They do not list the "break" current rating which is typically more difficult to design for. In the case of a crash and an electrical short/fire, the capacity of some switches is not enough to stop the 1,000's of amps of potential battery current.

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PostPosted: January 4, 2016, 9:52 am 
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I was just researching this yesterday myself. From what I have found SCCA states positive or negative lead. Must be accessible by the driver and easily from the outside by a corner worker. Where do you plan on mounting it or do you plan on running multiple to meet the rules? Since it must shut down all electrical power to the car, I think on the positive side would be better and as mentioned above you need to make sure the alternator cannot continue supplying power to the car. I guess if power is cut to the ECU which kills the spark, the engine will stop in short order thus stopping the alternator.

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PostPosted: January 4, 2016, 10:57 am 
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You either need a four-pole switch with a resistor for the alternator, or you need to connect the alternator to the battery and then put the switch downstream of that.

Disconnecting the alternator from its load without providing somewhere for the back current to go will damage the diodes. They don't usually bllow, they just get "leaky", and the alternator's charging capacity will drop a little bit each time, until it won't keep up with the charging requirements any more.

Unless you have a sanctioning body that requires the switch to be on the negative side, it ought to be on the positive side, to better protect the alternator. If it's just to disconnect the battery for storage it can be on either side, though.

NHRA techs normally check the function of the shutoff switch by having the driver start the car, then using the switch to make sure it actually shuts off.

There are some sanctioning bodies that limit the length of "hot" wire between the alternator and the switch; some of those guys have to run a relay in the alternator wire.


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PostPosted: January 4, 2016, 2:05 pm 
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Folks are correct about issues with current capacity for breaking a circuit and possible alternator damage. I don't think they really apply here though, the big issue to keep in mind is that it is very unlikely you will ever use this switch in an emergency. The first choice would always be the engine off switch on the dash.

In normal use to put the car in storage it won't be running and these issues don't apply.

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PostPosted: January 5, 2016, 4:58 pm 
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Mine's on the negative side. The big terminals stick into the engine bay and I'm more concerned about dropping a wrench on them than anything else. According to the gurus on the M.G. forum, the alternator won't be damaged by disconnecting the battery so long as it's plugged in and supporting the load of the ignition system. Not an issue for me either way, since the switch is under the dash and unlikely to be turned off while the engine is running.

From everything I've heard and read, from a pure popularity standpoint more switches are on the positive side.


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PostPosted: January 5, 2016, 8:22 pm 
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Disconnecting the battery on a running motor can ruin electronics as the alternator no longer has the load of a battery connected to act as a damper for the voltage regulator. The voltage regulator will go nuts trying to regulate the voltage as the output voltage (minus the battery) will rise and fall unnaturally at a fairly high frequency.

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PostPosted: January 5, 2016, 10:02 pm 
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I don't think they really apply here though, the big issue to keep in mind is that it is very unlikely you will ever use this switch in an emergency.
Which is exactly why I asked what his intended purpose was for.

I put mine in as a battery drain protection during non-driving seasons, like now. :( Since I didn't need to meet any racing requirements, I used the cheap, HF single circuit switch and put it in the negative side. 1) it eliminated 2 more crimp connections on an extra heavy wire. and 2) I don't have to worry about shorting out the wrench if I got reckless.

For those of you considering the HF switch, or similar low-end switch, I have a buddy that uses the same switch in his Formula Vee (no generator or alternator, just runs off the battery) He has had 2 failures of the switch over the past few years. So take your chances.

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PostPosted: January 6, 2016, 12:42 am 
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I do electrical and electronic work, and it's a standard practice to switch the wires that supply a circuit rather than the common or ground. When you switch the hot wire and turn it off, you don't have anything past the switch energized, so less chances of grounding something out and causing a spark or otherwise damaging equipment. It's the same reason that a fuse should be installed on the hot wire and as close to the power source as possible. A lot of automotive stuff is wired to switch the negative though, and it obviously works fine, but it's something I try to avoid when wiring something

I used a battery disconnect switch on the dash of my car. The positive lead from the battery goes to one terminal and every circuit for the car goes to the other one, including the alternator charge lead. The handle on mine is removable and is also the key for my car, with just a toggle switch to turn on the ignition and fuel pump, and a momentary push button to crank. But as was mentioned, a two pole switch won't shut the car off, so I still have to kill the ignition with the toggle switch.
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PostPosted: January 6, 2016, 11:23 am 
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When you switch the hot wire and turn it off, you don't have anything past the switch energized, so less chances of grounding something out and causing a spark or otherwise damaging equipment.


When you switch the battery negative wire, the same thing occurs. Cars are unlike other line-connected equipment. They are isolated circuits with no "earth" reference to worry about. When switched off, the chassis becomes isolated from the B-. Once the switch is opened, shorting from any "hot" wire to ground does nothing. Why do you think that virtually all major automotive repairs start with "remove the wire on the negative terminal of the battery"? Shorting a wrench to ground while doing this does nothing. Once done, you can become as reckless wrenching on other wires as you wish, just
don't short directly across the battery terminals.

Virtually all ECM controlled devices I have seen, provide B+ power to the device and switch the negative lead. This is done for different reasons. It is historically less costly to use a NPN transistor than a PNP or N channel FET vs a P channel. Not only are they less expensive, they are more efficient switches, more easily driven and more reliable. Relay switched circuits, well, they generally switch the + lead for the reasons you mention.

For the battery disconnect, either way works. As long as the switch is located very close to the battery, both are relatively safe installation methods.

Just for reference, this is my disconnect installation. The terminal from the switch bolts right to the chassis. The other wire on that terminal is the engine ground strap. The wire to the battery is short and easily accessed for removal when necessary


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PostPosted: January 6, 2016, 4:06 pm 
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Appreciate all the input, folks. I’ve much to learn in this area. To answer Chuck’s question: My goal is to have a battery disconnect to eliminate off-season battery drain and while I’m putting one in, have something that would pass tech as well. Seems that will require a dash mounted disconnect that also kills the alternator and engine.

I’m using a Honda alternator that utilizes an “ignition-on” +12v exciter line (as opposed to a “continuous” +12v exciter like most GM alternators.) Also planning a simple toggle on-off ignition switch with a keyless proximity lock on that circuit. Given that, a set-up similar to Kristian’s, but using a DPDT battery disconnect that also opens the ignition-on circuit should shut down both the ECU and the alternator exciter line in addition to opening the +12v battery feed. I also like Kristian’s approach of using the non-battery side of the disconnect as a +12v power distribution point.

Pegasus sells a 3PDT battery disconnect switch that looks interesting. https://www.pegasusautoracing.com/produ ... RecID=1464 This switch adds a third set of terminals and a grounding resistor to provide the alternator a loaded path to ground when shut off, as recommended by Bent Wrench and TRX.

Not sure I have this right yet, but hopefully getting closer… :?

rx7locost wrote:
Just for reference, this is my disconnect installation. The terminal from the switch bolts right to the chassis. The other wire on that terminal is the engine ground strap. The wire to the battery is short and easily accessed for removal when necessary
Chuck, very clean installation!

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PostPosted: January 7, 2016, 12:01 pm 
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seattletom wrote:
...My goal is to have a battery disconnect to eliminate off-season battery drain and while I’m putting one in, have something that would pass tech as well. Seems that will require a dash mounted disconnect that also kills the alternator and engine....


Don't forget SCCA requires it be easily accessible from the outside by a corner worker. If the master switch is on the dash close to the driver it may not be close enough for a corner worker. Some people install two switches in series one outside for a corner worker one inside for the driver. From what I have been able to gather the standard location is around the A pillar on the passenger side on a normal car which would be the scuttle / roll cage for us. Also don't forget to label it correctly if you want to meet SCCA requirements.

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PostPosted: January 7, 2016, 10:51 pm 
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rx7locost wrote:
Quote:
When you switch the hot wire and turn it off, you don't have anything past the switch energized, so less chances of grounding something out and causing a spark or otherwise damaging equipment.


When you switch the battery negative wire, the same thing occurs. Cars are unlike other line-connected equipment. They are isolated circuits with no "earth" reference to worry about. When switched off, the chassis becomes isolated from the B-. Once the switch is opened, shorting from any "hot" wire to ground does nothing. Why do you think that virtually all major automotive repairs start with "remove the wire on the negative terminal of the battery"? Shorting a wrench to ground while doing this does nothing. Once done, you can become as reckless wrenching on other wires as you wish, just
don't short directly across the battery terminals.


Yes, I had a brain fart there. I thought of that shortly after posting and thought I had edited my post to correct myself, but apparently not. I'm getting too used to how stuff at work is done. And the switch you have looms exactly like mine. It's not exactly a tricky one to defeat when used as a key.
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PostPosted: January 10, 2016, 11:42 pm 
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turbo_bird wrote:
I do electrical and electronic work, and it's a standard practice to switch the wires that supply a circuit rather than the common or ground. When you switch the hot wire and turn it off, you don't have anything past the switch energized, so less chances of grounding something out and causing a spark or otherwise damaging equipment. It's the same reason that a fuse should be installed on the hot wire and as close to the power source as possible. A lot of automotive stuff is wired to switch the negative though, and it obviously works fine, but it's something I try to avoid when wiring something

...
Kristian


The likely reason that SCCA specifies either Positive or Negative is because some cars utilize a Positive Ground power strategy, where the positive terminal of the battery is connected to the frame and is used as the ground. The end goal of the disconnect switch is to reduce the possibility of a short to ground (positive or negative) as much as possible with the switch by eliminating as much of the energized circuit as possible. The best approach is to splice in your switch to the circuit with the terminal of the battery that is not being used as the common ground. In many cases, there is a wire, or series of wires, coming directly off of the battery terminal that connect directly to a point on a grounding plane, such as the engine block, the frame, or other wide-reaching component. It is difficult to remove all grounds from the system without switching at the battery, which likely would not meet the SCCA requirements. When it comes down to it, however, a circuit needs to be complete to do anything. If the ground is removed, and only a short wire exists that still is attached to ground between the switch and the battery, then it really is no different than switching the power side. One is usually significantly easier to do than the other.

To clarify, when the alternator is running, there are technically two power supplies in the circuit, and they should be treated equally by the same switch (DPDT, or whatever is required in the racing spec you choose to follow). In other words, if you switch off the positive of the battery, you must also switch off the positive of the alternator DC supply. If you switch off the negative of the battery, you must also switch off the negative to the alternator DC output. This is why the grounding strategy used by the vehicle is typically the deciding factor for which side of the battery gets disconnected, because USUALLY, the ground of the DC output of the alternator is the frame of the generator, which is bolted to the engine, which is bolted to the grounding strap (or multiple straps) that connects to the side of the battery used as common ground. That is much harder (but not impossible) to limit with a switch.

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