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View Full Version : Battery Disconnect Switch, Positive + or negative side -, which is better & Why?



73Riviera
03-20-2007, 12:11 PM
Hi,

When installing a Battery Disconnect Switch,
Most info says to install it on the Positive +
side,
WHY?
Some say negative side -,
Which is better?
& Why?
I was thinking Neg side better, less chance of shorting or mishaps that could happen when adding 2 more terminals.

Thanks!

monkeyy337
03-20-2007, 12:52 PM
The NHRA rule book states it must be connected to the Positive side of the electrical system and stop all electrical functions. It must also be installed on the rearmost part of each vehicle and be easily accessible from outside the car body. The off position must be clearly indicated with the word "OFF". Hope this helps

73Riviera
03-20-2007, 12:58 PM
ok, if connected to the negative side, wouldn't it also stop all electrical functions?
This car is mostly for street use,
Are those rules for all classes?
as it seems to dictate a trunk mounted battery, to have the switch at the rearmost part of the car.

monkeyy337
03-20-2007, 01:17 PM
ok, if connected to the negative side, wouldn't it also stop all electrical functions?
This car is mostly for street use,
Are those rules for all classes?
as it seems to dictate a trunk mounted battery, to have the switch at the rearmost part of the car.
You didn't say that you have the battery in the trunk but I assumed you do? If infact you have the battery in the trunk then yes you must have a cutoff switch by NHRA rules. If this is a street car that you don't race at a NHRA track (some don't even inspect the car) then the choice is yours to do as you like. As for as connecting to the negative side, think about it. Most cars have more than just one ground wire going to the engine and turning off one would not be enough to cease engine/electrical system operation. If your going to the trouble to install one of these cutoff switches why not do it as the rules warrant and that way you only have to do it once.

73Riviera
03-20-2007, 01:25 PM
Some battery terminal connections have small leads on the + side too, so if those are still connected so is power right?
just like the ground, which is what i think you are saying about the ground lead off the Neg side terminal.

I'm really wondering Why most say to use the + side?
Is there a safety reason?

The thing is i do not like about the + side idea is having 2 more exposed HOT + posts that could possibly get arced to!

I know they make rubber caps for the posts, I'm thinking about getting those too of course, you'd think the switches would come with them since most install on the + side. :Do No:

69GS400s
03-20-2007, 01:26 PM
Always disconnect the positive.

The negative could still ground by any wire in the car whereas with the positive disconnected from the batt there is no supply of power.

....this is a poor analogy but - imagine your garden hose hooked up to the house spiggot on one end and the hand sprayer on the other end. If the spiggot valve is turned off, it doesn't matter whether the hand sprayer is on or off -> no water is going to come out

greatscat
03-20-2007, 02:46 PM
Hi,

When installing a Battery Disconnect Switch,
Most info says to install it on the Positive +
side,
WHY?
Some say negative side -,
Which is better?
& Why?
I was thinking Neg side better, less chance of shorting or mishaps that could happen when adding 2 more terminals.

Thanks!
You'll also need to wire the alternator to the switch or use a relay,as even though the pwer is off the alternator continues to provide powerand the engine may still run
gary

73Riviera
03-20-2007, 02:57 PM
You'll also need to wire the alternator to the switch or use a relay,as even though the pwer is off the alternator continues to provide powerand the engine may still run
gary

is that for switch on Neg or +? or either?

greatscat
03-20-2007, 03:04 PM
is that for switch on Neg or +? or either?
Positive,don't know about neg since my racecars are per NHRA specs
gary

Keith Seymore
03-20-2007, 03:08 PM
Always disconnect the positive.

The negative could still ground by any wire in the car whereas with the positive disconnected from the batt there is no supply of power.

....this is a poor analogy but - imagine your garden hose hooked up to the house spiggot on one end and the hand sprayer on the other end. If the spiggot valve is turned off, it doesn't matter whether the hand sprayer is on or off -> no water is going to come out

Well said, Alan. I think that's a good analogy.

K

Marco
03-20-2007, 03:55 PM
Darn!

I've been doing the negative cable for 16 years :Dou:

gymracer01
03-20-2007, 04:18 PM
I don't think the water hoes deal will fly in electrical theroy. If you open the circuit , it is open, period. I know the NHRA rules and I think the rule maker really don't know his electrical. It's hard for me to understand a deal that disconnecting the ground would not be just as safe but the rules are the rules. I guess if the car was rolling and something came in contact with the ground post of the battery and the postive was still on then it would not turn off when the safety guys got to the car and flipped the switch. Any way you could come up with a deal where the postive could be a problem if something conductive came in contact with the postive cable on the battery side of the kill switch. We just have to keep NHRA happy.
JIm N.

Keith Seymore
03-20-2007, 05:29 PM
The difference I see with the water hose analogy is this: if the spicket is still open and (due to a crash or whatever) something comes in contact with the spray nozzle, then the water flows once again.

If the spicket is closed, it doesn't matter; anything can happen to the nozzle and no water flows.

Said differently - if you kill the battery on the positive side then nothing will ever happen. If you kill the current by removing the negative side, then if anything comes in contact anywhere along that negative side then you now have a new complete circuit and power flows and you have a problem.

Make sense?

K

Nailhead
03-20-2007, 05:48 PM
On the surface it doesn't seem it would make any difference--if the battery is totally disconnected at either terminal the circuit is broken--end of discussion!

When disconnecting battery terminal always you should disconnect the grounded side first--negative in most modern vehicles--so if your wrench touchs ground there is no problem. Similar logic would suggest it does not make sense to add a switch on the ungrounded side, but I'll await an explanation from an NHRA techie, cause there probably is one!

John
:beers2:

greatscat
03-20-2007, 06:35 PM
On the surface it doesn't seem it would make any difference--if the battery is totally disconnected at either terminal the circuit is broken--end of discussion!

When disconnecting battery terminal always you should disconnect the grounded side first--negative in most modern vehicles--so if your wrench touchs ground there is no problem. Similar logic would suggest it does not make sense to add a switch on the ungrounded side, but I'll await an explanation from an NHRA techie, cause there probably is one!

John
:beers2:
The circuit may be broken,but if the motor is running and you have an alternator,the motor still runs.Haven't you ever disconnected the pos side of the battery with the motor running in a stock vehicle and the thing keeps going?If you have an alternator you need to remove its current also.
gary

Schurkey
03-20-2007, 07:13 PM
As I see it, the alternator/generator is the key to the + or - question.

If you connect the switch to the + side, and you have set up the wiring so that turning the switch OFF also disconnects the alternator field supply (which is also +) you've effectively disconnected the battery and the alternator/generator.

If you connect to the - side, you'll have an interesting time trying to disable the ground of the alternator. How are you going to insulate the alternator case and brackets?

The water hose story is inaccurate. Break the battery circuit ANYWHERE and no electricity flows. The difference is that there are essentially two circuits that need to be broken--the battery-powered circuit, and the alternator/generator powered circuit. (yes, they overlap a lot) The alternator is easier to disable on the + side than on the - side.

73Riviera
03-20-2007, 08:16 PM
So for doing repairs, if you have a switch on the + side, should you still disconnect the - side or not?

Maybe a switch on both would be the best bet. :cool:

monkeyy337
03-20-2007, 08:22 PM
Hi,

When installing a Battery Disconnect Switch,
Most info says to install it on the Positive +
side,
WHY?
Some say negative side -,
Which is better?
& Why?
I was thinking Neg side better, less chance of shorting or mishaps that could happen when adding 2 more terminals.

Thanks! I've got to know, is the battery in the trunk or still up front?

73Riviera
03-20-2007, 08:39 PM
I've got to know, is the battery in the trunk or still up front?

Still up front on this car, I did not move it.
I plan to put one in the rear on my GTO tho.

73Riviera
03-20-2007, 09:43 PM
As I see it, the alternator/generator is the key to the + or - question.

If you connect the switch to the + side, and you have set up the wiring so that turning the switch OFF also disconnects the alternator field supply (which is also +) you've effectively disconnected the battery and the alternator/generator.

If you connect to the - side, you'll have an interesting time trying to disable the ground of the alternator. How are you going to insulate the alternator case and brackets?

The water hose story is inaccurate. Break the battery circuit ANYWHERE and no electricity flows. The difference is that there are essentially two circuits that need to be broken--the battery-powered circuit, and the alternator/generator powered circuit. (yes, they overlap a lot) The alternator is easier to disable on the + side than on the - side.

Ah, now that's what I was looking for! An explanation so I could get an understanding why the + side was more often suggested.
Thanks!

gymracer01
03-20-2007, 09:52 PM
Gary is correct, the car will want to still run with the cable off if it has a charging alternator. This is NOT good for electronic parts. The alternator goes crazy and can damage electronic equipment. A for sure No No on modern vehicles. I heard of NHRA shutting off cars in tech with them running to test the battery disconnect switch and killing ignition boxes. I have never had one do that to me. School cars that we store of periods of time I put switches on them so the computer will not drain the battery. I always put them on the ground side so there is no danger of a cable touching ground. But if you race you should go for the positive.
Jim N.

73Riviera
03-20-2007, 10:35 PM
So it would be more user friendly maybe to use a relay setup to control the power from the battery and to the alternator, that i could wire up to be controlled from inside the car.
What do you think?

BadBrad
03-20-2007, 10:45 PM
Cars with MSD (maybe other types of ignitions too) also have power in the system without the battery connected. I've had my battery totally disconnected and accidently touch the positive cable to the chassis only to find voltage. Made the mistake of reconnecting the battery without restarting the car only to find the battery dead the next week. I believe the MSD proceded to energize its own capacitors and drain the battery.

Joe
03-21-2007, 11:15 AM
I can think of only one advantage to having the switch on the negative side AT THE BATTERY. If the switch is on the positive side and you were working around or on the battery and caused a short from the battery to ground an arc flash will occur. This will not happen with the switch in the negative lead because the battery will be completely isolated.

For NHRA if the switch is on the negative side in the case of an accident the battery negative post could make contact with the car. That would make the switch useless and keep the electrical system energized.

73Riviera
03-21-2007, 11:19 AM
Exactly, and the main reason why i was considering the negative side (at the battery, of course).
But I plan to get the rubber covers for the terminals and the posts on the disconnect switch.



I can think of only one advantage to having the switch on the negative side AT THE BATTERY. If the switch is on the positive side and you were working around or on the battery and caused a short from the battery to ground an arc flash will occur. This will not happen with the switch in the negative lead because the battery will be completely isolated.

For NHRA if the switch is on the negative side in the case of an accident the battery negative post could make contact with the car. That would make the switch useless and keep the electrical system energized.

Keith Seymore
03-21-2007, 12:18 PM
For NHRA if the switch is on the negative side in the case of an accident the battery negative post could make contact with the car. That would make the switch useless and keep the electrical system energized.

This is pretty much what I was trying to articulate....:Smarty:

Keith Seymore
03-21-2007, 12:22 PM
By the way, here is an incredibly in depth discussion about wiring the cut off switch so that the engine will not continue to run:

http://v8buick.com/showthread.php?t=98356

Beamer
03-21-2007, 11:37 PM
Electrical energy travels from - to +, with respect to each other. That is the reason if you are going to put a vehicle away for a while, you should disconnect the negative battery terminal to keep it from going dead. There are many instances when the positive terminal is removed and the battery was still able to drain trough the negative cable to the chassis.

As for which is better or safer, you need to analyze your purpose. For safety, I would agree that a positive disconnect would be better if the battery came loose and the terminal hit ground, you will get a nice weld spot. If the negative side hit ground, the circuit could be completed again and allowing who knows what to occur that was not wanted. If for battery storage, the ground disconnect would help aid from energy drainage.

Just my $.02

Mike