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

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.

 

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?

 

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.

 

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.

 

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.

 

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

 

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

 

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