Battery Master switch on Neg Leg?

PaulProe · #1 (**permalink**) 04-09-2007, 07:10 PM

I saw a post in another forum of a battery that had its master switch on the negative post. This is a link to that picture: http://www.robsdrive.com/cobra/my-co.../P3170022.html Is this acceptable practice?

I was thinking of a trunk mounted battery, with the positive going to the solenoid mounted at rear, then the normal lead going up to the starter. The negative lead would go from the battery to the master switch, from there to a ground lug on the frame. I would place a second ground strap from the engine to the frame.

Having the negative leads on the switch solves the problem of something accidently touching them.

???

Paul

Scufty · #2 (**permalink**) 04-09-2007, 08:47 PM

My shut off is also on the negative side if that helps ya at all

Bryan Wilson · #3 (**permalink**) 04-10-2007, 02:23 AM

The only way to go.
Cheers,
Bryan

Ron61 · #4 (**permalink**) 04-10-2007, 05:22 AM

Same way I had mine when I had one. Worked great.

Ron

Tommy · #5 (**permalink**) 04-10-2007, 05:35 AM

I can't see that it makes any difference where the circuit is interrupted by a cutoff switch. It's the interruption, not the direction of current flow that matters. For example, many British cars used to have positive ground systems.

Jerry Clayton · #6 (**permalink**) 04-10-2007, 07:32 AM

best way to do it as there isn't the high ampere load on the negative side--most of the switches on the market aren't rated high enough for continous duty on the high amp systems in most cars

And about half of the cars I've seen with the switch in the positive side will keep running with the switch in the off position--

Jerry

Yetiman · #7 (**permalink**) 04-10-2007, 07:27 PM

Quote:

Originally Posted by Jerry Clayton

best way to do it as there isn't the high ampere load on the negative side--most of the switches on the market aren't rated high enough for continous duty on the high amp systems in most cars

And about half of the cars I've seen with the switch in the positive side will keep running with the switch in the off position--

Jerry

There's an equal current draw on the negative lead, it is a complete circuit.

If your car keeps running when disconnecting the switched positive lead, it will still do it when disconnecting the negative lead for the same reason.
Having the exposed terminals on the switch at ground potential is a big advantage though if something metal contacts them and the surrounding grounded surfaces though (like the aluminum lined trunk compartment pictured).

fastraxsg · #8 (**permalink**) 04-12-2007, 08:54 AM

I've run my trunk mounted battery with a negative side ground switch for serveral years. It has a short lead from the negative terminal to the switch and another short lead to the frame. I have a large marine battery box with a small sealed cell in the box. I mounted the switch on the side of the box so the terminals are mounted in the box and the battery is fixed in the box so it won't slide around. If the box comes loose, the terminals won't short out to anything even if it turns turtle on me. Having the switch in the trunk is like a simple anti-theft device but can not be reached in case of an accident. Might consider a second switch on the positive side in the cockpit where you can reach it in case of an accident.
Don't buy cheap switchs for these.
For another simple anti-theft device, try installing a dimmer switch located some where under the carpet that cuts out the primary lead to the coil. Read that one in StreetRodder Magzine.

cobrashoch · #9 (**permalink**) 04-14-2007, 01:38 PM

Only way to go. If your car gets upside down or roughed up in a wreck any positive wire can make ground to the battery if things get rough by just touching the chassis anywhere. Going the other way (positive) requires a much higher rated switch, because everything is running through it in load condition. Buy the way, my car has 2 switches in line on the negitive side, one outside the car and the other is in the cockpit and requires both for the car to fire. Another thing you might consider is a tipover switch on the electric fuel pump. Really cheap insurance to help prevent a fire in a wreck.

Excaliber · #10 (**permalink**) 04-14-2007, 01:55 PM

On my old Austin Healey the cut off switch (mounted in the trunk next to the battery) was on the positive terminal side.

Oh, did I mention the postive terminal was also 'earth ground' to frame?

Lucas wiring, apparently doesn't know one side from the other.

Friend of mine (Backdraft car) has his cut off right next to the drivers seat on the back panel. Once he accidently hit the switch while entering the car, putting on his seat belts or whatever. Of course it wouldn't start and the 'green flag' was getting ready to drop. PANIC TIME!!! Took us a few minutes to figure out what the heck happened.

jwd · #11 (**permalink**) 04-14-2007, 01:59 PM

Quote:

Originally Posted by cobrashoch

Going the other way (positive) requires a much higher rated switch, because everything is running through it in load condition.

As stated before, it's the same load regardless of side.

Jerry Clayton · #12 (**permalink**) 04-14-2007, 02:15 PM

Try putting an amp guage in line on the negative and see----

Excaliber · #13 (**permalink**) 04-14-2007, 02:20 PM

Interesting observation Jerry, it would depend on the 'load' at time of testing to determine any substantial drop from positive to negative. But the theory is solid, less draw on the negative side. But in the case of a major 'fault' (short) it wouldn't make any difference anyway. I can't imagine the difference in 'draw' being enough to warrant a larger switch on the positive side, same switch should work fine in either direction.

cobrashoch · #14 (**permalink**) 04-14-2007, 02:24 PM

JWD - Try running a amp reading on a VTOM both ways at the battery. I agree it is the same in theory, but I can tell you from experience the readings will NOT be the same. Plus the negitive side is MUCH simplier to bypass wire non essentials such as a radio for presets, and other things. That is of course unless you have a fiberglass car. Fought this war for many years, don't think you can talk me into doing it the other way, but I'm open to suggestions.

olddog · #15 (**permalink**) 04-14-2007, 02:28 PM

I had to think a bit, but the switch opening on the positive side only disconnects the battery from the car's circuits. The alternator will continue to power the car, if the engine is running when you open the switch.

I am thinking that the alternator does not tie to ground, but I'm not certain this is true. Logically it should, but I don't recall connecting a ground wire to one and the brackets are painted. If it does not, then this is why the switch should be on the negative side, as it would kill the entire electrial system.

As for the current draw, as some have argued this both ways, the amperage draw is the same on both the negative and positive side of the battery, when the battery is providing the power. When the alternator is providing the power, I'm not certain, without seeing a drawing of the alternator regulator circuit.

I hope an expert can sort this out for us.

Tommy · #16 (**permalink**) 04-14-2007, 02:41 PM

Quote:

Originally Posted by olddog

I am thinking that the alternator does not tie to ground, but I'm not certain this is true. Logically it should, but I don't recall connecting a ground wire to one and the brackets are painted.

I'm no expert, but I'm pretty sure the alternator does not manufacture electrons. The alternator pushes existing electrons around a circuit. So a single wire alternator must be grounded through it's mounting assembly, IMHO.

olddog · #17 (**permalink**) 04-14-2007, 02:42 PM

Come to think of it, back when I was a puppy, I had a motorcycle that had a bad battery (by bad I mean shorted). I would use a small battery charger to provide voltage to power the coils and then kick start it. Then I could disconnect the charger and run it with no battery, as the alternator would keep power to the coils. I just couldn't let the idle drop too low.

So from this, I'm thinking the alternator has to tie to ground some how, even though I'm remembering the GM alternator/regulator system as only having one wire.

olddog · #18 (**permalink**) 04-14-2007, 02:44 PM

Quote:

Originally Posted by Tommy

I'm no expert, but I'm pretty sure the alternator does not manufacture electrons. The alternator pushes existing electrons around a circuit. So a single wire alternator must be grounded through it's mounting assembly, IMHO.

After thinking, I agree. You have to be correct.

cobrashoch · #19 (**permalink**) 04-14-2007, 02:52 PM

OLD DOG - A switch on the positive side would still kill the power to the engine electronics/coil, so that makes no difference, the engine would still die. Even one wire alternators make ground through the mounting brackets/bolts on the engine. Yes you are correct- if you somehow kept the alternator running regardless of if the engine is running, it will produce power.
That said, the negitive side is still the simplest side to switch without having to run a dual harness in a car if you go on the positive side.

Yetiman · #20 (**permalink**) 04-14-2007, 03:17 PM

Quote:

Originally Posted by cobrashoch

Going the other way (positive) requires a much higher rated switch, because everything is running through it in load condition.

You require a high current rated switch either way. The negative lead flows just as much current as the positive lead. It is a complete circuit. Put a 100 amp fuse in line on the negative lead and see if you can start your car on a cold day without blowing the fuse.

If all cars had a positive ground system, it wouldn't have even been such a bad idea. The biggest problems arose because it didn't catch on.