Why the negative cable?

[91Bear]

I've heard that if you need to disconnect your battery, you should disconnect the negative cable. Can someone explain why the negative instead of the positive?

[papabug71]

I've always been told to disconnect the hot side. If you disconnect the ground side & the cable hits something & makes a good ground while you're working on the truck it "could" be bad. Especially if you're doing electrical work.

When I do electrical, I usally take the battery out of the truck. Just in case.

[fordman]

the reason i was told is because when removing the negative and it sticks while coming off or the wrench grounds out. it wont hurt anything. i have also heard similar arguements about removing the positive while working on a vehicle. but i think the negative should still be removed first when doing any kind of battery work.

[1972hiboy]

I would have thought either one would accomplish the same thing. Remvoing the neg first may keep the sparks down. But really without path for electrons to flow back the the battery wouldnt pulling either one do the same function? Like Fordman its really best I think to just unhook both or remove the battery al together before doing and serious work.

[stephen44]

yes but ...if you remove the positive first and slip you can connect the pos post to ground via the wrench - and weld it and cause a fire - ask me how i know


so disconnecting the neg post is safer

for the older members here like me - this assumes you have a negative earth car - i remember back in the day - jumping two cars - one neg earth and one pos earth - and letting the two doors touch

[Thunderfoot]

Yes, the reason is that you are using a metal wrench to remove/loosen the bolt holding the cable and if you have the wrench on the terminal and it swings over and makes contact with the fender/metal body nothing will happen if you are removing the negative first. if you are doing the positive first and the wrench touches metal then you will see something like an arc welder and it can even WELD the wrench to what ever it makes contact with and blow the battery up at worst case.

Isolating the power either (+)/(-) will work but for safety removing the connections on the battery the reason above is the procedure to follow.


Side note: for using a battery "disconnect switch" as used for racing you usually put it on the Positive side, this is so that if the car gets wrecked that if some part of the body makes contact with the negative side of the battery you will be able to turn the electrical off, because you are switching off the power on the Positive side, and if the body was to make contact with the positive side of the battery then it would arc out and/or you still could turn off all the electrical circuits in the car buy disconnecting the power to the electrical (ignition, fuel pumps, etc) of the car with the battery disconnect switch.

[Thunderfoot]

I've always been told to disconnect the hot side. If you disconnect the ground side & the cable hits something & makes a good ground while you're working on the truck it "could" be bad. Especially if you're doing electrical work.

When I do electrical, I usally take the battery out of the truck. Just in case.

Once the ground cable is removed from the "battery post" there is no connection to any power and the cable touching anything, even the positive battery post, wont doing anything as both sides of the battery have to be hooked up in the circuit for current to flow. Same goes for the positive cable from the "battery post"...

Only thing is, as was stated above by us, is for safety of wrench proximity...

[mr_josh]

Once the ground cable is removed from the "battery post" there is no connection to any power and the cable touching anything, even the positive battery post, wont doing anything as both sides of the battery have to be hooked up in the circuit for current to flow. Same goes for the positive cable from the "battery post"...

Only thing is, as was stated above by us, is for safety of wrench proximity...

This is something I had to explain to a neighbor when I was working on my truck and he rushed to my "aid" in attempt to avert a fire that was just never gonna happen.

[fireguywtc]

Every fire department I have worked for also does this same procedure of removing the negative cable first when "safetying" a wrecked car. Like mentioned before, taking the neg. off first is a lot safer and eliminates the risk of arcing when we are taking off the pos. cable. We always remove both sides so we reduce the risk of fire and de-energize the airbag system if equipped. If I am doing some serious personal electrical work on my vehicles I always remove both terminals also just to be more safe. Thats just what I prefer though.

[stephen44]

on a similar note - do you all disconnect the battery (one terminal or two ?) before connecting a battery charger - i was always told not to do so can damage the alternator - the diodes maybe ?

[Thunderfoot]

on a similar note - do you all disconnect the battery (one terminal or two ?) before connecting a battery charger - i was always told not to do so can damage the alternator - the diodes maybe ?

I never do... A properly operating battery charger should not put out more voltage then the alternator does, if it does it will damage the battery. It would take a lot of voltage to damage the diodes (probably more then the wire could take) and the rest of the alternator is isolated by the diodes, if the diodes are bad (shorted) it could drain the voltage to ground but normally wouldn't hurt the alternator other then that the bad diode was bad already before putting the charger on it.

I have used a battery charger a lot at the track to help maintain the battery charge on my car, even though I have an alternator on it, and have never had problems.

[70_F100]

I was taught years ago that the reason for removing the negative (ground) cable first is because of the way current flows through the electrical system.

Removing the positive cable first can result in a small spark, which could then ignite the hydrogen gas that "lingers" around the top of the battery. Removing the negative cable first will not result in a spark unless there is a load on the electrical system.

I once asked an electrical engineer about this, and he was in agreement, so I guess there's a pretty good chance it's correct.

[Thunderfoot]

I was taught years ago that the reason for removing the negative (ground) cable first is because of the way current flows through the electrical system.

Removing the positive cable first can result in a small spark, which could then ignite the hydrogen gas that "lingers" around the top of the battery. Removing the negative cable first will not result in a spark unless there is a load on the electrical system.

I once asked an electrical engineer about this, and he was in agreement, so I guess there's a pretty good chance it's correct.

Either way will spatk, the only way to get a spark there has to be current flowing... don't matter which one you remove first if there is current flowing to some load in the system when you open/break the flow there is going to be a spark.

"Current" flows from Negative to Positive but if there is no load on the line then it has no flow... but as I stated above it don't really matter which way it flows for causing a spark, just has to be flowing when you try to interrupt the flow . This is why switches are rated for so much amp rating, as this is how well they can handle this interruption of the current without damage. If you look at some switches you will see that they have different ratings for whether you are using them for AC or DC applications because of the way the current works on each.

[fireguywtc]

I also highly doubt you will ever run across a point in time when you will have the conditions right enough to ignite hydrogen gas from the battery. Levels will always be to low (lean) for a burn. It would take some major event to create enough hydrogen to reach the LEL (lower explosive limit) in which it becomes rich enough to combust. Someone would have to be putting a major load in or on the battery, boiling the water with the caps off for an extended duration of time to maybe get that enviornment. Even then, the little bit of hydrogen would burn off in a flash.