Why do we disconnect the negative terminal?
- Thread starter LavaRand
- Start date
I wondered this myself years ago, and it has been apparently standardized what is + and what is -; this doesn't map directly onto the actual flow of electrons; i believe it is just "convention" as to what the + is. One is the ground and if you touch a tool to ground on a charged system (like hooking the battery up) you will complete the circuit.
AZjeff
$50 Site Donor 2023
Motor Trend explains why unless they're wrong:
https://www.motortrend.com/how-to/which-battery-cable-should-be-removed-first-but-installed-last/
https://www.motortrend.com/how-to/which-battery-cable-should-be-removed-first-but-installed-last/
I think the reason is simply that a wrench on the positive terminal can easily touch the ground, body, chassis or hardware and cause a short. A wrench on the negative won’t short to the body. And once the neg is disconnected no short will happen when working on the positive terminal unless……
Remember, your switches, such as a light switch, actually disconnect the positive! so it has nothing to do with the direction of current flow.
Remember, your switches, such as a light switch, actually disconnect the positive! so it has nothing to do with the direction of current flow.
If you have a battery in tight quarters in an older car with lots of metal around, turning the pinch nut with a wrench on the positive terminal while negative is still connected can certainly get your attention if the wrench touches the body work.
I learned that one pretty quickly on the MG, since the battery(or two batteries when new) sit down in wells behind the passenger seats and of course are almost totally surrounded by metal. I also installed one of the plastic caps over the positive terminal, which it didn't have when I first bought the car.
I learned that one pretty quickly on the MG, since the battery(or two batteries when new) sit down in wells behind the passenger seats and of course are almost totally surrounded by metal. I also installed one of the plastic caps over the positive terminal, which it didn't have when I first bought the car.
This and also think that when you reconnect the negative cable it will not spark and possibly cause issues with battery gasses escaping the battery and causing an explosion.
The positive cable will spark when you reconnect it.
JHZR2
Staff member
Either will spark if there is sufficient load.
The chassis of the car (or whatever) is all negative/ground.
Your wrench cannot short circuit from the negative terminal to anything nearby.
Conversely, taking off the positive terminal first, then hitting the chassis of the boat, WILL fry the electronics mounted on your Merc 25HP motor...
Your wrench cannot short circuit from the negative terminal to anything nearby.
Conversely, taking off the positive terminal first, then hitting the chassis of the boat, WILL fry the electronics mounted on your Merc 25HP motor...
I keep some of those terminal covers and when I pull the negative terminal I pop a cover on, same with the positive one. I also have some for a side post battery.
Arc strike:
Dropped the wrench when removing the nut from the negative terminal bolt. Woke me right up.
Dropped the wrench when removing the nut from the negative terminal bolt. Woke me right up.
If you're real careful with the positive cable not to get the wrench near anything metal while loosening the clamp, you would be fine. But if you do this often enough the odds will be against you so that when you're not paying careful attention to what you are doing and get distracted for a second, the wrench does touch something metal and all kinds of sparks will fly and cause damage to the car, the tool or to you. It's just as easy to disconnect either one so it common practice to do the negative first.
I think many have a wrench like that.
my last one was when loosening the negative clamp with a ratchet wrench, I went a little to far over and touched the exposed positive post. Scared the you know what out of me, and took a second to get the welded on wrench off of the terminal.
I now throw a rag over the positive if it does not have a cover over it when loosening the negative terminal, and make sure to use a regular wrench, not a ratching or a socket wrench.
Current flow is Current flow....The negative cable will arc just as much as the positive cable if there's Current flow/draw. You can usually smell a "gassing" battery....Smells like sulfur/rotten eggs.
Because the negative is connected to the body of the car, the engine, etc. If you were to bridge a wrench between the engine and positive terminal, you would create a dead short.
If you disconnect the negative, then such a connection would not result in a dead short. It's simply because the negative is connected to the engine and chassis. If the car had a positive ground, you would disconnect the positive battery cable.
I remember back in the early 80's watching my brother and his screwball friends put a new alternator in his Oldsmobile, and he was using a small wrench to remove the positive cable (first). He dropped the wrench and it landed right on the positive terminal on one end and the (grounded) metal support bracket on the other. Sparks flew everywhere and it then glowed cherry red for about five seconds. About 20 minutes later when it was cool enough to touch he had to beat the snot out of it to remove the welded remains. Good times.
Had he removed the negative first like he should have, the wrench falling and hitting the positive terminal on one end and anything other than the negative battery terminal on the other, that would have resulted in.....nothing.
Always, always, always disconnect the negative first, and put on back on last.
Had he removed the negative first like he should have, the wrench falling and hitting the positive terminal on one end and anything other than the negative battery terminal on the other, that would have resulted in.....nothing.
Always, always, always disconnect the negative first, and put on back on last.
I really hate the way every service seemingly begins with this, even for stuff not electrical... I pretty much only do it if removing the battery or alternator.
On older vehicles. Now days, everything goes through a BCM and switches are actually 'ground' input commands (if not on a bus) to the BCM...and then the BCM turns on your lights.
(This is why I can't turn on my headlights with the ignition switch off in my '08 Sorento.)
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