Battery disconnecting

[Steve-S]

Something that just crossed my mind .

Battery disconnect switches go to the positive side of the battery .
Yet when working on vehicles it is common practice to disconnect the negative side ... hmmm

Any one know the reasoning behind this ?

Steve

[mjw930]

I'm not sure what your common practice is but I always remove the positive side of the battery when working a car, boat, bike or any DC electrical circuit.

The conventional understanding of current flow is based on mistakes made in the 1800's. It was believed that electricity flows from a positive charge to ground, the reference point. It wasn't until electrons were discovered that scientists realized the convention was wrong. At that point it was too difficult to try and change the textbooks and literature of the era so it was modified to conclude electons flow from positive to negative but current flows the opposite direction. While this is massively flawed it was the best compromise they could come up with.

What's really happening is the fact that earth or ground is the reference for 0v. Electrons will only escape a battery if the opposite side of a circuit has a potential, meaning it has fewer (or more) electrons than the negative terminal. The only way you can flow current through a circuit only connected to the negative terminal is to complete a circuit between a point with either a more negative or more positive potential. Since you can't get more negative then "ground" it's unlikely any current will flow through a device (or person) connected to the negative terminal of a battery. If you were to remove only the negative lead from your battery the potential of devices connected to the positive side of the battery would be higher than ground so any conductive material, like your hand, that came in contact with the circuit would cause electrons to flow from the ground into the battery through it (you).

This is why when you wire batteries in parallel (positive to positive, negative to negative) the voltage stays the same but the current doubles. You haven't increased the potential (difference between positive and negative), you've only increase the amount of current that can flow. When you wire batteries in series you increase the potential (voltage) but maintain the current.

SO, you remove (or combine) the positive side of a battery to manage the FLOW of electrons relative to ground.

[Steve-S]

Wow , that was awesome !
I had to read through it four times , but I get it now .
Excellent explaination .

[NICE PAIR]

I gotta headache. ;-)

[Euroski]

I always just removed which was easier. But, the thinking is if the (+) cable was removed and U dropped a wrench which hit the body and (+) of battery things SMOKE! Now if the (-) removed and U drop the wrench on the said above example then nothing happens. But as we all know with electrons and bad luck sh!t still can happen. Also it's easier to have a battery switch to switch (+) on multi battery systems than switching (-). I heard having poor grounds on multi battery system could lead up to having 24v being supplied to consumers.

I'm done... time to go boating.

[Robby321]

I always just removed which was easier. But, the thinking is if the (+) cable was removed and U dropped a wrench which hit the body and (+) of battery things SMOKE! Now if the (-) removed and U drop the wrench on the said above example then nothing happens. But as we all know with electrons and bad luck sh!t still can happen. Also it's easier to have a battery switch to switch (+) on multi battery systems than switching (-). I heard having poor grounds on multi battery system could lead up to having 24v being supplied to consumers.

I'm done... time to go boating.

Correct. If only shutting off current from a batt like a disconnect, either side will work. But working on the motor, Neg should come off.

[mjw930]

Correct. If only shutting off current from a batt like a disconnect, either side will work. But working on the motor, Neg should come off.

I diasgree but do whatever you want. Just don't complain when ECM's and other electronics start acting up.

[Robby321]

I diasgree but do whatever you want. Just don't complain when ECM's and other electronics start acting up.

Depends on just WHAT doing and what yer working on and for what. I been doing what needed since the 50's, and never a problem. And my dad was a wizard, Electronics Tech as a super wrench. Any can do what they want, but I'll keep on doing it my way as worked on more damn engines than I could even think of. The new junk all putered out now I simple take to someone who knows .."what to do" as I'm so old school its beyond me to mess with it.
Gotta know yer limits........

[mjw930]

Depends on just WHAT doing and what yer working on and for what. I been doing what needed since the 50's, and never a problem. And my dad was a wizard, Electronics Tech as a super wrench. Any can do what they want, but I'll keep on doing it my way as worked on more damn engines than I could even think of. The new junk all putered out now I simple take to someone who knows .."what to do" as I'm so old school its beyond me to mess with it.
Gotta know yer limits........

It's all good. Like so many things in life, there are multiple ways to get the same outcome.

[Robby321]

It's all good. Like so many things in life, there are multiple ways to get the same outcome.

Or maybe some ya don't want and ya learn fast too! Back 70's my XS Switzer I put a new batt in. Easy right? HA! So I put the cables on and got a heck of a spark on one terminal. Said.."Thats not right"? Sure wasn't as reversed the Neg/Pos. YEP! Fried the switchbox!

Reminds me Steve Wright. "Got a electric bill from the power company the other day. I sent it back. Told them I never seen electricity, so I'm no paying for it"

HA!

[BarryStrawn]

Disconnecting the positive is fine for car repairs but for safety, always disconnect the negative first. If you wrench on the positive terminal with the negative hooked up then any connection to a ground will flow a few hundred amps. And there are lots of grounds everywhere under a car hood. So a touch to ground with the wrench you are using on the positive terminal bolts, or that wrench to a ring or wristwatch or whatever can be really unpleasant. Anyone with common sense should be able to avoid a touch to ground but mistakes do happen.

There is no electrical potential between the battery ground terminal and all the exposed underhood grounds so it is much safer to disconnect the negative terminal first.