Does it matter order of battery disconnection in a car?

[becrux]

The big spark doesn't happen until the short circuit is broken. When the short is established current begins to flow, thereby creating a magnetic field. You can think of this induced field as electric 'momentum'. When the circuit is broken, the energy contained in the field has to go somewhere, and a much higher voltage is induced by the breakdown of the magnetic field. the spark will jump that larger gap, aided by the plasma.
>Maxwell's Equations<

[GILow]

Ok, becrux sounded a lot more convincing than my Star Trek science.

Ecos gets my vote for the most poetic explanation.

Wotname, you gonna put me out of misery?

[Wotname]

I will accept becrux's answer as correct, he clearly knows basic electrical theory ; I will expand on it below.

I was disappointed other's didn't give it a go - maybe they thought the exercise was too simple (or perhaps too hard ).

I take my hat off to Ecos for having a go - especially as I understand, he doesn't have an in-depth electrical background and his final answer was worthy of a beer

GI, I think you know the theory but perhaps forgot to apply it this time ().

So 12V (DC) isn't enough PD (potential difference) to generate a spark of any size. If you want a long spark, you need many many more volts. A spark plug has a gap around 20 to 60 thou and needs something like 30,000V to bridge it.

Short out a fully charged 12V battery and you will draw at least couple of hundred amps. This creates an magnetic field around the conductor (basic electrical theory ). This field is proportional to the size of the current. Break the circuit (either by removing the short or the having the conductor fail - melt etc) and the current creases to flow, thus the magnetic field around it collapses.

A magnetic field cutting a conductor creates an electrical charge and the charge created is proportional to the size of the magnetic field and the rate of change of the field. Again, basic electrical theory - as in how a generator or alternate works - conductors cutting a magnetic field (or vice versa - of course).

So remembering the magnetic field existing around the conductor was large (as the current flowing was large) and the rate of change of the field is extremely rapid (i.e. going from a large value to zero) as the conductor is removed, the potential difference generated in the conductor will be many hundreds thousands of volts (for a short period of time). This is enough to ionise an decent sized air gap. The ionised air (or plasma) has a very low resistance (compared to the un-ionised air) an allows current to flow again from the battery though the conductor and plasma (or ionised air gap). Once current is flowing again, the PD returns to the terminal voltage of the battery (i.e. 12V) and the current can be maintained as the plasma las a low value of resistance. The current will be maintained until conductor overheats or the battery is discharged (i.e. flat) or conductor is removed.

Way way back in the day, I used to installed 50 V, 2,000 AH batteries backed up with twin 500 A charging sources and these had large uninsulated bus bars with only a small air gap for insulation between the +ve and -ve busbars (never be allowed these days ). A metal tool dropped across them would glow red hot for a second or two before disappearing in a shower of sparks. The batteries hardly grunted although the charging sources would peg the meter and afterwards the busbar would show signs of arcing. The was never done intentionally

As other have posted, metal jewellery was left outside the battery rooms.

Unfortunately I never had the chance to access to submarine battery rooms but I guess they were much much bigger!

[becrux]

I was trying to keep it simple, due to the late hour, but I appreciate the expansion.
Well said, sir!
The "shower of sparks" image persists...

[Wotname]

Originally Posted by becrux

I was trying to keep it simple, due to the late hour, but I appreciate the expansion.
Well said, sir!
The "shower of sparks" image persists...

Simple yet accurate - maybe better than the long winded explanation but now we both so that should satisfy most readers.

[Wotname]

Originally Posted by Ecos

How about creating a capacitor?

Originally Posted by Wotname

^^ sorry no capacitance involved but here's a hint - it is something similar.

What is similar is that inductance is the key and inductance is the alter ego of capacitance. Any thing that a capacitor does, inductors do the opposite. For instance, capacitors pass AC and block DC while inductors try to block AC and pass DC. Series inductance is additive while parallel capacitance is additive and so on.

Just in case you were wondering .....

[ShoreFun]

One other thing to consider is the spark when you reconnect.

If you have a really big spark then something that draws a lot of current was in circuit at the time of connection.

So then you have to ask what. Some devices have capacitors which get charged rapidly and cause an in rush current. If you are sure that you did not leave something on and you get a big spark then maybe something in the circuit is wrong.

Something I look at because over the years I have trouble shot many different eletrical issues.

A note about a wrench and a battery.
I did volunteer rescue for a number of years. Once I went to a garage fire. The owner had severe burns.
He was trying to start his 60's convertible. He had a glass with gas in it to pour down the carb. He kept a wrench on top of the battery next to the glass. He accidentally moved the wrench into the battery and knocked over the glass of gas on the battery and himself.
The whole garage was very fully involved in a very short time. It is really cool watching an acetylene tank send a 10' tall flame up helping the fire cook up better.
The moral of the story, respect the battery.

[GILow]

See!

I KNEW it involved plasma.

And Klingons... I am sure there were Klingons in there somewhere...

[parkstone bay]

On an old car I had once, the battery hold-down clamp had rusted through. So I decided to locate the battery in place with a lashing of monel seizing wire. All went well until I accidentally shorted + and - posts with the wire. It went instantly white hot, and I have the brand on the palm of my hand to this day. Professional sparkies often cut their spanners ("wrenches" to our cousins) down in length, so that they aren't long enough to span + and - posts. For the same reason, always have a wood or plastic lid on your battery. That way, when you drop a long enough conductor into the battery compartment, it can't short out. Yes, remove finger rings and other jewellery, always, before working on electrics.

[Dave9111]

A battery story.

A few years ago I was working on my sailboat while it was in the cradle for spring prep. About 200 ft from me, two guys were working on a twin engine power boat. They didn't seem all that skilled and were yelling and swearing a lot, at each other, and the boat. They did not appear to know what they were doing.

So I wasn't surprised when they started yelling and swearing even louder at some point. I kept just working. Soon afterwards they were gone, but two batteries were sitting near the transom of the boat. The batteries looked new, even from a distance. So I walked over to see why they would leave what looked like two new batteries.

Indeed they were new batteries, but the tops of the new green and white Interstate group 27 batteries were all but melted off. Molten lead was everywhere. I could see most of the plates in both batteries and there was a lot of battery acid missing. Only remnants of the lead battery posts remained.

My guess is that they somehow connected plus to minus and minus to plus which resulted in a dead short for both batteries and chaos, melting, yelling and swearing ensued. I've never seen such a mess. Fortunately they didn't catch their boat on fire, but their swift departure may have been a due to a needed trip to the showers or hospital! I never heard what happened to them.

[parkstone bay]

Originally Posted by Dave9111

A battery story.

A few years ago I was working on my sailboat while it was in the cradle for spring prep. About 200 ft from me, two guys were working on a twin engine power boat. They didn't seem all that skilled and were yelling and swearing a lot, at each other, and the boat. They did not appear to know what they were doing.

So I wasn't surprised when they started yelling and swearing even louder at some point. I kept just working. Soon afterwards they were gone, but two batteries were sitting near the transom of the boat. The batteries looked new, even from a distance. So I walked over to see why they would leave what looked like two new batteries.

Indeed they were new batteries, but the tops of the new green and white Interstate group 27 batteries were all but melted off. Molten lead was everywhere. I could see most of the plates in both batteries and there was a lot of battery acid missing. Only remnants of the lead battery posts remained.

My guess is that they somehow connected plus to minus and minus to plus which resulted in a dead short for both batteries and chaos, melting, yelling and swearing ensued. I've never seen such a mess. Fortunately they didn't catch their boat on fire, but their swift departure may have been a due to a needed trip to the showers or hospital! I never heard what happened to them.

The words "Oh" and "Sheeee - ite" spring to mind!

[alexholden1]

On newer cars this is really critical. The mechanic who works on my MB related a horror story about a customer of his that had Advance Auto replace the battery in his Mercedes. They obviously didn't follow protocol and the car would not start after the new battery was installed. It trashed the EMM computer on the car. A $3,000 mistake to save a few bucks on a battery! Always disconnect the negative lead first, then the positive. Reconnect the positive first, then the negative.

[hellosailor]

On modern cars you have a separate issue, not related to safety at all. Some, probably all, of the new cars rely on battery power to keep the many computer settings intact. These can range from your radio station memory and volume controls, to the "fully open" window positions and basically everything that is driver-adjustable. And, the car may have a "learning" transmission or other engine controls that have adjusted to your driving pattern, but those get wiped out without power as well. Unless the car maker has used non-volatile memory ($$) or installed an ultra-capacitor to keep those circuits powered during battery changes.

That's not related to the safety issue. The USN used to have some fairly gory photos on one of their web sites, showing what happened when a sailor went to work on a 12v battery and his wedding ring made the short from positive to ground. The polite phrase is "traumatic amputation".

I got lucky, I only *lightly* welded the bezel of my watch one day.

Just for reference, one typical car battery (Group 24) in average condition can put out over 3000 amps in a dead short. That's what blows off fingers and makes "disconnect and shield the ground" a good idea, no matter what the battery is in. Twelve volts may not be lethal, but 3000+ amps can really do a number on you.

[Wotname]

Yep, never never underestimate the amount power of the stored energy in the humble car battery - it's just waiting to get out

[hellosailor]

I look at it this way: The 16" gun on a battleship could throw a shell roughly the weight of a VW Beetle roughly twenty miles.

But even that little Beetle has enough "stored energy" in the tank to throw itself maybe 250-300 miles. Pretty much like the battleship firing a nine gun salvo, but all stored up in that little Beetle.

VWG has also made a lot of magnesium alloy engine blocks, and when one of those catches fire...Oh yeah, more stored energy in that, too.