Explosion of lead-acid battery

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michaeltay

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Hi:

I would appreciate your assistance for the following questions.

1) Why are batteries in vehicles (sometimes) grounded? What is the purpose? What are the advantages/disadvantages of grounding or not grounding?

2) A lead-acid accumulator exploded when a metal bar fell over it. What could cause the short circuit? Did the metal bar contact simultaneously both the positive and the negative terminal? Or can the short circuit occur with the bar touching only one terminal (positive ort negative) and ground?

Thanks in advance.

Michael Tay
 
I'm by means qualified to discuss auto wiring in depth, but I assume they are grounded mainly for cost savibgs. By grounding the battery, the auto mfg. is able to use the metallic car body (frame) as one conductor thus not having to run two conductors to each electrical apparatus.

The short can occur anytime the currrent has a path between the positive and negative. If one is connected to the frame, then a path can be established between the frame and the other post. Thats why most folks disconnect the grounded post first. It's easier to allow your tools to touch the frame as you are disconnecting the non-grounded post.
 
Lets be clear, no automotive electrical system is intentionally 'grounded'. (That would require a very long conductor back to the grounding electrode and would create a big mess at intersections. ;))

Any vehicle electrical system I have ever seen has one side of the battery bonded to the vehicles frame/body. Some much older cars bonded positive now I think all bond negative.

Now I fully understand that you will see this referred to as either 'Negative ground' or 'Positive ground' but that is more of a slang term then the reality of the situation.

As far as the short circuit assuming a negative bonded system all that would have to happen is a tool falling between battery positive post and the vehicles frame or bodywork.
 
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what adds to the confusion about "grounding" of automotive batteries is when aftermarket manufacturers push products such as "grounding kits" (US) and "earthing kits" (overseas). the frame of the car is the return path as has been mentioned, for cost savings.
on automotive electronic forums i've read of people who've replaced their oem batteries with aftermarket not correctly matching the size and not using insulator's on the posts. upon closing their hoods they dead shorted the battery causing the post/s to explode thru the hood. not good!
 
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I can only add to what has been said so far.

1. Pure economics by using the vehicle frame as a conductor. Otherwise manufactures would have to use much more distribution wire and hardware including fusing both polarities to every electrical device in the vehicle. By bonding to the frame only the un-bonded polarity needs a fuse or breaker.

Only one disadvantage I can think of is noise produced by the ignition system riding on the frame and wiring. It was a real problem in the 80?s when the first generation of High Energy Ignition systems was introduced

2. That has already been pretty well covered all you have to do is short the un-bonded polarity to the frame or opposite terminal as they are both the same point electrically.
 
You guys have pretty much covered the questions, but just to reiterate, the body is used as a conductor because it is conductive, and it's easier to intentionally use the body than it is to insulate from the body.

This is similar to the single-wire electrical distribution systems (SWER - Single-Wire Earth-Return) that uses the earth, rather than a grounded conductor, to complete the circuit.

By the way, the reason we use a negative-ground system in vehicles is that the ignition spark is more effective at igniting the fuel-air mixture than with a positive-ground system.

It has to do with the direction of the spark, between the spark-plug tip and the ground prong. I don't remember which way the spark jumps with the negative ground, but that's why.
 
Elecrons being negaitive charges flow from negative to positive, hence the spark will be from outer rim of plug to center. I can't see it will make any difference to ignition though and believe the real reason is to reduce corrosion in the auto body frame/chassis.
 
If you dont bond one side of a battery system (pstive ground has shown to be best) and a conductor contacts conductive parts that are relatively close to the current flow and or the opposite polarity insulated conductor, a capacitance charge could build up, till the charge exceeds the insulations volatge rating, cause a short and then a fire...in sort
 
cause's corrosion?

cause's corrosion?

okay i've heard that argument before. what's the substantiation on postive "ground" cause's corrosion compared to negative "ground"?
 
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Limey Pete said:
Elecrons being negaitive charges flow from negative to positive, hence the spark will be from outer rim of plug to center. I can't see it will make any difference to ignition though and believe the real reason is to reduce corrosion in the auto body frame/chassis.
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In some older cars, positive "ground" was used which resulted in a negative voltage on the center electrode which tended to run hotter than the other electrode. Being hotter, it released electrons more easily than would the other electrode. That is, it functioned like a cathode in a vacuum tube.

In modern cars, the ignition system is far more sophisticated, and there is no need to use a positive ground.

The auto engineers do not follow the NEC, so if they call the chassis a ground, then it is a ground. I think too that in modern cars, most if not all the lighting and accessories do not use the chassis as a return path.
 
mgd said:
okay i've heard that argument before. what's the substantiation on postive "ground" cause's corrosion compared to negative "ground"?
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I'm not sure but the following article managed to confuse me in suggesting the opposite is true:

http://engr.smu.edu/~levine/ee8320/positiveground.pdf

Actually, reading the article again especially pages 3 & 4 regarding "electro-plating" it now seems to make perfect sense, negative "ground" would quickly diminish conductor sizes through chemical action, and corrosion.
 
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