Many moons ago, my father taught me that when removing or refitting a car battery, always disconnect the positive lead second, and reconnect it first.
The reason?
If, whilst swinging the spanner round to do/undo terminal posts you touch the bodywork or engine, no short circuit will be completed. Think about it, and he's right.
I gave this advice to a friend at the weekend, whose car has a poorly accessible battery. A few minutes later, when, with cold and numb fingers, the ratchet handle was dropped, and free-wheeled down to touch against the bulkhead whilst still connected to the positive post, he was somewhat grateful not to have an exploding battery, or fire, on his hands!
Just thought I'd pass the trick on.
|
Many moons ago, my father taught me that when removing or refitting a car battery, always disconnect the positive lead second, and reconnect it first.
I always thought this was just common knowledge when changing a battery?
|
|
Strange as it may sound positive should be removed first.reconnected last.reason being positive flows to negative.but then in most cases either will do without any damage as no current flow.At least that is what I remember correct if wrong?
|
Your correct in that no current will flow. However, the positive to negative flow is a bit misleading, as electron flow is negative to positive.
Correct no damage will be done to the cars electronics, however if you touch the spanner to the chasis whilst undoing the positive terminal with negative still attached, you get fireworks.
You only disconnect positive first if you've an old positively-earth (someone correct me, sounds like a contradiction :-S )vehicle.
|
ALWAYS disconnect Earth (aka Ground / Chassis ) lead first and reconnect last.
Reason: risk of short circuit and explosion of battery gases if spanner connects vehicle body to live terminal.
|
|
Thats exactly the first post says.
|
Thats exactly the first post says.
??? only if negative = earth
However, the positive to negative flow is a bit misleading, as electron flow is negative to positive.
>>
??? do electrons really flow? electrons in this pc and house electrical circuit come from power station? electrons from my pc now going to your pc?
|
Sounds like it's more appropriate for the 'I have a question thread.'
Yes. There are electrons in the wires that move around, that's what current is. Although current conventionally flows from positive to negative, in fact the electrons go from negative to positive.
Earth = ground. Many cars use the car body as a return to the battery for electric components such as lights.
The positive terminal sits at +12 volts. The car body sits at 0 volts. Lucky really that it's that way round, otherwise you'd get a shock every time you touched the car body whilst standing on the ground.
Top tip, be very careful never to drop a spanner between the positive and negative terminals.
|
'Lucky really that it's that way round, otherwise you'd get a shock every time you touched the car body whilst standing on the ground'.
Err, not likely! 12V is far too low to receive a shock. You really won't start to feel anything until you get above 50 - 60V. Electrical safety standards reflect this, being only concerned with DC voltages of 50V and over.
Try to the classic trick of putting a 9V battery across your tongue. 9V is only just enough to give you a sharp tingle - and that's across a small, highly conductive gap with plenty of nerve endings! Just 3 volts more across the length of your body won't register.
|
|
|
Sounds like it's more appropriate for the 'I have a question thread.' Yes. There are electrons in the wires that move around, that's what current is. Although current conventionally flows from positive to negative, in fact the electrons go from negative to positive.
>>
??? question was tongueincheek. ask physicist electronic electrical engineer and he/she tell you electrons do not flow!!!
|
Oh yes you do, Jazzmag - as in fact you point out. Of course you don't feel it otherwise you'd never be able to attach the red lead to the postive terminal. Just because you don't feel it doesn't mean that you aren't getting a shock - however tiny. You'd feel it if you kissed your car. You'd be constantly discharging the battery as well - not least as the current runs to ground through the tyres.
So as I wrote, it's jolly lucky they do it that way round, isn't it.
Dalglish. Yes indeed electrons do flow. Very sorry your tongue was in your cheek as it means you were WRONG!! Your third url even links to a site that shows the electrons flowing.
You may be thinking of alternating current. With AC, the electrons merely oscillate around a mean point. With DC, they do more round the circuit. It's a very slow progression. Whilst the electricity will travel at the speed of light (cue pedantic discussion of the speed of light in copper wires) to the lightbulb when you press the switch, the individual electrons will take a very long time before they flow through all that wire. Think of it like an escalator with a person standing on each step. When one gets on at the bottom, another instantly gets off at the top step. However it takes ages for the person who has just got on at the bottom to reach the top.
|
A friend at work who was our authorised sub-station man used to work for the Yorkshire generating board and told us they have what they called a "Cow test". This is the amount of leakage allowed from overhead pylons.
Where the cow comes into it is that apparently they have a low tolerance to electric for some reason. Perhaps having four legs to earth is an handicap.
And of course the power stations dont use a return/neutral/earth cable but use the ground.
|
Dalglish. Yes indeed electrons do flow. Very sorry your tongue was in your cheek as it means you were WRONG!! Your third url even links to a site that shows the electrons flowing.
mapmaker:
3rd url = demo of conventional simplified misconception.
from www.amasci.com/miscon/whatis.html :
What is electricity? This question is impossible to answer because the word "Electricity" has several contradictory meanings. These different meanings are incompatible, and the contradictions confuse everyone. If you don't understand electricity, you're not alone. Even teachers, engineers, and scientists have a hard time grasping the concept.
Obviously "electricity" cannot be several different things at the same time. Unfortunately we have defined the word Electricity in a crazy way. Because of the contradictory meanings, we can never pin down the nature of electricity. In the end we are forced to declare that there's no such stuff as "electricity" at all!
agree to disagree. hope you do too. but make you happy, i woz wrong, you woz right.
|
Hi Mapmaker,
I guess we differ over semantics; I interpreted you to mean \'shock\' as in \'ouch, that hurt!\', the medical definition of electrical shock being, \'the passage of an electrical current through the body that can cause cardiac arrest and/or burning.\'
I guess you mean the formation of a leakage path to earth, resulting in someone being able to sense the flow of current. I still must say I\'ve never felt anything on many occasions over the years, where I\'ve accidentally \'shorted\' both terminals together with my hands, or with feeling anything flowing through my feet - even in the rain.
I guess I\'m not just a sensitive sort of guy. I\'m sure my would agree!
|
Jazzmag: I concede if you're taking me too literally then my description wasn't too helpful. But your 9v battery example is a shock, and that doesn't concur with your medical definition.
Dalglish.
Any fool can write incomprehensible tosh like that website. Do you really think electrons don't flow under DC current? Or what?
|
.Any fool can write incomprehensible tosh like that website. Do you really think electrons don't flow under DC current? Or what?
>
I thought the first two were pretty good. The third was pretty simplistic. IMHO they're not incomprehensible nor written by fools but this is a very difficult topic to get your head round, very difficult to explain in non-specialist language and they're a pretty good attempt.
I think I'll stick to threads on mats.
|
|
|
|
|
You are right.It is standard to disconnect the neg terminal first.Just so accident`s like that dont occur.The same applied to the positive earth in years gone.Those that haven`t a clue do so anyway correct or not.
|
Having at last established which terminal to undo first, don't undo anything until you have connected up a back-up voltage.
On many cars you risk:
a) losing the radio code, or worse,
b) upsetting the computer/electronics
|
Here's how it's done at "Hawkeye Motors" on a 1996 Citroen Synergie with no reliable record of the radio code -
1. Cobble together a 12v supply from an old mains phone charger and a cigar-lighter plug.
2. Turn ign key to accessory setting to energise the cigar lighter circuit and connect up Heath Robinson power supply.
3. Disconnect negative terminal and clean inside surface with Stanley knife. Disconnect positive terminal, clean likewise, and hide it a Marigold glove
5. Remove battery clamp and battery
6. Fit new battery and clamp
7. Refit positive terminal
8. Refit negative terminal
9. Coat both terminals with Vaseline
10. Remove power supply, test radio and switch ign off
11. Go and claim smug cup of tea off hard-pressed Mrs H
Hawkeye
-----------------------------
Stranger in a strange land
|
Thanks Hawkeye - I knew it wasn't as simple as just undoing the terminals and letting them dangle.
I like the Marigold glove - easy to forget that with the back-up supply connected, the positive terminal is still live.
Here's another bit of live-wiring trickery. How to undo the thick live wire to the starter motor, eg for replacement, while maintaining back-up voltage? Wrap the spanner in a rag (glove even better) and then pop the live cable up an old radiator hose.
|
|
|
|
|
