Disconnecting a Battery Cable

If you truly want to be safe you will disconnect the negative lead first and then the positive. Almost all potential for damage is then eliminated unless you drop a tool across the battery posts. This is also a good time to clean the connections and coat them with dialectric grease. Hook them back up in reverse order, positive first.

GA for a 100% correct post.......

If you disconnect the positive first then accidentally touch the frame will short circuit the other jumped battery because the chassis is (-). Also disconnecting the black (-) lead which should be clamped to the bumper (part of the chassis) will prevent sparking any Hydrogen Gas near the battery caps. I new a person that checked the level of his battery acid with a lighter and BOOM went the battery in his face. He was OK lucky for him.

Thanks for the comments. Everyone commenting so far has made good points and tcmtech got 5 points for his answer. I don't remember seeing anyone with 5 points before.

I am sitting here contemplating the answers and my mind started thinking about the old cars we had when I was a kid. They were 6 volt and if I remember correctly some of them were positive ground as opposed to nowdays where everything is negative ground.

Randy

Randy

I think we are talking about controlling and what is your type of system -ve or +ve control so if your system is -ve control i.e +ve ground so you should disconnect the -ve pole and also wrt the +ve control systems.

the aim of that to prevent any current drawback from any devices on the system

Thank you

The answers are given above are all correct, and yes, if you have a positive ground, then you want to remove the positive cable first...

Another thought to bear in mind when working on batteries or jumping vehicles:

TAKE OFF YOUR WEDDING RING! I knew a man who was working on a battery and slipped (or wasn't careful) with the wrench and got his ring between one terminal and the wrench while the other end of the wrench was still touching the other terminal. That gold (a pretty decent conductor ) got red hot instantly and did not cool down until he was done dancing around the shop and found a bucket full of water to plunge his hand in. The lucky thing was that the battery somehow did not explode as well from the sparks - his whole upper body would have probably wound up looking like his ring finger... To this day he does not have to wear a ring finger to prove he had one on one day in his shop!

Metal watch straps can do the same, I have a friend with the scar to prove it still after 35 years......

If you are working on any part of the vehicle there is a danger of short circuiting the 12V system either by careless application of a tool or from a loose, disconnected 12V supply cable. Most electrical equipment will have the return path to battery via the vehicle chassis so if you disconnect the battery negative terminal it should make all the 12V circuits safe to work on, (unless you are really careless and short out the battery terminals) but it still allows you to do fault finding on the electrical system.

All the electrical components will still have their positive supply cable attached and you can then measure the +12V to any adjacent chassis part and trace open circuits defective switches/relays, intermittent wires, etc.

If you are going to disconnect the battery don't forget the radio will probably lose it's access code - you do have the code written down somewhere???

All the electrical components will still have their positive supply cable attached and you can then measure the +12V to any adjacent chassis part and trace open circuits defective switches/relays, intermittent wires, etc.

Not really. The +12v is no longer 12v with respect to ground, which you have disconnected. Also many circuits will not work at all for troubleshooting because the associated relays will not operate.

The primary reason for disconnecting the ground (negative) lead first is to avoid the short and dramatic sparking and heating that can occur between the wrench used for disconnecting the positive cable and any metal object connected to the chassis. If you can avoid this accidental contact possibility, then disconnecting the positive cable would work well, because you will have interrupted any possible circuit involving the battery. For many troubleshooting operations, however, you need to have the whole system energized.

Back in the 90's if you removed the positive battery cable and allowed it to contact the chassis you would damage the computer. Don't recall the manufacturer but it was a well known problem among the car audio install community.

I will leave the welding your 5/16" wrench to the frame as a given............

Having been idiot enough to do the worst of disconnecting
batteries in my youth I can speak only from experience.

First NEVER disconnect either terminal after charging the
batteries, especially if the cell caps are open. The risk of
explosion is very real (been there done that) and since the
face is usually in close proximity, guess what gets it!

Second, If you need to disconnect it, I would use a very
small spanner (or screw driver for top cap) and remove the
positive terminal first, such that the whole car is now safe
to work on without fear of shorting at any point.

Last, remember on some modern cars now, disconnecting the
battery also, disables other circuits, like the radio, alarms, etc
and these will need to be re-set, - do you have the manual handy?

There is a way round this by inserting a parallel battery to maintain
these circuits, but if you need a complete disconnection that's no good.

Take care. It looks simple enough - until they blow up in your face!

jt.

It's what you learn after you know it all that counts.

Second, If you need to disconnect it, I would use a very small spanner (or screw driver for top cap) and remove the positive terminal first, such that the whole car is now safeto work on without fear of shorting at any point.

This is considered very unsafe. Most spanners are conductive and batteries are often in positions where it is too easy for the spanner to contact any metal object round the battery. If you are working on the negative terminal such contact has no negative effect. If you are working on the positive terminal without having disconnected the negative terminal, there is a clear hazard, that can lead to exploding batteries, burns, etc.

No manufacturer of a modern car recommends removing the positive terminal first.

Guest (#6) brings up an excellent adder that don't think anyone else mentioned. Which brings me to say that it very well could have been myself with the ring thing he mentioned. I had done exactly that with demonstration to another how the installation/service procedure should NOT be done, with of course the proper method. For some years, I had been using Craftsman rachet wrenches (suggested then and now and are only about 5" in length, well less than the one he was using (like 10"). Well, in any event, I opted to show him how he could do with his wrench: "place your hand across the negative terminal just in case". Oops, damnit! Forgot about the ring. Did indeed weld between the wrench and pos. terminal!. Dance? Yes. Big time. Did luckily get the wrench and my finger free from the disaster. I could have lost that finger indeed. The ring still shows the pit marks.

2-way radio shop circa 1980 something. Sound familiar Guest #6?

Thanks everyone for your comments. I will now look at the flow of electricity with a different perspective. I also wonder how I've not had one of those shorting/learning experiences in my 50 plus years of backyard monkey wrenching. Just lucky!

I originally was just concerned of why it mattered as to how you opened the circuit. Never even dawned upon me about shorting the thing out and the associated problems that could cause.

My Thanks again.

Randy