Bob F Posted February 5, 2009 Report Share Posted February 5, 2009 When removing a battery which battery lead should you remove first? Negative first. Positive first. Either one, it doesn't matter. You should never remove the battery leads. Quote Link to comment Share on other sites More sharing options...
Bob F Posted February 6, 2009 Author Report Share Posted February 6, 2009 Quite a few opinions on this one, but the safest answer, and one you should definitely apply to removing your car battery, is to remove the negative first. Here's why. The roots are from the cars but bare with me. All circuits need a complete loop of wire from one terminal of the battery through the circuit itself (eg: the lights say) and back to the other battery terminal. So basically two wires are needed for each circuit - a "flow" wire and a "return" wire. In order to save on lots of unnecessary wiring, all cars with metal bodies use the chassis of the car as one of the wires. This is called "grounding" or "earthing" in car electrical parlance and thus saves 50% of the wiring (and associated expense) that would otherwise be needed. (Thus fully fibre-glass boats need lots more wire). Now to the crux..... you will only get a big bang if you connect the two terminals of the battery together. So if you leave the negative lead connected and, whilst disconnecting the red positive lead, your spanner happens to touch the chassis while its other end is around the bolt on the positive, you will have connected the two terminals together through your spanner and it will become a permanent part of your chassis as 300 amps flow through it and weld it to the car! You will also probably let go of it very smartly as its temperature reaches that of the Sun!! Conversely, if you disconnect the negative lead first, thus also disconnecting the chassis from having anything to do with the battery, then your spanner can be round the positive and touch the chassis with no ill effects. There is no need to disconnect the positive terminal of the battery unless of course you are taking the battery out for some reason. If you do undo both, always remember, UNDO THE NEGATIVE FIRST. When you have undone the negative lead, tuck it safely out of the way so that it can't suddenly spring back and touch the negative of the battery again before the work is finished. And do undo the negative everytime you work on the electrics. A minute's extra work is better than a fire. Now some will argue that it doesn't matter which lead you remove first on a boat constructed of GRP or wood because there is no metal chasis which the negative is connected to. This is true, but there are circumstances where you can still short out the battery. For example, say you have a two battery set up with a 1-2-BOTH battery switch, and the battery switch is left in the BOTH position. If you remove the positive lead from battery #1 first and leave it dangling and it accidently touches an earth somewhere (for example the engine block or an earth stud or the earth on either battery), there will be an electrical connection through the battery switch to the live positive terminal of battery #2. And hey pesto - messy pants time!! So, safe practice when removing a battery on a car or a boat is to REMOVE THE NEGATIVE FIRST. Now, what about when you reconnect the battery??? Quote Link to comment Share on other sites More sharing options...
Jim Posted February 6, 2009 Report Share Posted February 6, 2009 I should say it's the reversal. Connect the positive first. Well done Bob. - - - I made the mistake of replacing the screw caps on a battery that I was charging years ago. I went in the house for a cuppa. A little while later, I was connecting the battery back up and the spanner slipped, and caused a short accross the terminals. There was a massive bang, and I got a face full of plastic shards and sulphuric acid. I was very lucky not to lose my sight. So, be carefull Jim Quote Link to comment Share on other sites More sharing options...
Brian Posted February 6, 2009 Report Share Posted February 6, 2009 Unless it's a classic car with positive earth. Then remove the positive first. Quote Link to comment Share on other sites More sharing options...
Rob Posted February 6, 2009 Report Share Posted February 6, 2009 "a face full of plastic shards and sulphuric acid. I was very lucky not to lose my sight." Lucky you only lost your looks hey Jim!!! Quote Link to comment Share on other sites More sharing options...
Wight Magic Posted February 7, 2009 Report Share Posted February 7, 2009 wot looks? Quote Link to comment Share on other sites More sharing options...
great white Posted February 7, 2009 Report Share Posted February 7, 2009 Nice one Bob. If you dont believe it try it. [in a safe well ventilated space.] Usually if you disconnect the positive first you get a spark Negitive first you do not. As a I believe that a charging battery gives off hydrogen gas you do not want many sparks around Charlie Quote Link to comment Share on other sites More sharing options...
Jim Posted February 7, 2009 Report Share Posted February 7, 2009 Thanks Rob & Dave ! Jim Quote Link to comment Share on other sites More sharing options...
Bob F Posted February 7, 2009 Author Report Share Posted February 7, 2009 (edited) Oh Charlie, you just stole my next question. Yes, the gas given off is Hydrogen, which is why it is very important to ensure the batteries are well vented. Edited February 7, 2009 by Bob F Quote Link to comment Share on other sites More sharing options...
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