When using jump leads they say you need to connect the positive (red) terminal first.

I have tried to understand this, why not the negative first? As the negative is invariably the earth, the common ground, even if the cars are insulated from the ground, still, I cannot see why the positive first.

I have imagined it may have to do with "what things you can accidentally touch with the jump lead terminal and cause sparks", but it seems the same risk either way.

any ideas?

One word comes to mind "SAFETY".

Lead-acid batteries produce flammable hydrogen gas while being charged. This highly explosive gas, generated within the cells, will expand and seep out of the vent caps. A cigarette, tool, or spark from any source could ignite the gas, causing the battery to explode.

1) Connect the first cable to the positive (+) terminal of the good battery; then attach the other end of that cable to the positive (+) terminal of the dead battery.

2) Next, attach the second cable to the negative (-) terminal of the good battery, and make the fourth and last connection to a clean metal part, such as the engine block of the car being energized, rather than to its negative battery terminal. This completes the electrical circuit, as if it were connected to the dead battery, but if sparks are produced, it serves to keep them away from any explosive battery gases.

ozzy

I knew about the battery exploding and the danger of sparks, but I had not imagined to connect to the block or chassis. Thanks for the explanation.

many years ago, my brother and i went off to collect a derelict and unwanted car that need loving. in trying to start it we had jumpers on and off and what not. i dont remember all that we actually did but i was in the car and my brother moved a cable and somehow the gas from the charging process in the battery ingited. BOOM!!!! it blew the top of the battery straight up, well not all of it but a piece as big as the back of your hand. my brother 's head was right under there and i flew out of the car positive that he would have acid dripping off his face, but miraculously nothing. and oddly the battery actually would take and hold a charge with the hole in the top. astounding.

i have always had a whole different view of batterys since then... respect... and wary of them.

the above description of how to do it is excellent.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I had the same thng happening - the battery exploded half-way through charging it, one of the connecting clips must have moved and produced a spark. Initially I heard a loud bang and then smoke and smell. By the time I went around to check I could not see anything obvious. Then I saw liquid on the ground and tracing it back it was the battery that had split in half, still intact at the bottom, so held together, and still capable of running the car...