Dumdum moment (involving jump leads)

[Greg66]

Our D2 A8 chews through batteries. Today it spat out its latest one.

So I look in the handbook for the S4 long enough to find out where the battery is, hook the batteries up and start the S4. The cables are smoking immediately. Oops. Turn off the S4, disconnect and resolve to ring the RAC, assuming my jump leads are a bit too cheap for this job.

So then I look at the S4 handbook a bit more. It says that when the S4 is jump starting another car, the negative lead should not be connected to the negative terminal on the dud battery, but to the engine block on the dud car. I've never come across this before. Anyone know why it says this?

Bit impractical for me, as the A8's battery is in the boot, and I'd need v v long leads to pull this trick off were I to try it.

[no_RS]

Can only assume it is because the extra cable between the dead battery and the engine increases the load on the S4 which it may not like. Anyway why were the cables smoking? Would expect them to get warm but not smokin', that sounds like more of an issue as it would appear you are shorting your S4 battery out and will end up with 2 dead batteries.

[Greg66]

Smoking = heat = current is too high for the cable (and they were hot!). That much school physics I do recall. Cheapie jump leads, I suppose.

It's not an extra cable. It's leaving the circuit open and earthing it (so googling tells me). Just not sure why it's necessary in this case, because I've jump started cars before with straight +ve to +ve and -ve to -ve without issues.

[nihc]

Hi,

They tell you to make the last connection to the engine block so that it's a safe distance away from the battery when it sparks. Otherwise, you could trigger an explosion with any hydrogen gas hanging around.

Chris.