Jump starting car query

The message from "Bill" contains these words:

Two reasons.

Firstly, there will be a lower resistance if you choose a bit of the engine rather than the negative terminal of the battery. This is because otherwise you have to pass current along the earth-strap which may well be what's at fault in the first place.

Secondly, and it's a minor consideration until it happens to you, if you connect in the correct sequence - positive first, then negative to chassis, any sparks that may occur happen last and away from the battery. Batteries have been known to explode from sparks setting off the hydrogen - it's rare but apparently extremely unpleasant.

Thanks Guy for the explanation, I had read about the minute risk of explosion but wasn't sure if that applied to modern sealed batteries.

The point regarding 'low resistance' makes perfect sense too.

bill

The risk is not minute, I have managed to blow up several batteries, the results are very unpleasant, but luckily for me not too serious, but they could be lethal.

mrcheerful

The risk is minute unless the battery has just been or being charged, where a local spark with the battery in an unventilated space becomes rather dangerous.

I was lucky where in my case just the caps were sent vertically at speed with acid sprayed everywhere!

Or if it's just been heavily discharged, as that can also cause battery gassing.

It's the order of connection. Connect red leads first, then the black to the donor car battery terminal earth, then to an engine earth on the 'flat' car. The idea is that any spark will not happen close to the battery with the possibility of igniting hydrogen from it. But it's a tiny possibility in the open air with a flat battery.

Only if there is a faulty cell(s) as the heavy discharge actually tries to re-charge those.

I did mine rather differently. I forgot it was on charge in the corner of the garage as I set to with the angle grinder... blew the casing to bits, and my charger too!

Pretty well all exploding batteries tend to happen indoors where the hydrogen can accumulate - battery charging areas in garages are always favourites. IMHO it's pretty unlikely outdoors where most cars are jump started.