If you’ve got hydrogen around the battery on the dead vehicle then you probably don’t need to jump start it! (Assuming the battery is not goosed, and you’ve not been trying to use a charger on it!). Explosions are real, although less common with modern maintenance free “sealed” batteries.
Don't forget to have the jumper idling BEFORE you start connecting things....or you may find you've 2 flat batteries and no running car.
If a car battery is really flat it's best to connect the jumper and leave it a minute or 2 (jumper just revving slightly) to attempt to get a bit of juice into the flat battery. Particularly helpful if your jumper leads aren't very thick.
Also make sure the cars aren't touching. That doesn't need saying though really...I mean we don't go around scuffing into cars carelessly too often.
Connecting the jump lead to the earth on the engine, if that's accessible, it's not only safe but it cuts another bit of copper out the circuit and potentially gets the voltage up further as a result. Sometimes every bit helps.
As for why to rev the jumper, I have had the jumper drop revs and nearly stalled as the last connection was made. The sudden demand from the alternator, and jumper being a small car. Diving in the open door and giving a few revs saved it.
Also make sure the cars aren't touching.
Why would that matter? They have a common earth via the cable and plastic/paint are insulators. (Genuine question, have I missed something?)
Depends, the battery on a 3 litre straight 6 diesel should start pretty much anything instantly. The battery on a 1 litre 3 cylinder petrol might not even start a big motorbike... Needs to be running. A bit faster. Also means you can catch things if the voltage dips too far when you start the other car.That battery can supply enough current on its own (otherwise the good car wouldn't be able to start either), which is why I said that you shouldn't really need the good car to be running at all.
For maximum shits and giggles one of my previous employers had a requirement that you should be able to jump any diesel engine in our range using a 24v truck battery safely. Due to the number of outfits sending out truck mechanics if a diesel car called for recovery (in the 70s). No idea if it was true, but we all used to go outside to watch them testing, slamming the starter pinion into the ring gear with almost twice the speed it needed, and the (factory standard) jump cables going all shiny as they started to overheat. Thankfully deleted when we did a major overhaul in the early 00's.
