Jump starting (according to haynes)

[827Roverman]

Firstly, Happy New To Everyone, I treated myself for Christmas with a new Haynes Service & Repair manual (my old one was rather dog eared). Today I had a look at it for the first time, I spotted something I thought a little strange (to me anyway) On the section of jump starting from a booster battery, It tells & shows you how to connect the jump leads, all common knowedge so far, But the it goes on to say "Start the engine using the booster battery, and run it at idle speed, Switch on the lights, rear window demister, and heater blower motor, Then disconnect the jump leads in reverse order, Then turn off the lights etc" I have jump started many cars over the years without switching on lights, heater etc, My question is, what is the need, or purpose of doing what Haynes say regarding lights etc ??

[Laird Scooby]

Originally Posted by 827Roverman

Firstly, Happy New To Everyone, I treated myself for Christmas with a new Haynes Service & Repair manual (my old one was rather dog eared). Today I had a look at it for the first time, I spotted something I thought a little strange (to me anyway) On the section of jump starting from a booster battery, It tells & shows you how to connect the jump leads, all common knowedge so far, But the it goes on to say "Start the engine using the booster battery, and run it at idle speed, Switch on the lights, rear window demister, and heater blower motor, Then disconnect the jump leads in reverse order, Then turn off the lights etc" I have jump started many cars over the years without switching on lights, heater etc, My question is, what is the need, or purpose of doing what Haynes say regarding lights etc ??

The idea is David so that if the battery has lost a cell and therefore won't present a load to the alternator, the lights etc will give some load so the alternator doesn't blow itself up. Usually not necessary but obviously trying to cover their backsides against any comebacks.

[827Roverman]

Originally Posted by Laird Scooby

The idea is David so that if the battery has lost a cell and therefore won't present a load to the alternator, the lights etc will give some load so the alternator doesn't blow itself up. Usually not necessary but obviously trying to cover their backsides against any comebacks.

That electrickery again, hear what you are saying, David.

[Forrest]

There seems to be no end of slightly different sage advice for jump starting a car these days. How lucky I must have been never to blow myself up or damage anything all those times in my youth when I just connected positive to positive, negative to negative; fired up both cars in order and then took the jump leads off.

The advice to have some load switched on seems to be to dampen voltage surges associated with sudden load changes on the alternator. Presumably, if using a donor car rather than a booster battery the same risk applies to the donor vehicle as well. Some other online guidance, e.g. the AA, advises only removing jump leads with all engines turned off. The crux of the problem seems to be that a voltage surge from the alternator when removing the jump leads can be enough to fry the ECU, which could be expensive to fix.

[Laird Scooby]

Originally Posted by Forrest

Some other online guidance, e.g. the AA, advises only removing jump leads with all engines turned off. The crux of the problem seems to be that a voltage surge from the alternator when removing the jump leads can be enough to fry the ECU, which could be expensive to fix.

That AA advice would defeat the object of a jump start because you'd be switching off two cars, one with a charged battery, the other with a near flat battery to remove the jump leads. The car with the near flat battery would then need jump starting again!

[martin calva]

Originally Posted by 827Roverman

I have jump started many cars over the years without switching on lights, heater etc, My question is, what is the need, or purpose of doing what Haynes say regarding lights etc ??

Makes no sense to me. Disconnecting the battery from the other car is not going to make any significant change to the voltage across the battery of the car that has just been started. Sounds like advice from somebody following some sort of mechanic's folk lore but without any understanding of why.

[Forrest]

Originally Posted by martin calva

Sounds like advice from somebody following some sort of mechanic's folk lore but without any understanding of why.

My thoughts too.

Whilst on this subject does anyone know of any real life cases of people having hydrogen explosions due to arcing when connecting/disconnecting jump leads? This is apparently the origin of the advice to connect the negative to an earth point remote from the flat battery.

Perhaps they’re the same people who have fires at petrol stations through careless use of a mobile telephone.

[Laird Scooby]

Originally Posted by Forrest

My thoughts too.

Whilst on this subject does anyone know of any real life cases of people having hydrogen explosions due to arcing when connecting/disconnecting jump leads? This is apparently the origin of the advice to connect the negative to an earth point remote from the flat battery.

Perhaps they’re the same people who have fires at petrol stations through careless use of a mobile telephone.

I've not had batteries explode during jump starting but i have seen the result of a battery expldoe on the car during trying to start it, had a battery explode in my face during charging and had another battery explode during charging. Makes a hell of a mess.

I wouldn't take any risks while connecting for a jump-start, too fond of being able to see what i'm doing!

[Wagon Sailor]

Originally Posted by Laird Scooby

I've not had batteries explode during jump starting but i have seen the result of a battery expldoe on the car during trying to start it, had a battery explode in my face during charging and had another battery explode during charging. Makes a hell of a mess.

I wouldn't take any risks while connecting for a jump-start, too fond of being able to see what i'm doing!

Do you know why this happened?

[Laird Scooby]

Originally Posted by Wagon Sailor

Do you know why this happened?

Faulty batteries, certainly on the one that blew up during starting.