Flat battery - DPF warning

AdrianC

And how does an alternator at the other end of a pair of jump leads "spike" in a different way to the alternator that's on the engine of the car you're trying to start?

tedted

because the car your trying to start needs more power quickly as its battery is low, if you put the leads on and leave it for about five minutes the battery will take a surface charge and would be ok but many people put the leads on and try to start it straight away

TimSynths

tedted said:

because the car your trying to start needs more power quickly as its battery is low, if you put the leads on and leave it for about five minutes the battery will take a surface charge and would be ok but many people put the leads on and try to start it straight away

I didn't know this- will bear in mind next time I need to do it.

AdrianC

tedted said:

because the car your trying to start needs more power quickly as its battery is low, if you put the leads on and leave it for about five minutes the battery will take a surface charge and would be ok but many people put the leads on and try to start it straight away

That will indeed draw a lot of current down the jump leads, but it won't increase the voltage.

So how is that alternator putting out more than 14.5v, and why doesn't having that much voltage in the system affect the car that's jumping, where that higher voltage is coming from...?

And does it make a difference if the jumping is coming from another car, or from a jump box?

HansOndabush

AdrianC said:

I'd love to hear the physics behind connecting a battery at 12.5v to a battery that should be 12v but isn't, giving a "high voltage spike".

You're thinking 'schoolboy' electrics, see here:

https://electronics.stackexchange.com/questions/24830/why-is-there-risk-of-overvoltage-when-jump-starting-a-vehicle

Examples of blown electrics:

https://www.portsmouth.co.uk/business/jumpstarting-my-car-left-me-aps650-bill-1221119

http://forum.a8parts.co.uk/showthread.php?t=3980

Damage ecu posts 16 and 21:

http://www.audizine.com/forum/showthread.php/295951-Jump-starting-an-Audi

Blown air bag module:

http://www.vwaudiforum.co.uk/forum/showthread.php?150126-Brake-and-Air-Bag-Warning-lights-af
ter-jump-starting

Blown alternator + ecu?:

http://www.vwaudiforum.co.uk/forum/showthread.php?51896-Damaged-Ecu-Due-To-Jump-Leads-Spike-
Golf-Mk4

AdrianC

"In automotive electronics, it refers to the disconnection of the vehicle battery from the alternator while the battery is being charged."

Just as well nobody's doing that, eh?

Once the started vehicle is running, its own alternator is putting out charge.

As for those examples. My money's on connecting the wrong way round. I'll take a fiver on "person makes stupid mistake" over "breaking basic laws of physics" every day.

HansOndabush

AdrianC said:

"In automotive electronics, it refers to the disconnection of the vehicle battery from the alternator while the battery is being charged."

Just as well nobody's doing that, eh?

Once the started vehicle is running, its own alternator is putting out charge.

As for those examples. My money's on connecting the wrong way round. I'll take a fiver on "person makes stupid mistake" over "breaking basic laws of physics" every day.

Quite frankly you don't know what you're talking about. Read up on inductive loads and connecting and disconnecting power, then come back and try again.