Jump starting - have I created a problem?

[nathanhenry]

The problem with jump starting is "load dump". What happens is you get the car started (so far so good) now the alternator (which can push out 200A or so) maxes out charging the flat battery. Still OK. Now you disconnect the first battery/car and this is where the problem occurs. As you disconnect the battery the load being charged reduces but the alternator is running flat out. The result is a big voltage spike for a moment until everything settles. That spike can destroy car ECU's, pretty much any ECU on the car.

If you jump start from a running second car that car sees a load dump when the jump leads are disconnected so it can be damaged too!

There are various ways to try to minimise the spike - like turning on the rear demist - before disconnecting the jump leads, or letting both vehicles run for a good while before disconnecting the leads. However in a car with a big battery, powerful alternator and lots of ECUs jump starting is a risky business. https://alzaabiautocare.com/blogs/how-to-jump-start-a-car/

Ran across a damsel in distress today in the parking lot. Dead battery (she thought) and indeed it would not turn over and there were no lights. I was a little leery about jump-starting her car with my TDI, just worried that with all the dang computers onboard something on my car might object to the voltage spike or drop that might accompany jumping another car. So I spent a good long time looking over her wiring and checking her fuses. Found a burned-out fuse so I didn't have to jump-start her after all. But my question remains: are there any concerns about jump-starting a car with an '09 or '10 TDI (aside from the usual issues of jump-starting Best Practices)?

 

[markjay]

Ran across a damsel in distress today in the parking lot. Dead battery (she thought) and indeed it would not turn over and there were no lights. I was a little leery about jump-starting her car with my TDI, just worried that with all the dang computers onboard something on my car might object to the voltage spike or drop that might accompany jumping another car. So I spent a good long time looking over her wiring and checking her fuses. Found a burned-out fuse so I didn't have to jump-start her after all. But my question remains: are there any concerns about jump-starting a car with an '09 or '10 TDI (aside from the usual issues of jump-starting Best Practices)?

Opinions are divided.

Some say 'don't do you it, you'll fry the electronics'. Some say 'it's OK if you run the high beam for a while before jump-starting'. Others say 'nah, it's perfectly safe mate, been doing it for years'.

Personally, I wouldn't jump-start a car from another car, because I'm not sure if it's OK or not to do that, so why chance it.

 

[DSM10000]

Opinions are divided.

Some say 'don't do you it, you'll fry the electronics'. Some say 'it's OK if you run the high beam for a while before jump-starting'. Others say 'nah, it's perfectly safe mate, been doing it for years'.

Personally, I wouldn't jump-start a car from another car, because I'm not sure if it's OK or not to do that, so why chance it.

We have a jump pack in both of the main cars. I have used one for starting the Range Rover, 2 friends cars and to help a lady whose car wouldn't start and was stuck in a Salisbury car park I followed the instructions on the units and there were no ill results I am aware of.

 

[geoffus]

The problem with jump starting is "load dump". What happens is you get the car started (so far so good) now the alternator (which can push out 200A or so) maxes out charging the flat battery. Still OK. Now you disconnect the first battery/car and this is where the problem occurs. As you disconnect the battery the load being charged reduces but the alternator is running flat out. The result is a big voltage spike for a moment until everything settles. That spike can destroy car ECU's, pretty much any ECU on the car.

If you jump start from a running second car that car sees a load dump when the jump leads are disconnected so it can be damaged too!

There are various ways to try to minimise the spike - like turning on the rear demist - before disconnecting the jump leads, or letting both vehicles run for a good while before disconnecting the leads. However in a car with a big battery, powerful alternator and lots of ECUs jump starting is a risky business.

Old wives tale, no such thing as a spike when jump starting a car.
When you disconnect the donor battery the car battery is still connected, this acts as a suppressor. The alternator will not go into freefall and the voltage regulator will control any voltage increase.
You are getting Amps and Volts mixed up and no-one has ever simulated a voltage spike in a jump charge situation.

 

[jasonh]

Old wives tale, no such thing as a spike when jump starting a car.
When you disconnect the donor battery the car battery is still connected, this acts as a suppressor. The alternator will not go into freefall and the voltage regulator will control any voltage increase.
You are getting Amps and Volts mixed up and no-one has ever simulated a voltage spike in a jump charge situation.

Unfortunately this isn't true, probably due to only a basic understanding of physics and electronics.

 

[ALFAitalia]

Its fine.....lots of years in the car game....the only time I've seem damage occur is either by connecting the terminals the wrong way around (more common than you might think.....then they blame something else so they don't look stupid!) and/or not letting the donor car tick over a while to get a bit of power in the flat battery before turning it over.....but even then it only usually causes lots of fault codes to ping up....easily corrected with the right diagnostic tool. When you connect another battery to your car all you are doing is effectively creating a bigger/higher voltage battery for the dead car......Not putting the neg terminal directly onto the battery terminal (use another clean earth point ....like the block itself) will help with current flow and clean starting too.
Most of the tales of doom you hear are not first hand because they didn't happen to him....it was "his mate Dave"!!!

 

[geoffus]

Its fine.....lots of years in the car game....the only time I've seem damage occur is either by connecting the terminals the wrong way around (more common than you might think.....then they blame something else so they don't look stupid!) and/or not letting the donor car tick over a while to get a bit of power in the flat battery before turning it over.....but even then it only usually causes lots of fault codes to ping up....easily corrected with the right diagnostic tool. When you connect another battery to your car all you are doing is effectively creating a bigger/higher voltage battery for the dead car......Not putting the neg terminal directly onto the battery terminal (use another clean earth point ....like the block itself) will help with current flow and clean starting too.
Most of the tales of doom you hear are not first hand because they didn't happen to him....it was "his mate Dave"!!!

Exactly, the fictional “spike” does not exist.

 

[geoffus]

City and Guilds time served Electrician in the aircraft industry, studied electrical installations and electronics at Coventry, Rugby and Nuneaton. Distinction in electrical and electronics City and Guilds.

 

[markjay]

Its fine.....lots of years in the car game....the only time I've seem damage occur is either by connecting the terminals the wrong way around (more common than you might think.....then they blame something else so they don't look stupid!)...

Someone did both - connected the wires the other way around, and then denied it - right in front of my eyes last year. The damage was four figures.

 

[SpikyMikey]

Great

Reported

 

[SpikyMikey]

Reported

Thanks Admin.
That was quick