Stupid battery/jump start question...

[geoffus]

When the leads are connected there's no problem. When the leads are disconnected there's an open circuit, even at the precise moment of disconnection. As per Ohm's Law V=IR, therefore I=V/R. When R is infinite I=0.

But there never is an open circuit, the car’s batteries are always connected, so there is no risk of spikes or even high voltages.

 

[geoffus]

If you are expecting the car battery to absorb the voltage spike it won't absorb all of it. The voltage at the car's ECUs could be very different to the voltage at the battery (momentarily). Let's say there's 100A flowing and the battery internal resistance is 100 milliohms, the difference is 10V, that's 10V on top of the 14V that was there at the moment of disconnection, i.e. 24V which can be too much for some ECUs.

Sorry didn’t understand any of that, you can’t charge a battery at 100amps and anyway the alternator would not allow a voltage of 24v. The regulator would control the current and the voltage, just by disconnecting a donor battery would not cause a sudden surge. The main battery is still connected so no reason for a high voltage to be produced, this is why a regulator is fitted.

 

[m80]

These things used to be so simple.

The complication here is having a stupid type battery.

 

[ptruman]

Well this got all exciting
In any case, I acquired a little cheap portable battery pack of that there Amazon which has a pair of plug in crocodile clamps and a special adaptor. It started it fine, so I'm charging it back up ready for tomorrow. I shall leave said unit in the car - as it has a light and is basically a USB portable power pack as well.

 

[Trickythemerc]

It got so exciting I had to send for more POPCORN

 

[jasonh]

There's more.

To explain for anyone who's not clear there's an SAE standard on this you can buy:
J1113/11_202303: Immunity to Conducted Transients on Power Leads - SAE International

And an an ISO standard you can buy ISO7637-2:
ISO 7637-2:2011

But here's a simple extract from Texas Instruments Application Note (Texas Instruments make chips and surge suppression devices):

 

[geoffus]

It does say when the battery or significant load is accidentlly disconnected.
But we don't disconnect the vehicles battery, it stays connected. The good donor battery is not a significant load.
The batteries are connected with jump leads, the operator then starts the car, he revs it a couple of times to make sure its ok. the cars alternator the begins to stabilise and charge the vehicle battery at 14.4v and 20 amps or there abouts. The operator then walks round to the bonnet and removes the fully charged donor battery, the engine continues to run and the alternator carries on charging with its designed parameter.
There is no reason for a sudden spike when the fully charged donor battery is disconnected.
Or am I missing something ?
How would you know if a spike was created? You would have to connect an ossiliscope to the battery terminals whilst removing a donor battery.

 

[colinallcars]

It got so exciting I had to send for more POPCORN

I’m reaching for the razor blades ….

 

[ALFAitalia]

^ LOL!! . Like lots of threads on here.....people like to drag up the theoretical worst case scenario......sure, it COULD happen but in reality it doesn't happen and most don't know anyone the worst case has happened to (wait now for the "my mate Dave knows a bloke who".......type examples to use as evidence!!!). I've jump started dozens of cars ....never hurt one. Even when they were new cars.....(my old boss used to like displaying the forecourt stock cars with the tailgates/boots open......often forgetting to remove the boot bulb!.....so I got a bit of practise.......!!!) I'm surprised no one has questioned whether jump starting affects your insurance and that maybe you should tell them just in case.....another very popular post!!!!

 

[SpikyMikey]

It got so exciting I had to send for more POPCORN

 

[Bobby Dazzler]

it doesn't happen and most don't know anyone the worst case has happened to (wait now for the "my mate Dave knows a bloke who".......type examples to use as evidence!!!).

Depends upon your definition of “my mate Dave” as they’re are both literal and urban dictionary meanings.

My mate Dave (urban dictionary meaning) has had a car jump started by an independent roadside recovery outfit at the request of Mercedes Breakdown Assistance twice, and on both occasions they connected the dead car to their truck (donor) directly and fried the reversing camera module.

On one of those occasions something else was fried, but I can’t remember what it was specifically right now. On both occasions Mercedes dodged meeting the cost of repairing it on the basis that there was no way of confirming whether modules were damaged by jump starting, or had already failed and caused the battery drain.

I’m fairly sure that it was jump starting that caused it.

 

[jasonh]

How would you know if a spike was created? You would have to connect an ossiliscope to the battery terminals whilst removing a donor battery.

I have personally seen a number of dead modules on cars after they have been jump started, mostly on BMW's, definitely glowplug controllers and Bluetooth MULFs.

I have also read the fault codes on those vehicles and seen a number of "over voltage" codes from other surviving modules.

I have seen over voltage codes on Mercedes too after they have been jump started.

If people on here want to jump start cars using other cars, I have no problem with that I'm just trying to explain the facts.

For those who don't understand, or think it doesn't happen, then I'd say 5 out of 6 people say Russian Roulette is perfectly safe ;-)

 

[geoffus]

How do you know the dead modules were caused by jump starting?
I still don’t believe jump starting will cause spikes if done correctly, I physically don’t understand how disconnecting a donor battery can cause the alternator to over voltage a closed circuit.
My previous background is electrical and electronics within the aircraft industry.

 

[Bobby Dazzler]

How do you know the dead modules were caused by jump starting?
I still don’t believe jump starting will cause spikes if done correctly, I physically don’t understand how disconnecting a donor battery can cause the alternator to over voltage a closed circuit.
My previous background is electrical and electronics within the aircraft industry.

Not sure whether your comment is regarding my post (#31). If so, then unfortunately I don’t know or else I would have pushed Mercedes harder to pick up the cost of replacement modules.

The likelihood of two reversing camera modules failing and resulting in battery drain seems slim. Unfortunately the second was only just out of warranty from from being replaced the first time.

 

[geoffus]

Not sure whether your comment is regarding my post (#31). If so, then unfortunately I don’t know or else I would have pushed Mercedes harder to pick up the cost of replacement modules.

The likelihood of two reversing camera modules failing and resulting in battery drain seems slim. Unfortunately the second was only just out of warranty from from being replaced the first time.

Sorry my comment was regarding Jasonh post.
Interesting to note that the modern cars he has reported on have had failed batteries, and that all required jumping, also may have had failing alternators or other issues that may have caused the high voltage issues in the first place.

 

[jasonh]

How do you know the dead modules were caused by jump starting?
I still don’t believe jump starting will cause spikes if done correctly, I physically don’t understand how disconnecting a donor battery can cause the alternator to over voltage a closed circuit.
My previous background is electrical and electronics within the aircraft industry.

Do you actually have any electronics qualifications though - if you did you should understand.

 

[geoffus]

Yes I do, City and Guilds, both in Electrical Installations and in Electronics.

 

[jasonh]

Here's a timely post, make of it what you will. I'd say they did have a key problem now they have a dead FRM module due to the jump start.

Mini Cooper key issues from hell

 

[saff]

So here's a layman's question. This took place yesterday evening. Bro in law phones to ask if I have a starter pack as his partner's 2.2 diesel Freelander has a flat battery. I dont but offer jump leads. He has a 2015 ML250 by the way. He then remembers he has RAC home start and goes with that. When the guy is there bro mentions the jump lead option and the RAC man warns him that it would have done £5K damage to his ML. Would there be any truth to that? Steve

 

[geoffus]

I'm sure we can both find examples of jump starts killing various modules and such on the internet, we can also find evidence of jump starting causing no issues at all.
Its easy for a garage to say it was your fault by jump starting, thus causing your issue, maybe the issue was there before, maybe the damage was done continually trying to start with a exhausted battery, maybe the jump leads were crossed polarity ? We all know trying to start your Mercedes will throw up lots of codes and fail warnings on the dash if the battery is low.
I have yet to see anyone prove whilst jump starting a car, then removing a donor battery would cause a voltage spike, removing or adding a load may increase the current but the voltage is regulated and would not rise significantly.
To prove a spike did occur you would have to connect an oscilloscope to a battery whilst jump starting with a donor battery, then removing the leads and recording the voltage at the exact moment.
This of course would never take place in practice, so its based on theory. This exercise would may have to be performed hundreds of times to create the illusive spark, I've yet to see this.
We will have to disagree on this occasion.