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[terry paget]

Friend of mine has a small Kia, with a battery labelled DO NOT JUMP START. He reckons his car cannot be jump started, I reckon it means his battery is too small to jump start another car. Who is right?

[Lazydocker]

Read the manual 

For about 18 months there was a directive to “Apply no exterior power” to all new BMW diesels. Not sure why, believe it was because of the risk of a power spike.

[Andy B]

Read the manual 

For about 18 months there was a directive to “Apply no exterior power” to all new BMW diesels. Not sure why, believe it was because of the risk of a power spike.

The Mercedes forum seem petrified of jump starting a Merc, often quoting 'power spikes'. The expert indies though are apparently exempt when using their jumper/booster packs 
Won't the very presence of a battery on the 'jumpee' prevent a 'power spike'? 

[Nick W]

The Mercedes forum seem petrified of jump starting a Merc, often quoting 'power spikes'. The expert indies though are apparently exempt when using their jumper/booster packs 
Won't the very presence of a battery on the 'jumpee' prevent a 'power spike'? 

Ferraris were the same, with customers frightened into having their shiny underused toy recovered back to the dealer just because the battery was flat again. So you'd struggle for ages getting it out of the garage and onto the truck, and the same unloading it. Once the paperwork was done, the yardman would the start it with same booster you have and park it normally. A complete waste of everyone's time, which could normally be solved with regular use of a £50 smart charger.

[Doctor Gollum]

With BMWs they should be jumped via the engine bay terminals or risk blowing a relay pack in the rear fusebox

Most vehicles with remote mounted batteries have this requirement and the appropriate terminals clearly identified in the engine bay

[Viral_Jim]

Read the manual 

For about 18 months there was a directive to “Apply no exterior power” to all new BMW diesels. Not sure why, believe it was because of the risk of a power spike.

"Accepted online wisdom" (whatever you consider that to be worth) is that you need to put a decent load on the battery before disconnecting the car that's providing the jump. So turn on heated read/front screens, blower motor and headlights.

I see the logic, not sure how effective it will be though. I also remember reading that batteries have to be coded to the car so that they charge correctly. Really don't under that one 

[2boxerdogs]

With BMWs they should be jumped via the engine bay terminals or risk blowing a relay pack in the rear fusebox

Most vehicles with remote mounted batteries have this requirement and the appropriate terminals clearly identified in the engine bay

. 



Merc ones are easily identifiable under the bonnet, the battery is located under the rear seat. Manual gives all instructions.

[Andy B]

With BMWs they should be jumped via the engine bay terminals or risk blowing a relay pack in the rear fusebox

Most vehicles with remote mounted batteries have this requirement and the appropriate terminals clearly identified in the engine bay

. 



Merc ones are easily identifiable under the bonnet, the battery is located under the rear seat. Manual gives all instructions.

Or in my case, under the driver's seat 

[Andy B]

With BMWs they should be jumped via the engine bay terminals or risk blowing a relay pack in the rear fusebox

 ......

Not doubting you ... just asking. Why? Surely if the donor car is switched off before connecting jump leads, it's the same as connecting a new battery to the dead car 

[Bigron]

here's my guess, Andy: donor car's battery would be fully charged after a run and has a high terminal voltage, donee car's battery flat and high internal impedance, so it won't pull down the (high) terminal voltage of the donor battery - hence a large voltage spike that has potential to upset the electronics/micriprocessors of rhe donee vehicle?

Ron.

[JasonH]

I know exactly why the BMW's can't be jump started, particularly diesels, the reason is voltage spikes. The alternators are typically capable of putting out 170A and are electronically controlled. When the jump start battery is disconnected the change in load can cause a very short voltage spike in excess of 18V. There are two modules, the Bluetooth MULF and the Glow plug Controller (all electronic, no relays here) that basically go pop the moment their supply exceeds something like 18V....

An "expert" would, if doing it properly, pull the fuses for at least the Glow Plug Controller before jump starting.

[Doctor Gollum]

When I did a volunteer stint with Surrey Police, jump starting was one of the potential jobs... The fried electronic unit is about the same size as a 16gb ram board and melts quite spectacularly costs ££££ too...

[YZ250]

Just had a look at my bmw hand book out of curiosity. With reference to jump starting there is nothing unusual mentioned, with the exception of allowing a diesel to run for ten minutes prior to using it as a donor car.
As mentioned, they do have a +live post clearly marked with a big red cover under the bonnet but the starting procedure is pretty standard. It does say not to have the donor car running until the jump lead connections are complete but apart from that nothing any different.

[Bigron]

As jump-starting is a fact of life with cars of whatever vintage, surely the designers of modern car electronics should have incorporated a simple over-voltage protection to their sensitive circuitry? It IS easyto do!

Ron.

[Doctor Gollum]

When I did a volunteer stint with Surrey Police, jump starting was one of the potential jobs... The fried electronic unit is about the same size as a 16gb ram board and melts quite spectacularly costs ££££ too...

Should add, that I was shown a frazzled one as a practical demonstration of what happens if done wrongly, ie by connecting directly to the battery