jump starting 350i
Discussion
The short answer is "No Problemo"! Why?
I dont believe it could ever be a problem, after all, when the two batteries are put together correctly (ignition off) the good battery will start to charge-up the empty one and the two voltages will immediately stabilize to what ever it wants to be, depending upon the "state of charge" of both batteries.
Now, when the ignition is switched on, the car electronics will see a slightly "lower than normal voltage" (12 cell) battery, rareing to go.
The voltage on cranking will still drop to about 9-10 volts, there will be no voltage spikes of any significance OTHER THAN those normally induced around the LT coil circuit and the car is protected from them anyway, by the local ignition suppression condenser and the massive inertia of the battery circuit.
So, apart from old wife myths perpetrated by garages, who want scare off the "tyro" jump-starting person so they get more breakdown spondulics, the "worry" as described is unfounded.
Now, I do believe, you cannot do this with modern cars because their AirBag ECU's sit there, live, on a flat battery and can be triggerered by a rapid change in battery voltage.
Ooops!
I dont believe it could ever be a problem, after all, when the two batteries are put together correctly (ignition off) the good battery will start to charge-up the empty one and the two voltages will immediately stabilize to what ever it wants to be, depending upon the "state of charge" of both batteries.
Now, when the ignition is switched on, the car electronics will see a slightly "lower than normal voltage" (12 cell) battery, rareing to go.
The voltage on cranking will still drop to about 9-10 volts, there will be no voltage spikes of any significance OTHER THAN those normally induced around the LT coil circuit and the car is protected from them anyway, by the local ignition suppression condenser and the massive inertia of the battery circuit.
So, apart from old wife myths perpetrated by garages, who want scare off the "tyro" jump-starting person so they get more breakdown spondulics, the "worry" as described is unfounded.
Now, I do believe, you cannot do this with modern cars because their AirBag ECU's sit there, live, on a flat battery and can be triggerered by a rapid change in battery voltage.
Ooops!
I had no problem jump starting my 350i wedge. It fired up straight away. I'd had a problem starting it on a twin battery set up after leaving it stood for a month. It turned over so much it totally flooded, washed the bores with petrol and lost compression. Needed a tot of oil down each plug hole and a tow start to produce clouds of smoke which cleared after a spin round the block. Causing problems with the electrics by towing is nonsense.
I think the stories kicked off in the late 80s when there was a large expansion in the used-car market (and, coincidentally, an increase in engine management-equipped cars). Lots of large car lots full of cheap bangers mostly bought at auction. What would often happen is that Joe Punter would come in to look at that Bargain! BMW 635CSi - only £50! and of course it wouldn't start because it had been stood so long the battery was completely dead, ie unrecoverable. So Simple Simon, the boss's favourite, would wheel the boost charger over and connect up. Unfortunately most of those big powerpacks were of lamentably poor design, essentially just a transformer and big rectifier, no voltage regulation. As any fule know, the full-wave rectified output of a transformer is somewhat higher than the secondary winding, ESPECIALLY off-load. So when you turn the ignition on, for the few seconds before you turn the key, the voltage hitting the ECU could be as high as 22v, more than the onboard regulator was designed to cope with, and it was THAT spike that took out ECUs. If the battery was merely 'flat', the load applied to the booster as it started to take charge current would bring the voltage down to something acceptable, so no ECU problems.
Jumping from another car, as HJ2 says, should produce no nasties. The impedance of a lead-acid battery is very low, so any voltage peaks would be swallowed by the battery. A completely dead battery, however, won't offer this protection, but as long as the 'jump' vehicle is running properly its charging system should cope with the electrical load of the dead car as if it were its own. If that makes sense
Jumping from another car, as HJ2 says, should produce no nasties. The impedance of a lead-acid battery is very low, so any voltage peaks would be swallowed by the battery. A completely dead battery, however, won't offer this protection, but as long as the 'jump' vehicle is running properly its charging system should cope with the electrical load of the dead car as if it were its own. If that makes sense
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