Jumpstarting ought to be easy; connecting battery terminal to battery terminal. But apparently, BMW provide battery terminals under the bonnet, with the battery in the boot, and specifically warn not to connect booster cables directly to the battery. Anyone vouch for this? The car in question is sitting in a garage, and I don’t think my 25mm thick cables are going to reach around to the front of the car. The car is going to be difficult to move, as the driveway is loose gravel, and this M3 has those huge tyres.
Hi AT, the only reason I can think of is they are being cautious about excess hydrogen being given off in the enclosed environment of the boot during the rapid charging that happens when jumping, and sparks from the jump leads setting it off. Possibly they also feel you’d get a more direct connection when the battery was low by excluding the car’s battery cables.
Otherwise, electrically, it would make most sense to jump battery to battery imho as the terminals under the bonnet can only be parallel connections to the battery posts.
I did google it as it got me intrigued (I had a BM with battery in boot but never even saw it in 5 years). First thread up more or less says no problem either way but I wouldn’t hook up negative to the bonnet hinge like one poster there or you may find it welded up 
http://www.bmwland.co.uk/forums/viewtopic.php?t=107261
Hi, not the same BMW, but we own a E61 5 Series touring, which I’m sure I read in the manual, has seperate battery terminals for boosting. Can’t remember why I’m afraid. But I was always told years ago on my MK2 Golf GTI that the sudden power spike from flat would fry the onboard computer, so there was a particular technique for boosting the battery, fortunately never had to try it out. Not much help I’m afraid, but it looks like it might be a common BMW thing.
You can jump start BMWs batt to batt thats ok, I have done it plenty of times with no problems.
As for the golf mk2 GTI frying onboard computers, that was only on the digifant injection ecu and as long as earths are good theres no issue there either.
If you do choose to jump start from the battery I would make sure all the consumerable were turned on before conecting the batteries this stops the high peak frying the ecu’s why not just charge the battery up over night
Alan
Thanks for the comments. In the end, it proved to be not a problem; just a bit hesitant given the possible costly ramifications.