Quote:
Originally Posted by Oldmcst
It kinda blows my mind that there would be terminals to a battery located in the trunk that are under hood as well.
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All BMWs, not just Ms, have had their batteries in the trunk for decades to improve weight distribution and allow them to achieve the legendary 50/50 front/rear balance. The reason that there are terminals under the hood is to make it safer if the car needs to be jumped and more convenient.
Older lead acid batteries emitted dangerous hydrogen sulfide gas which is toxic and flammable. A spark from connecting jumper cables incorrectly could ignite the gas causing an explosion. This is why you always connect the red (positive) leads first and then connect the black leads last, preferable to a metal ground in the car, not directly to the battery. Using the front battery terminals enforces this and eliminates this danger, although with modern batteries this is less of a concern.