If you are doing your own valve covers you are probably a pretty adept DIY, but mainly for the NOOBS out there I will reiterate BMW battery care.

BATTERY DISCONNECTION
Gently loosen the NEGATIVE terminal on the battery. Make sure it is loose enough that you can easily remove the negative terminal clamp from the negative battery post. The negative battery cable has a sensor system integrated into it known as the IBS or intelligent battery sensor that monitors incoming and outgoing charge as well as battery temperature. It is very expensive, so handle it gently and DO NOT pry the battery cable clamp up and off the battery post with a screw driver, which can easily damage the IBS. Once the cable is off, carefully insulate the clamp terminal with a rag or other insulating item and secure with a tie wrap or rubber band.

Then gently loosen the POSITIVE terminal on the battery. Again, make sure it is loose enough to easily remove the positive terminal clamp from the positive battery post. The positive battery cable has the BST or battery safety terminal integrated into it. The BST contains an explosive charge which is set off during an accident, disconnecting a portion of the car's power (mainly to the engine to prevent under-hood fires due to ruptured gasoline lines). Power to the communications system and interior lights are maintained on a different power line. The BST is expensive and can be damaged with rough handling, so DO NOT pry the clamp off the terminal. Once the cable is off, carefully insulate it to insure it does not fall back and touch the positive terminal of the battery, which would reenergize the power system.

When work is completed, replace the positive battery terminal first and tighten it down, then replace and tighten the negative battery cable.

After reconnecting the battery you will need to reset your clock. Your active steering system may also throw an error. After starting the car turn the steering wheel all the way to the right to lock, then rotate the steering wheel all the way to the left to lock, then go on a short drive and the error should clear.

 

Thank you so much bimmerfan52

 

I would like add, the bolts to the battery terminals are stainless steel,, if over tightened ( which is easy to do )the nut will seize to the bolt, and impossible to remove. the only way is to break the bolt and a new cable is 300 bucks... I suggest to get some anti sieze lube and apply a generous amount on the terminal bolt threads before reinstalling the battery terminals. It will prevent the nut from seizing on the bolt if u accidently over tighten the nut..

 

+1 Good addition!

 

If the negative terminal cable is removed, why then would it be necessary to disconnect the positive terminal?

 

Removal of the positive cable was included in an abundance of caution, and for those noobs who might be swapping their battery out to point out that both cables need to be handled gently.

You are correct that once either cable is removed from the battery electricity cannot flow. The negative cable is the best candidate for removal first, because it is directly connected to the frame of the car (ground) and a slipped wrench cannot cause a spark.

Working with a battery is different than working with electricity in a building. In a building, simply making a connection between the "hot" line to any ground will cause electricity to flow, because a voltage potential is continuously supplied by the power company. But a battery is different in that it stores energy chemically, which is converted into electrical energy only when both terminals of the battery are included in the circuit, causing the internal plates in the battery to release electrons. Connecting just the positive terminal of a battery to a ground will create no electrical potential.

In the past, one mechanic I worked with would remove and insulate both cables. Another would remove only the negative cable, but insulate the negative terminal on the battery with a plastic cap, which is the most foolproof method. If you save the cap that covers the battery during shipping it can be reused to cover the negative terminal when the negative cable has been removed. Both mechanics over the years experienced tools or metal parts set by themselves or other carelessly in the engine compartment that shorted the negative post of the battery to the ground of the car. A fully charged battery stores a tremendous amount of energy, which if accidently shorted while you are changing out a starter or alternator can be a startling and painful experience.

The BMW battery's location outside of the engine compartment lessens greatly the chance of tools or parts being set on top of it, but I guess my old habits die hard.