Originally Posted By: crw
I don't know about the 5 or 6 series. Replacing the battery on a 3-series E46 is as simple as opening the trunk, opening the battery compartment and swapping it out like any other battery. (The car apparently doesn't know that you are not using special BMW dealer amps.)
Being in the trunk, I have a suspect that the battery is subjected to less environmental stress than being under the hood. The cables are never corroded. Perhaps they last longer in this situation? Oh, there I go again rambling on about that german engineering.......
For those exact reasons + weight distribution is why it's back there.
Your E46 is what we call an i-Bus vehicle. The electrical system is the same as any other traditional vehicle for all intents and purposes. All communication, coding and programming data is transmitted through two wires between the limited number of modules.
Starting with E6x vehicles they went to what they call Most-bus. The electrical system is very "intelligent." They don't waste power running the alternator all the time, that's silly! They've got a module that tells the engine control unit that it wants juice, then the ECU asks the alternator for it. That sounds better, right?
The ground cable for instance, is a digital multi-meter. It reports (to a module) battery charge level, number of starts, trips, trip distance, key-off draw current WHILE it's parked, how long it's parked for, etc. So the system very much wants you to tell it when you've got a new battery, as well as the Ah rating and type (AGM or standard). And if you want to retrofit to the newer AGM batteries from a standard-type, you had better inform the modules!
The modules all communicate to input sensors via normal wires, but the modules communicate with each other via fiber optics. They can "talk" with each other at a much faster rate, making the car safer and more efficient. But you're left with eighty something (on some cars) control modules. They all need to be on the EXACT same program number (i-level) for the car to work properly, if you have a module missing or inop then no data for you. If all your modules work AND communicate properly then you can program the car to tell it that it now has the better battery in it, and it will take it easy on the alternator output requests. Important to do! As smart as the system sounds, it isn't smart enough to realize (of it's own accord) that it has a new battery, and the risk is that the system asks for the same alternator output as it did with your six year old battery. Seeing over-voltage faults stored throughout a vehicles control units is somewhat common to see in conjunction with a parts store house brand battery in somebody's trunk.
