why battery registration is crucial
This is what I learned, the hard way, via a trip from hell:
The battery charging, like most other systems on these cars, is computer controlled, hence they are demanding of more information and are very sensitive to proper voltages.
Specifically, two aspect needed for the smart chargers within, are the battery type and the CCA rating. Absorbed glass mat batteries have differing charge rates (profiles) and discharge profiles than traditional lead-acid batteries. Either type can be profiled in the car but some shops report strange problems with non-AGM types.
Our '05 745Li refused to blow on high, except on defrost. The problem was cured with a new AGM battery, despite the 'old' lead/acid being rather new and registered to the car, with mileage when done, stored in the car's computer.
As a protection strategy, the car will "load shed" under given circumstances, two of which are dropping system voltage and rising coolant temperature. What system is shed (shut off) is prioritized; first being (in my case) minor volume changes or pops in the left audio channel, progressing within minutes, to ABS warning, Stability control warning, transmission in protective mode, dash light dimming, then engine shutdown via EFI, etc. I saw all this recur withing a 5 minute timeframe +/-. The car will lock in a high gear in the trans protect mode, perhaps making it impossible to climb small inclines.
Also, the car locks in park with E-brake set, when shutdown, having towing implications. There is a release handle, difficult to find, under the dash, above the driver's left foot (US) accessed thru a keyed cover. Pulling the cord, releases both, allowing the car to roll (ensure it is secured otherwise, if needed). Now is a good time to know exactly where to find it, as not all tow truck drivers are aware. One used a jumper box to start my car; needed to shift from Park.
Given the above, I learned why the battery type and rating is important, similar to why my old-school battery chargers are unsuitable for AGM batteries, outside of jump starting. Some independent shops know the above, many do not. Some prefer BMW brand, perhaps b/c of ignorance, but I have seen good result from non-BMW brands of high quality. The Palm Desert shop that replaced the L/A with AGM did not reregister, probably causing many headaches on my recent road trip.
Sadly, BMW provides us with virtually no information about charging state (voltage) until the computer-generated idiot lights begin. Enroute, I finally rigged up an indicator to read accessory plug voltage via the newly acquired DVM. Instant info was there, telling me (and a good technician) that the system was not charging properly, post-'fix' and after a road test indicated effective repair. That same "rig" was crucial to my knowing the system was not slowly approaching shutdown, and that headlights and other added loads were being borne by the alternator. The plug with two wires cost <$3.50 at a truck stop where I performed a full recharge. Simply working the DVM probes into the plug's leads, then wrapped with electrical tape, proved an invaluable tool. Such devices, professionally made, are available.
I will never take a long trip again without jumper cables, a digital volt-ohm-meter and the plug for cockpit readings.
FWIW, the onset of load shedding is at 12.8vDC while normal running voltage is 14.1 +/-.