I had left my vehicle for about 3-weeks while away on a vacation, and when I got back, it popped up a message that it had gone into battery save mode. It 'forgot' my stored radio stations, the seat position memory, and probably a few other things. It did start, and seemed to work fine afterwards, though. A couple of months later, after having some issues with my rear hatch switches, I took it to the dealer, and they replaced the battery and replaced the switches for the hatch. In my case, I had occasionally noticed the hatch wouldn't pull itself fully closed for a long time every time, and once in awhile, I'd get in, and get an indication that the hatch was open. I think the defective switch was activating the system occasionally, and if left long enough, draining the battery. This is the second defective switch I've had on my vehicle...the first one showed up a day after I bought it - the emergency brake switch would sometimes say it had failed and wouldn't turn off (but it actually did, but didn't know it). There are few switches in this vehicle that actually perform the function they appear to directly...in most cases, they send a signal to the computer, and the computer completes the actual operation. This allows the switches to be smaller since they don't need to carry much of any current, but makes life miserable for control issues when one is intermittent.
FWIW, I replaced the headlight switch to get the rear fog light functionality, and the small plug to it had wires probably at 24g.
I researched chargers awhile ago, and ended up picking up a Bosch C7 unit that's quite slick. About the same price as the one BMW sells (if you shop carefully), but has more features.
Last edited by jadnashuanh; 01-25-2013 at 12:20 PM.