Search cluster test...your car will tell you the voltage of car off and on...s/b approx 12.x V off and around 14 when on (plus or minus)...I'm at 13.7 V running, so I'll be needing an alt (or voltage regulator) someday...but I seem to be getting enough to keep the battery charged for now.
Anyway, cluster test, or multimeter (measure bat voltage off and on)...or autozone/etc.
...and find key...I love proximate causes, and the different key is certainly that...so don't go too crazy yet on replacing stuff...and you'll want a new key anyway, so go ahead and either search aggressively for it or order it and see what that does for you.
Upcoming NYC dates: Jan 8 and 27. See Facebook for other shows and PM me if you can make one...love to see you there!
Mangofy your E46!