I have a 2011 50i so I probably have not had the experience of having a battery to fail,,,YET! I have seen the dreded "battery discharge" screen on my message board, BUT I refuse to get a battery tender, or anything else that resembles a battery charger! Not being a snob or anything, I think that BMW should build a vehicle and electrical system that does not discharge like these do, reading some of the complaints. I have not seen these types of complaints on any other car to the degree that I see from BMW,,,and I really don't know why. Our systems are not anymore complicated than Mercedes, Porche, Audi, Lexus or any of the other high end vehicles. Sounds like BMW has exectrical systems that are on par from the early Fiat/MBG days. Hey I love my 50i, but I'll be damned if I am going to put a trickle charge on my vehicle for issues that BMW should resolve!! JMHO!