CTEK 3300 is PERFECT.
amazon.
I just picked up a ctek 7002 as well for my car with two batts in her.
CTEK Battery Chargers - The World's Smarter Battery Charger BMW even uses them but re-brands them with their logo

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I swear, my cars are like a girlfriend.
Sometimes its a rough ride, sometimes its smooth motorin'.
Sometimes she doesnt like how i treat her and sometimes i dont like how she behaves.
BUT at the end of the day, she loves it when I am inside her.

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'91 850i
'05 X5 4.4i
'09 Clubman S