I was reading up on the fuel filter heater recall that was performed on 09 X5 35d's and came across the following. Perhaps this is why the battery was replaced in the X I am looking at.

"The fuel filter heater, which is used for warming up the diesel fuel during vehicle operation at low outside temperatures, could experience an electrical overload condition and become permanently activated," said NHTSA in its recall summary of the problem. "The fuel filter heater could remain activated even when the ignition is off and lead to a dead battery. The unit could also overheat, possibly leading to a vehicle fire."

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Profeshenal spellar