Today, the battery in my '08 X5 drained in 20minutes, and I couldn't start.

I wondered why it happend, and only thing I found I parked my car in a parrking lot and turned off the engine and left the light switch at 'AUTO' position, and the low-beam light remained 'ON' for the 20 minutes. Is the 20 minutes long enough for complete battery drain or is there something wrong with the battery?

Thanks