I hope your not leaving him in there too long. You did say he "could be in there long enought to run down the battery". That sounds like a long time to leave a dog in the car. If the weather is anywhere near warm, the interior heats up very quickly and many a dog owner has found out the hard way that it gets hot enough to kill a dog very quickly. Also, it's a good way to have your windows broken out by someone trying to rescue the dog thinking it's in distress. Just a though.

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2007 X5 4.8i Space Gray
2001 X5 3.0 Titanium Silver