According to apple these are the operating environment specs for the ipod classic:

Operating temperature: 32° to 95° F (0° to 35° C)
Nonoperating temperature: -4° to 113° F (-20° to 45° C)

So obviously warm is bad for the battery. Now what I have had happen to mine is that when it gets really cold, the battery drains all the way and then the ipod will not turn on until it is charged on the dock. The 6FL adaptor is not able to start it.

So now that it got cold at night in N. Va I do not leave my ipod in the car anymore.

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07 X5 3.0si - 06 330i