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If your main battery goes dead, then your backup battery takes over. However, you shouldn't have to worry about this as long as you keep it in the charger at night (you shouldn't run the battery below 40% anyways, and that's plenty of comfort room).
The backup battery also takes over if you remove the main battery. I have heard varying accounts of how long it lasts. The best guess I have heard is 20 minutes. However, I doubt you'll ever need to have the battery out that long.
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