If you own a Windows Phone 7 device you might have noticed that there’s no way in charging the handset’s battery with your phone being turned off, the way you could with older Windows Phone versions and even Android. If it is already turned off and you plug in the charging cable, it will automatically power up. If you are already charging and want to turn it off, the device will boot up again.
While this could cause headaches for some of you, we need to understand that this behavior is by design and that there’s nothing wrong with your unit, according to a Microsoft Answers thread. Regardless of the make and model you are using, this phenomenon will be there as long as you run Windows Phone 7. There are three reasons said to be the judgment for phones behaving this way: first, the charging of the battery is controlled by the operating system; for proper charging, Windows Phone 7 has to be booted up. Second, there’s the wireless sync feature where your device will automatically sync with your computer’s Zune software only if the phone is plugged in. Third one is less viable though and invokes the reason of Alarms not being triggered while the phone is powered off. Previous Windows Phone versions didn’t trigger the alarm while off and neither does Android but BlackBerry devices do.
While there’s nothing you can do about the charging-while-on aspect, you can either have your phone on Airplane mode or Silent if you don’t want to be awakened by overnight phone calls, emails, text messages and so on.
Source: Microsoft Answers