I am using android via virtualbox in my windows 7 PC. My android x86 home screen always shows a "Charging 54%" message before I unlock it. It never increases or gets down. Why is it always "charging 54%" ?
Since your virtual machine doesn't have a battery, it uses a fake battery sensor to provide the required battery information to the OS. While it's possible to make it read any amount of charge, and to change over time, it's convenient to use a fixed amount somewhere in the middle of the range, since values close to 100% or 0% can cause the Android OS and apps to take special actions (such as closing down apps) to help preserve the battery).
For your use case, it might make more sense for it to report that the device is on mains power, since there isn't really a battery at all, but they've probably set the default behaviour based on the official Android emulator. Since the official emulator is primarily used for testing and developing apps, it's better for it to report battery power, to make it easier for app developers to test how their apps behave on battery power.