One day my phone was plugged in, charging, in another room. I looked at my watch to see if it was charged yet. Then I realised my watch didn't have that information. So I wrote a thing. Now it does.
Watts App reacts to your phone being connected to or disconnected from a charger. It shows a notification with the current battery level, updated every minute, while your phone is charging. When the phone is disconnected, the notification goes away.
This was written for Android Wear users, since notifications are automatically mirrored from the phone to the watch. You can still use it without a watch, it's just a bit pointless. But hey, if it works for you, go nuts.