A free app for Android, by Nicolas POMEPUY.

If you want to know how much battery life you have left on your Android Wear watch, or if it is time to charge it, then you should download Wear Battery Stats. It is a companion app for your watch and it is completely free. You will be able to monitor the battery level of your device in real-time. It works on all Android Wear devices, including those from Samsung, LG, and Moto 360.

It works as a service, so it does not need to be running all the time. You can easily check the battery level of your device at any time. If it is time to charge it, then the app will let you know and you can start the charging process.