AFAIK, you can't see an indicator or percentage in Garmin Connect (Mobile), though this would be a neat feature for people who don't wear their watches except for activities. A low battery warning on your phone would be even more useful. You may want to post this in the Software forums, since it applies to all devices.
Push and hold the menu button. The battery level will be shown on that screen with icon and percentage. It can only be viewed on the device and not in GCM.
