When it's time to charge Glass' battery, you'll see an empty battery icon on the display. Use the micro USB cord included with your Glass with a USB wall charger (not included) or your computer to recharge the battery. While charging in an inactive state, an empty battery icon on the display will animate whenever the display is activated.
The Power LED
While Glass is charging, the LED on the curved back of the device, near the power button, will pulsate slowly. It will remain solid when Glass is fully charged.
Checking battery level
You can check the battery level while using Glass by swiping all the way to the Settings bundle at the far left of your timeline.
Battery Charged indicates that your Glass is fully charged.
You can use the included USB cord to charge Glass, but you'll get more power directly from a working wall outlet if you use a wall charger (not included).
Best charging practices
Make Glass inactive when not using it
Putting Glass in an inactive state will lock the touchpad, disable incoming phone calls and turn off notification chimes. This will save battery while you're not using Glass. Enable On Head Detection from the Settings bundle to put Glass in an inactive state automatically whenever you take Glass off.
Hard power down
While Glass is plugged in, pressing and holding the power button for 15 seconds triggers a hard power down; the LED light goes off and Glass isn't charging. Glass will not charge when hard powered down.
Charge Glass overnight
While charging and within range of a Wi-Fi network, Glass will automatically update to the latest software and sync any pictures and videos to your Google+ Auto Backup. It's good practice to charge every night to make sure that Glass is always up-to-date and your photos are there waiting for you in your Google+ albums when you wake up.
Powering Glass after a long period of disuse
If you've not charged Glass for a couple of days / weeks, you may find that it will take longer to charge Glass. In this state, it's normal to not see the empty battery icon appear on the display or that Glass will not turn on when you press the power button. Grab a cup of coffee and return in a bit and Glass will turn on normally.