When your keyboard is plugged in and the red light is constantly on, this means the keyboard is still charging. When that light shuts off, the keyboard is 100% charged and you should be able to pair it to your iPad.
Please note that If your backlight keys are always on the battery may drain significantly
faster. This is a bit common as you would not notice the backlight during the day. To conserve battery, be sure to only use the backlight keys when needed.
To check the battery life manually, press and hold the 'Battery' key until you see a light flash on the power key.
Green = 75% - 100%
Yellow = 25%-75%
Red = Below 25%