Here's a link to one on the Great Hobbies site...
http://www.greathobbies.com/products...od_id=HOBP0356
There are also a number of devices such as the AirAlert which plug into your receiver and have a series of lights to indicate battery voltage or flash lights or start beeping if your receiver voltage drops too low...
http://www.greathobbies.com/products...od_id=HOBP0335