http://www.futaba-rc.com/faq/faq-4yf-q738.html

From futaba,

"Radio System FAQ
I don't understand the red and green battery LED's on my 4YF, how do I know when my batteries are charged and when they are getting low?"

"Your batteries are sufficiently charged when the green LED comes on, on your transmitter. As you are using the transmitter BOTH LED's should be on steady. As your voltage gets low the red LED will start to blink as an indication that you need to charge."