After each shift a green indicator light on each component turns on. Once battery charge is below 25%, the light turns red. Below 15% the light starts to flash red. Sensors in each part ensure that the components go into sleep mode automatically when you aren’t riding to limit battery trickle down.
The indicator lights for battery life:
Rear derailleurs:
Green LED: 15-60 hours of ride time remaining
Red LED: 5-15 hours of ride time remaining
Flashing red LED: Less than 5 hours of ride time remaining
Front derailleur:
Green LED: 22-90 hours of ride time remaining
Red LED: 9-22 hours of ride time remaining
Flashing red LED: Less than 9 hours of ride time remaining
Shifters:
Green LED: 6-24 months
Red LED: 1-6 months
Flashing red LED: Up to or less than one month
See the SRAM RED eTap® 2x11 System Overview video for a visual of the indicator LEDs.