How do I know when to charge the derailleur or replace the shifter batteries?

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 System Overview video for a visual of the indicator LEDs.