Originally Posted by
grumpus
According to the manual when charging the derailleur you'll only get two LED colours, blue for charging and red for error. It sounds to me that you're getting momentary red until the connection is established, then blue as it tries to charge, then nothing because charge is already complete, and fully charged is indicated by ... nothing. Seems like poor design, what's wrong with green for good, Shimano?
Sometimes it helps to have a Garmin Edge that you can connect to the DI2 system. You can add a data field somewhere to display DI2 battery status, that’s where I check mine,