Originally Posted by
colnago62
I am not sure the need for a spare battery, unless you have a older/cheaper head unit that doesn’t communicate with an electronic system. I have a Low Battery notice that pops up on my Wahoo when the battery is getting low. The wire connection would be internal, which makes it very doubtful that there would be any kind of issue.
Not the connections with the derailleurs or the external junction box.
FWIW, I've worked on hundreds of bikes with Di2, and the only times a connector had failed was due to crash damage..