Is the 12-sped Di2 charging cable (EW-EC300) supposed to have one pin sticking out more than the others? I ask because mine does and because on my last ride I got a “low battery” warning even though I’d charged it after the ride before. When I went to charge it again, I noticed the light went off shortly after I plugged in… sure enough, it wasn’t charging. After a few tries I got it to stay charging, but I’m wondering if it’s just a bit tricky or if my cable is bad. (Cable is $50 which is kinda ridiculous)
I did a google image search and it looks normal. Maybe the other end, that plugs into the transformer/brick, isn’t well seated.
I’d think it’s a ground/common connection to guard against arcing over time? Some types of USB cables have connections that are made early in the insertion process. That is supposedly why Apple’s Lightning cables suffer burned contacts (according to an electrical engineer) 
. For for the people that might want to know ‘why’.
Thanks, looks like my cable is normal.
Try a different USB power supply. Some will overheat and turn off if you put too much load on them.
Quick way to check is to see if the power supply is hot to the touch when it stops charging.