Originally Posted by
johngwheeler
This does highlight the major shortcoming of Di2 though - if the power fails, you're stuck in whatever gear you were in
AFAIK - bad news if you're a long way from home stuck in 50-11 and have hills to climb
I've no direct experience with Di2 but I believe if the battery fails you can shift the derailleurs manually to what ever cog and chainring you choose and they will stay there. You still have only one gear but it's a lot better than the highest or lowest as a default.
It's like having a shift cable break, you can tie the broken end off to hold a given gear or run in the rear derailleur's high limit screw to keep it in a larger cog.