Believe it or not, depends entirely on how much you change gear. I believe front uses more power than rear.
Would you go on a four hour ride in Essex 20% on your di2?
Why chance it? I think you'd probably be fine for four hours, but surely the simplest thing to do would be to charge it first.
Should be fine in theory, but bear in mind that the front will automatically stop working at 10%.
@snoops started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
Believe it or not, depends entirely on how much you change gear. I believe front uses more power than rear.