I've always been curious as to what happens if you run out of battery so I did that earlier this year. On a ride this spring my Wahoo Bolt warned me that my Di2 batter was low, but I ignored it and went on a ride anyway. When the battery got to a certain point the front derailleur stopped working but the rear kept going. I was able to do another 15 miles home in that condition. I figured I pressed my luck as far as I wanted and charged the system from there. Haven't charged it since.