The battery takes a while to die, between 600-1,000 miles is normal life of a battery charge. There are also charge lights. If you are really worried about it, the batteries are small, think old school Nokia cell phone battery. On a long ride, put it in your seatbag.

The derailleur also stays in the position it was in before it died. So if you are starting to see a red blinking light, keep it in an easier gear in case it dies.

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I've only seen Di2 on a Cervelo P4 we built up in the shop. It was ubber cool and amazing to look at. The coolest thing was the self trimming feature the front derailleur has. When you are in that 3rd or 2nd to last cog, it self trims the front derailleur.

For pictures -----> http://arsbars.com/?s=di2