Yes, from what I read about Di2, there is plenty of warning, and even once the low battery warning comes on, you have 500 miles in reserve! I'm pretty fastidious about charging my electronics, so for me, I don't think that would be an issue. As Crankin says, and as I read, the battery life is crazy long. Still, not a bad idea to take a backup battery on every ride, just in case.

After breaking a shifter cable on one of our long rides and having to limp back to the hotel for 40 some miles in a single-speed mode, I can see the virtues of Di2. Among others, this article by a female triathlete made me lust after it:

http://angelanaeth.com/once-you-go-di2/