I have the older 6800 Di2. When the Di2 status light goes from solid green to blinking green, it's below 50% charge.
I just check every few rides, usually while riding along, by holding the front shifter button for a couple of seconds. Once I see blinking green, I recharge soon.
Yes, I've had the front stop shifting on a ride--oops, solid red! I just forgot to check.
There's no advantage to letting it go lower, the battery doesn't need to be discharged for long life. I finally replaced my original seatpost battery after more than 5 years. I was having to charge about every 300 miles when the original was old.
After more than 7 years, and more than 25000 miles, everything still works perfectly. The shift buttons are fine, and the rear derailleur has no play in the pivots. It's still fast and precise.
Last edited by rm -rf; 04-24-21 at 07:07 AM.