Hi. I have the same phenomena with my D5. It gradually goes from 100% to 60% over most of the day and then suddenly drops from 60 to 30, just within 20 minutes or so and then drops to 10% or lower in the next hour.
I had big problems with even making it going through most of the day.
But, I found out something. This watch (and probably yours as well) HAS TO BE CONNECTED TO THE INTERNET AT ALL TIMES.
Without internet connection, it will, at some point, start looking for something, or tries to connect to something and will not give up.
Also, if you activate location services, it drains your battery real quickly.
In my case, the solution was to allow internet over Bluetooth on my smartphone and on the watch, activate the the use of internet over Bluetooth.
Since then, my watch is always connected and I get much better battery life.
Hope this helps you.