I have the same problem. Using Ultegra Di2 for more than 2 years now. Used to charge every 1500 km's or so, or sooner when going for a bigger event. Since this week my battery discharges within 6-12 hours. Started after a very wet ride, but my dealer says that cannot be the reason. Dried all connections, but problem persists. Maybe the battery has run out of life, which would be a disappointment after just 2 years. Ordered a new battery (want to have a spare anyway !!) and will see if that fixes the issue. Will let you know in a few days.