I had exactly the same problem in the early months of 749s ownership. In the end, after numerous calls to the supplying dealer (DLS) over this time, I eventually lost it - I rang the dealer & told them the bike was on the back of an AA truck on it's way to the back to them & that I wanted it fixed
First of all, they said they were too busy to even look at it, then when they did eventually agree to actually look at it, they said there was nothing wrong with it, but they had replaced the battery anyway. Of course, when I got it back it was exactly the same
In the end I went & bought a new battery, charged it for 48 hrs before fitting it to the bike & it's been fine ever since. Needless to say, I have never spent another penny at DLS
I'd say you need a new battery & it is definitely covered under warranty (with the right dealer
)