If it is a sealed battery, and hence doesn't have the problem above, it could have some shorted out plates in the battery. Has it fallen over recently? Crud (bits of plate fundamentally) gets stirred up when that happens and the cells can short across it cutting down the amps available.
The usual giveaway is that switch on gives the usual array of lights on the speedo etc but ignition makes all the lights go dim and flicker because there ain't enough amps available for everything.
If it is that then it is a new battery. BMW batteries are, if I recall correctly, warrantied for 2 years so worth going back to query that on a 2006 bike.