I had my battery on my 1150RT replaced after 6 years of loyal duty, with the bike sitting outside Summer and Winter. Although it still started fine every time, I – and my techie friend with me – decided that it was time to put in a new one.
Lazy bugger as I am, I had it done by my ‘private bike technician’, but one can easily do it oneself of course.
For the sake of brevity (by me) here are some interesting LINKS for you to find that information!
From XS-Adventure.com: http://xs-adventure.com/motorcycle/bmw-R1150RT-battery-replacement