My BMW f800gs battery is constantly draining despite the fact it's connected to a battery tender 24/7

There are all sorts of high performance rotors for this bike on line and through BMW.
https://www.google.com/search?q=F800gs&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a#q=F800gs+front+rotors&rls=org.mozilla:en-US:official&tbm=shop

Buy a battery tender maintenance battery charger to keep the battery fully charged and extend the life of the battery. Google "Battery Tender"

Is the battery any good? Tender literature indicates it is for maintaining battery level not charging a discharged unit - maybe that is the issue.

First thing, how often do you ride it? If you don't ride it every day or atleast every other day the battery is going to lose power, even if its brand new. The best thing to do is to get a Battery Tender which you can find online or at your nearest motorcycle shop. Also if anything is constantly on it is constantly draining the battery. I would get a battery tender. They are useually around 23.00 up to 40.00. Good luck!!!!

If you have a BMW style power outlet for accessories such as heated clothing you can use the battery tender to charge your battery through that connection. I have an '04 R1150RT which I've been charging this way since I bought it. It has a 15 amp fuse which is adequate for charging with a low amperage charger link the Battery Tender or Battery Tender JR. Don't use the connection to jump the battery as the starter will pull too much current through the 15 amp fuse. You can connect the SAE type Battery Tender leads to an SAE to BMW male outlet cable to make the connection. Alternately, you could wire a cable to the batter with an SAE connector on it and bring the SAE connector end out under the seat and plug it in that way. You can store the SAE end under the seat when not in use and not worry about it shorting out since the exposed metal end of the SAE connector is the ground connection.