I think its hard on the bikes charge system to use it to charge a battery back up, but its been done a ton of times to no apparent bad result, so, fire that dude up & ride a while,,

Id have to pop the tank & check all the connectors, if the battery checks good.

Get a Volt Meter,,3 bucks at Harbor freight, Volt Ohm Meter,,

What year model? Has it been kept inside? Starter i most likely fine, You thing the ground is fine, Im not sure, Connectors under tank,, may not be okay, lets start there.

First, break the ground loose & clean under there, make sure you have a good ground. Look at the connections on the starter,, heck,, break them loose,, make sure theyre good,, Youve got a little gremlin eatin you alive,, Its not a big deal,, but it a pest,, we will solve it,, just work thru potentil hassle after potential hassle till you get it,,


Lay a hand ON the starter & hit the button, if you feel the starter engage & the engine dont spin,, the decomp is the problem.
OR, the electricals arent passing enough current,, feel the ground cable, if its HOT, youve found resistance,, Hot side, do the same,,

Daaang,, I wasnt expectin a fuse either,, sorry I didnt go there,,

Don't feel too bad, SGT...  ...an 06' is due for a battery anyway... ...