If your battery and charging system are good, there should be no need to charge it every night. So I would assume something isn't right with one or more of those components. I would not put a smaller battery in it, I'd fix the system. If you need to replace your battery, Deka makes a good one http://cgi.ebay.com/ebaymotors/Deka-ETX15L-Powersports-AGM-Battery-100-NEW-/270282472155?cmd=ViewItem&pt=Other_Vehicle_Parts&hash=item3eee173adbI have somewhat of the same problem, and I'm a beginner so fill free even if it is novice. What I have been told by a “mechanic” is that I should put it on a slow charger every night. Now for the most part I have and I haven’t had any problems but if I go 3-4 days without putting it on the charger I’m dead in the water.
So my questions would be; is what I’m doing going to cause any problems, other than the inconvenience. Also what I have learned by just another biker is that you could put a smaller battery in it. Yes I have less cold crank amps but it would be easier for the charging system to maintain the battery.