I'm going to give a reply that is admittedly not very expert, but I believe the Rotax generator is good for about 21 amps with no problems. The problem is much more likely to be the Ducati voltage regulator. I have heard that much over about 13 amps for a prolonged (whatever that means) period of time can overheat and damage or reduce the life of the regulator. If I were you, I would figure out a way to limit the charging amps to less than the 18 you mentioned. My 912uls system with the EarthX 680C never draws more than 10-11 amps charging and my regulator lasts a long time. Just my 2 cents.