Your bike is not charging the battery. You need to replace the stator. When you take a reading or get someone to if it´s not reading 12 or more, you need to replace it. Yours is most likely reading around 6 since the bike is failing at night with just the headlight gone. I don´t know much about the process of changing the stator on the XS1100 even though I have one. Actually I have never had to do anything on mine bought last May 2009. Dealer serviced. Good luck.