I highly recommend you contact Cygolite. I found their contact info on their web page and conducted a conversation entirely in email. I explained that my light didn't hold a charge after two years. They said send it in, and they will charge around $10 or $15 for a replacement battery. I sent it in. They sent me a replacement light and charged me $0.

I've heard only good things about this company, and my story is one of them. Their stuff is made in the US. How can they do that? It's sturdy stuff, and the mounts are really smart. Li-Ion batteries sometimes don't last as long as expected, so the smart thing is to replace them in this way. I have no hard feelings about it at all.