I would just like to add that purchasing a 2nd battery now as a "back up" may not be a real good idea. If you are buying it as something you may need frequently on some of your longer rides, that's another matter.
These batteries have a shelf life. You are way further ahead buying a new one when the original starts getting dicey on you. The Reention batteries are available from several different sources, so they will be available for quite some time, even if one or 2 of them are no longer in business when it comes time to replace....