From what I recall, the battery light on the Voyager is meant to warn of an electrolyte low condition in the battery. If one buys a battery that isn't designed to work with the sensor then the light stays on. You should be able to look at the battery and see if the sensor is in one of the cell filler holes. If not, that's your problem. Or it could be doing what it's designed to do and telling you about a low electrolyte level. Assuming the PO bought a battery not designed for the bike then there is a 'fix' to fool the sensor.
Here is the AVA site for tips and techniques. About midway down the page is a paragraph on the battery sensor thingy....
http://www.amervoyassoc.org/XIITipsBody.htm You may want to think about joining that group. Good bunch of people.