The battery could be bad or the converter could be bad. Check the voltage from the converter without the battery connected. If it is significantly higher than 14.2 volts (16+volts) then the converter has failed. However, it is most likely the battery. Check each cell with a hydrometer. This should confirm a failed cell. However, if it a maintenance free battery you can't check each cell. In that case check the battery voltage. If it is less than 12 volts several hours after charging then it is bad.
As far as why goes the only thing I can say is God didn't make this stuff....man did. And as such can fail at anytime. How old is the battery?
Chuck - Sparta, TN
2012 Jay Flight 22FB, 2 x Honda EU2000i
2013 GMC Yukon XL Denali AWD
EDUCATION is what you get when you read the fine print.....
EXPERIENCE is what you get when you don't.