First thing, you need a meter. With it off measure the voltage on battery. Then with charger on it should be reading after a few minutes in the 13-13.5 range. Sometimes if battery is completely dead it could be less than that but you should see a difference between off and on voltage. If that is ok then most likely the battery is bad. If voltage dosnt come up then charger is most likely bad. If charger is working take battery to auto parts store and have it load tested. Have you checked water in battery?