The batteries charged up fine. Lights and acc all work when on. In ignition all I hear is a hum. This has happened in the past and for no apparent reason or specific amount of time it'll start up fine. I'm not complaining that it eventually does start but id like to know why when I haven't done anything but wait and I really need to depend on it starting the first try every time! I'm engine illiterate and don't even know where or what to start checking. Help?!