My battery and ox light keep coming on at the same time. They stay on for awhile then go out again.  I have the van parked now and while I have been checking the voltage at the alternator the lights have not gone back on. I think they will go back on again when I drive. The voltage at the alternator was 13.5.  
This may be a coincdecence but the OX light was on for my drip down yesterday and I crawled under the van to shut it off.
Any suggestions and if by chance my alternator is bad how far can I drive on a battery without the headlights? I have a full Aux battery too.
Thanks,Bryan