The only two things I can add, to all the above good posts. .
1), when it's hot out, the alternator output can be as low as 13. 5 Vdc and still be fine, when cold, like near 0, it will be up at 14. 5Vdc.
2) your headlights will do a fine job of loading your batteries. with the engine off, your battery will float down from where the voltage WAS with the alternator charging them. They should float slowly down (the better the battery the slower this is, this is "self discharge from the float voltage to plate terminal voltage"), but never pass 12. 6Vdc.
No turn on your headlights, if your batteries hadnt floated down to 12. 6Vdc yet... with the headlights on, you should now be seeing a steady 12. 6Vdc. If the batteries go right down to something less, usually it will be at or less than 12Vdc... one or both of the batteries have a part of a cell bad.
If the voltage just slowly keeps discharging like a tire slowly deflating... the batteries are just out right weak and tired.
A good battery will stay at 12. 6Vdc for atleast an hour with your headlights on.