How low should the voltage on a car battery get after sitting over night?
It should not go down at all! The battery must be able to retain the current for longer periods and must maintain a voltage of 12V. Sometimes it does go higher (or lower due to prolonged inactivity) by a small margin, but it should be able to crank up the motor without any problems.
More tips on battery service – http://www.automotix.net/autorepair/repairtips/electrical_repair.html