...Actually the Volts should be 12.2 Standing and 13.8 Max.(when on 'The Charge'), With the engine running at 3000 RPM! This will lower from 13.8 as the Battery becomes Fully Charged. You should Never see 14. Volts! Over 14. and you have a regulator problem, and Your Battery will Boil Dry. NOW! Lets talk Battery! How old is it? I Bet That is your Problem. Older than 3 Years and You can expect it to start going bad. The Battery must do Three things!
'Take' a Charge.
'Hold/Maintain' a charge.
and then be able to Deliver Full Amperage to the Starter, on Demand.
I had this same problem, although it took One to Two days, of 'Sitting', before the Battery was incapable of turning the Starter; even though I would disconnect it with a Master Switch to keep it from 'Draining' overnight. That Battery was 5 years old. Amazing what a Brand-New Battery will do for You. Do what Larry suggested, and get a Voltmeter and test that thing! If the Volts go UP when the RPMs are increased from idle, there is Nothing wrong with the Charging System!! Also! Check the Voltage While Cranking the Starter! If they drop below 8.5 While Cranking, Buy a New battery and get the Highest Cold Cranking Amps that You can FIT into the Car!...