The battery dosn't power anything when the alternator and rectifier are online.
The battery is purely for emergency power and starting the engine.
Not necessarily according to Mr. Cessna. From the POH of the 172RG I mostly fly:
The airplane is equipped with a 28-volt, direct-current electrical system. The system uses a battery, located aft of the rear cabin wall as the source of electrical energy and a belt-driven 60 amp alternator to maintain the battery's state of charge