Use a Voltmeter to check if the battery is being charged correctly.
Before testing the charging voltage to the battery, ensure that the vehicle’s transmission is in park or neutral (manual transmission) and that the parking brake (aka emergency brake, hand brake) is firmly applied.
With the engine running connect a Voltmeter across the car’s battery terminals and measure the voltage.
Be safety aware as the engine is running
It should measure between 13.8-14.5V DC.
If it is less than 13.8V DC the battery will not be charged and will actually discharge trying to keep the engine running. The problem is either with the alternator or the voltage regulator.
Not sure with you’re vehicle (you don’t state the year model) whether the voltage regulator is integrated with the alternator or is part of the PCM.