Possibly failing battery, possibly failed alternator, possibly slipping Alternator belt, possibly loose connection from Alternator.
With the engine running, check the voltage at the battery with a decent meter, should be 14.4 V or thereabouts., which would suggest the alternator is trying to charge it.
You could also check the current drain from the battery when the car is stopped, ignition off and everything switched off. Should only be a few milliamps if there is an alarm fitted; anything more suggests a fault. You'll have to eliminate circuits one at a time maybe by pulling fuses.
jhaslkdjhalskjhd, July 2018