voltage...

[dindy]

my friend of mine is worried about some issue with his voltage. he got 1.0 auto 2001 yaris 4 cylinder. on normal idle his voltage is 13.5 but it will increase to 13.7 when rev the engine. when he turns acceseries on, like headlamps , heater blower and hazard lights, it goes down to 12.9 volts on idle, with 13.0 on reving....i noticed too that his battery is on black mark, means it need recharge....think it is from the battery or alternator problem? i almost forgot to say, when he stops on idle u switch on the hazard lights, his car it will go like rev up and down gjust a little bit and the gauge cluster will dimm a bit and bit on again. any answers? thanks and happy holidays.