Sudden Voltage Drops

In the summer when my car's huge fans are trying to cool down the engine and the AC is running as well, some times the voltage drops and an Orange light pops in the panel. I have to quickly switch off the AC and dim the lights to conserve energy .... if i switch off the car shortly after that the battery goes dead and cant start. The same happend last summer and then it didnt happen again until now when the heat wave is back.

I have tried to get the battery changed but the voltage seems to be fine. Is it my generator not making enough power for the car or shall i try to change the battery which is behaving pretty normal.