Alternator not maintaing voltage

[STUART_T]

Hi Everyone,

I just want to ask a question, about a problem I have with my voltage output from the alternator.
When I have the fans, lights, heater fan, stereo and wipers on my voltage according to the gauge drops to 11v. On a recent outing the voltage dropped enough that when I went to start the car after stopping a service station, nothing, the car would not turn over. With a push start and all eccesories off the battery gained enough charge and it started fine at the end of the journey.
I have fitted a new alternator (100amp Delco universal) which is identical to my old unit. When I measure the ouput from the alt it is 13.6v with no accessories running, the same at the battery terminals. As soon as I turn headlights, fan the voltage drops. Should the voltage from the alt maintain a voltage higher than 13v even with acc's on. The battery is about three years old that is my next replacement item. I have checked my earth connections from the battery and relocated the position and checked continuity between the engine and chassis and it looks fine. But apart from replacing the battery I am lost. I have a volt guage and an idiot light which is also the excitor wire, the globe is only 2w and I believe I should have a 3w but I am going to add a resistor 15ohm 10w across the globe connections, as a safe guard if the lamp blows. Any help would appreciated before I take it to an auto electrician.
Thanks
Stuart