What makes my battery voltage drop down if I have a new battery and a new alternator?

I have a 1999 Grand Marquise. In hot weather (AC on) and more when in traffic jams after long city driving, my battery voltage drops down almost to a quarter or below of the battery voltage scale.
I have installed a new battery which just bought from here:www.power-battery.com.au and a new alternator but to no vail. Battery is only getting about 13.2 volts. What could be the problem? is there the problem with the battery?
Thank you for your help and prompt repsonse.
Regards,
john

 

It could be the voltage regulator best practice when changing a alternator you change that to.

 

OR

You could get a newer car

 

Check the main ground wires for contact surface rust.


99 marquis, nice. COBRA?

 

Make sure the battery cable running from the battery to the starter isn't touching a hot exhaust manifold, this happens sometimes and melts the insulation giving the battery a slight short.