I just put a optima yellow top 34/78 in my 2006 fx4. Now the battery light is on in the truck.When i run the rpm's up the voltage stays at 12v. Where it used to go up to 14v. Any ideas why? thanks

 

Sounds like you blew the fusible link for the alternator. Did you make ANY sparks while changing the battery?

On a side-note:
Why did you put in an Optima? They have less CA, less CCA, less reserve minutes, & less warranty than a MotorCraft at ~1/2 the price. Both are made by Johnson Controls.