Ok my C.E.L turnd on and I search the code and it tells me its a sensor under battery? I have a yellow top optima if that helps

 

There is a battery temperature sensor in the battery tray. It could be rotted from years of battery acid leaking on it.

 

You know what now that I think about it the battery before the optima was leaking battery acid!

 

That temp sensor tells the PCM the battery temp to control the charging rate. It may have been bad before and cooked your old battery. Most people don't even know it's there.

 

You know what that's exactly what happened my old battery started to leak acid

 

That's the problem with today's systems. It will show you volts but not charge rate. When cars had ampmeters, you knew if the alternator was putting out or not. Now you only know something is wrong when system volts start dropping.

 

But everything was fine till I did a head swap a few weeks ago, could it be that i forgot to re ground the chassis cable to the cylinder head?

 

did you check the Sensor/Harness to see if it is corroded at all?