1999 ford taurus battery drainage

we have a 1999 ford taurus 3.0 v6 we can start & run the car for 15 minutes then the car loses charge & we had the altenator checked & it was good we checked all the wiring it all was ok all the fuses are good could you please give some idea what we can try now thank so much
You need to measure the battery voltage using a digital voltmeter across the battery with the engine idling and all accessories off.
You should see between 13.5v and 14.8v. If you don't, there is a problem with the charging system.
 

thank you so very much we'll be sure to let you know if it worked
That wasn't a fix. I'm just trying to determine if the charging system is working or not. I suspect it is not so we will have to do further testing to determine why.