Battery light stays on, battery gauge says ok, battery goes dead, Why???

you may have what is called a key off current draw. Something circuit is still draining battery with the key off. connect an amp meter to battery wires, then start pulling fuses, until the current draw stops, then that circuit is suspect. Sears used to test charging/battery systems for free. You may want to check with them. It seems you missed the voltage regulator. Did you check if the running voltage is at least 13.7-14.2volts, If not the regulator is suspect. As per usual, check all connections, grounds, etc. Good luck.

Go to most any major auto parts house and they will run an ODB-II test on it for free.

Where is the alternator fuse located at on a 1999 Ford expedition?

I have a 2009 expedition el the battery light came on car wouldn’t start checked the battery it was good the altinator was good checked all fuses and relays they were good pulled apart the ignition everything was connected spent 4 hours and all it was was the car key it’s self put the spare key in and it started right up but the other key still won’t work