My bet is going to be place on a loose/poor ground connection. I had very similar symtoms a while back, thought the battery had gone (It was more than 3.5 years with the OEM battery between 120F and -6F starts and prolonged use) replaced the battery with a red top and had the same set of issues, inestigated the starter made some repair to the wiring harness still had the issue, took the alternator off (not a fun process in the dead of a northern winter with no garage) alternator tested fine, put it back on put everything back together and the car actually ran much better but at too low a voltage, replaced a corroded ground strap and the charging voltage is now in the proper range for more than a year. I attributed my issues to a loose alternator connection and a corroded ground strap (the one on the passenger side of the engine bay in front of the motor mount). Battery replacement masked the issue.

Installed my 3rd Valeo rebuilt alternator from O'Reilly along with a new battery, cleaned the ground strap connections and the bus connections on either side of the engine compartment, and the ground under the #3 fuse panel. After reassembly, battery voltage is only 12.06 V at idle. Took alternator out & this time had O'Reilly bench test it, and it checked good (13.6V at idle, 14.5 at RPM limit (of the tester) How can I check to see if various components are loading down the system to that extent? What can I disconnect & still run the engine without blowing up the ECU or something else?

 

Installed my 3rd Valeo rebuilt alternator from O'Reilly along with a new battery, new belt & tensioner, cleaned the ground strap connections and the bus connections on either side of the engine compartment, and the ground under the #3 fuse panel. After reassembly, battery voltage is only 12.06 V at idle measured at the battery terminals. Took alternator out & this time had O'Reilly bench test it, and it checked good (13.6V at idle, 14.5 at RPM limit (of the tester) How can I check to see if various components are loading down the system to that extent? What can I disconnect & still run the engine without blowing up the ECU or something else?

 

reviving a thread from 2013 . . .