Alternator slowly not charging
Any help on this would sure be appreciated. I have a 1996 4 cyl Ranger. Issue is, I was driving to work and the ammeter indicator started wiggling around. It started dropping, and when I revved the engine, it would pop back up- but eventually, even that wouldn’t work and it died. I was stuck without my tools, so I got a ride to the local Walmart and dropped a new battery in. Made it home, but the same thing happened as I pulled into my driveway.
Charged the battery, got out the voltmeter, battery is at 12.4 V when not running, started the engine and started at 14V, then slowly over 3 minutes with lights, AC and radio on, dropped down to 11V. Turned off engine. Took alternator out and had it bench tested at NAPA. They said it was fine. As a test, went to a Ford salvage yard and picked another one up that was also tested fine, put that one in. Same exact issue.
Checked all my fuses, the box in the engine and the panel in the cab, everything checks out fine. Seems that the alternator charges fine, until it starts to heat up. At the top of the morning, I can drive for about 20 minutes before the voltage drops.
Any thoughts? Thanks in advance for the help!