My ranger has the battery light on and both alternator and battery tested good. Had another battery from other car put on and battery light had gone off. So put the old battery back on and light came on again. Got it tested again and still testing good. Anyone have any thoughts or advice?

 

Welcome to the forum

What engine in your 2010 Ranger, 4.0l V6 or 2.3l 4cyl?

They used different charging system wiring

Battery light on means there is a difference in voltage between alternator/voltage regulator and vehicles voltage

YOU need to test battery and alternator wires with a volt meter

Who tested the battery and alternator?
if it was at an auto parts store then they were not actually tested

 

It is a 2.3L. I got it tested at an O’Reillys battery and alternator on the vehicle. they said battery had 12.65 volts and alternator was running at 13.9-14.3 volts.

 

Well if you drove the Ranger to O’Reillys then they couldn't actually test the battery, battery needs to sit for at least 4 hours to test its voltage, 8 hours+ is better, i.e. sitting overnight

The test with engine running is good, but may be a little low just after starting the engine, expected is 14 to 14.8v but 13.9 to 14.3 is OK, as long as its higher than battery voltage, in this case 12.6v
But they should have raised the RPMs to a steady 2,000 and held it there for 30 seconds or so to make sure voltage went up and then dropped back down after 5 seconds, that means the voltage regulator IN the alternator is working

Since swapping out the battery to a different one and Battery Light went off, I would say the battery cable, positive or negative, can not make good contact with the "old" battery, or the battery terminal's internal contact is poor/failing, I would expect this to cause Starting cliches but not always