Nissan Repair: 1991 Pathfinder alternator bad or battery bad, volt ohm meter, amp light

Question
I have a 91 Nissan Pathfinder, automatic, 4x4. I was driving home tonight and it was cold (10 degrees F), and my lights suddenly started to dim and the voltage meter on the dash was dipping slowly below 1/2 way charged, then 1/4 charge.  I stepped on the accelerator and it puttered until the engine shut off.  I had a friend jump it twice in order to get home tonight, each time it held a 1/2 way charge for a couple miles, then the same thing, the engine died.  Each time the amp light on the dash started up, also sometimes the check alternator light came on.  I am currently charging the battery in the cold garage tonight to test in tomorrow's 20 degree weather.  How can you tell if the alternator is bad or the battery is bad?  Does the alternator charge the battery only after the motor gets up to a certain RPM?  I've never had electrical problems before, not even during last winter.  The battery has always been strong.  The dash lights started lighting up about a week ago then going off intermittently.  Thanks for any help on this!  Marcus

Answer
Marcus,

It sounds like the alternator is going out and not putting out enough to change the battery.  If you get a volt / ohm meter and put it on the terminals, while the vehicle is running you should get 13 to 14.2 volts if you rev the engine to a little over 1000 rpms.  My bet is the alternator is not putting out sufficient voltage to run the truck and provide a little extra to keep the battery charged.