Bad Alternator? or bad batteries.....

[Sideways240sx]

So monday mornign the car turned over a little more then usual, but started. I just assumed it was the cold. Car sat outside all day, i drove about 20 minutes to work, and parked the car. 8 hours later the car started right up, and i drove home. Now tuesday morning, it was a little colder today, the car would barely turn over, and it didn't start. Instead of screwing with it, i jumped in the other car, and went to work. I came home today, jumped the car, and drove it, 10 minutes to autozone. They tested the alt, and battery on the car. The battery was almost a little to low on voltage to test, but seemed fine, the battery only charged for 10 minutes. Then he tries the alternator. It was reading about 12.1 volts, at idle, about 750rpm, and at 2,000 rpm's it goes up to 14.4 volts or so. As far as i can tell, isn't this correct? He told me the alternator was probably bad, and wasn't charging the batteries fully, and after several starts the batteries went dead. This makes sense to me, but i want to make sure this is what it is, before i put a 280 dollar alternator on the car. I again checked the voltage at the alternator with a volt meter and get low 12's at idle, and 14 + at 2,000 rpm.


I checked all the cables, there all good, with 4 gauge going from alt to battery.

I say batteries because there are 2 stinger sp1000 batteries in the car. the one in the back is for stereo stuff, with a 150 amp circuit breaker to disconnect the back battery from the fron stuff.

The alternator is also an upgraded 135 amp unit i bought maybe 2 years ago. Both batteries are also around 2 years old.

Thanks, in advance, Chris