My car has had some trouble starting and I noticed that the battery guage is showing low about 10 volts. I know that the guages are never right so I went to Oreilly's and checked the battery and the alternator myself. The battery was only showing a charge of 50%. The alternator was also showing low volts. I just replaced the battery 2 weeks ago. So I pulled the alternator to get a new one. While it was off they tested it on the tester inside the store. It showed that the alternator was fine. They also pulled out one of their new ones and it showed the exact same volts. So I decided to put my old one back on and now it's doing the exact same thing. Could their tester not be putting enough load on the alternator outside of the car or what? Should I just go and get a new alternator?

 

I'd check terminal wires and junction posts first.
Also check and accessories that were added to the car (EWP, E-cutout, amps, etc...)
A loose cable or bad ground could be causing your issue.
Replacing a perfectly good alternator probably won't fix your problem.

 

I checked all the wires, terminal, connections and they are all fine. All the accessories are fine too. I haven't added any new ones in over a year. I thought that Oreilly's tester might not have been able to put enough load on the alternator, so could the alternator really be bad? Last time I tested it it was on the car while it was running the alternator only put out 11 volts.

 

You could always take it elsewhere for a second opinion.
It would not be the first time an autoparts store was wrong.

At least O'Reilly's alternator will come with a lifetime warrenty.
It wouldn't be a complete waste if your stock one wasn't the problem.