Battery warning light on

I recently had a battery warning light come up on my car. I changed the alternator. The battery light went off for a whole day and then the next day it came back on. We suspected the alternator was bad, so my boyfriend went a bought a new alternator. Not even 5 minutes of driving and the battery warning light came back on. We decided to change the battery next. So two alternators and one battery later we thought the problem was fixed. After 20 minutes of driving, battery light returned. I had an automotive electrical technician come and check out the vehicle. He tested everything and aside from a fluctuation in the alternator, everything tested fine. Battery, ground wire, wires in ECM. All tested fine. I asked another mechanic what he thought. Apparently my boyfriend got an alternator that had 140amps and not the 120amps that my car is supposed to have. He suggested we get the right alternator and replace the belt. Something tells me this is too simple of a fix considering this will be my third alternator. My car drives perfectly fine and other than the warning light I wouldn’t suspect anything to be wrong. Also no check engine light. Any suggestions? Everyone seems to be stumped and this is really stressing me out.