Power Loss after 30-45 minutes

[Bsluper]

I have a 1994 reinell 2000 brxl with a 5.0 cobra EFI engine. The boat has been starting just fine but recently after 30-45 minutes of running i notice a loss of power (electricity), the GPS/Depth Finder seem to be at half power (the display dims) and whenever I adjust the trim the engine cuts out. any ideas??? and thank you for your assistance!

 

[vasy]

Check ground at alternator and general, check Alternator output when cold and after 25-30 minutes, as it can be degraded with heat due to age, check your batteries, cold and hot.
To me, voltage regulator is a big suspect here....

 

[Bsluper]

Thank you for your help! The alternator was replaced last year but I will still check it to see what's going on with it, along with the batteries. I will go out to the marina tomorrow morning to test.

A few questions about testing;
Alternator - to confirm the black lead of the voltmeter just goes on the alternator house and where do I connect the red lead? (I included a photo of the alternator so you see what I see)
Voltage Regulator - I have no idea how to test that, any insight for me?

Once again, thank you for all of your incredible help!!!

 

[Bsluper]

I made it out to the marina this morning, does anyone else know how I should test the alternator and voltage regulator?