I noticed a few flights back that upon start and early into the flight my system voltage was 14.2, right where it should be. As the flight time increased, the voltage decreased down to 13.8. If I loaded it up by turning everything on it would hold at 13.7. I’ve changed the alternator (ND 14870(14824)) twice and they all 3 can’t have the same problem. I’ve cleaned the main ground strap from the engine to airframe and other ground wires with no affect. All connections have been pulled and wiggled and can’t find anything loose. Next I’m going to check resistance of the main feed from the alternator to see if it may be internally corroded. If that is not it, I’m stumped. Where should I look next?