Another thing to check is the buss bar between the battery relay and the ground power relay in the nose by the battery.
This buss bar, along with the large wire going back to the start relay behind the hat rack, are made of aluminum. The buss bar is subject to corrosion, which will create some resistance between the battery relay, and the ground power relay. The wire going to the back of the ship attaches to the ground power relay.
Remove the wire and buss bar, clean or replace the buss bar, make sure the terminal and wire are in good condition, and see if that helps. Also, there is a junction block above the pilot’s head that the big wire goes to before branching off to the back. Had one of those get loose somehow once, and we had an intermittent battery charge problem. Took a while to figure out that one. May want to check that too.