...Perhaps temperature is an issue here??
Good point, Paul. In addition to the increased resistance of a heated winding, there's the magnetic flux loss of the rotor core (Curie temp).
In the mid-70s, we had a lot of complaints of charging insufficiency from the new batch of Honda CB550-fours. 4-cylinder ignition, the new always-on lighting, and heavy battery recharge rates, were challenging the charging system.
I found the charging system to be barely adequate when cold, insufficient when hot, even when the regulator was bypassed, full power to the excitor (field coil). I found the excitor's resistance had increased by more than a half ohm when hot. If I quickly replaced the hot excitor with a cold one, leaving the engine hot, the charging improved, to just barely adequate.
I wrote a field report to American Honda motor co., detailing the issue.
They quickly responded with a new excitor (field coil) that had about 1/2 ohm less resistance than the OEM coils, and bumped up the part number suffix (revision).
The new coils solved the problem, and were available on a non-public-disclosed warranty program. All new factory bikes were refitted with these new coils.
And, that's my only claim to fame...