Voltage: at 14v it is very hard to be efficient.

• diodes loss: car alternator operate around 14v, with 0.6v for each diode pass you have 1.2v loss: almost 10% loss only this one.

• windings: you loose a lot of power over resistive loss in the winding because of the high current for the winding size (you could compute this loss with the winding gauge at a given current)

• Connections: it's very easy to loose a few percent of efficiency at this current and voltage: a 0.1 ohm connection at 65A would be 6.5v across, loosing almost 50% of the power!

• core loss: at higher speed core loss are probably increasingly important (big guess trying to explain the graph)

I think we could get a far better efficiency just by operating the same alternator at a higher voltage.