Great explanation!

The reason for the inefficient iron core generator, mainly not to mess up with a readily available 220V single phase generator, by simply running a wire from it's output socket to a 1:4 or 1:3 transformer, so that when the resulting single phase AC passes through the rectifier, it gets within a reasonably efficient charging frame.

Yes, single phase & iron core transformer, both add to inefficiencies. So, what about directly connecting the high A/C line (220VAC) into the battery pack (after rectification)? Wouldn't that be comparatively efficient?

I know that a 3-phase PMA turning within the 50-60VDC Voltage frame is the most efficient way to go, but that would need some re-engineering work to an off-the-shelf generator.

It is preferable to either go for no retrofitting at all, or designing a completely custom-designed & built engine/generator.

To comply with the second route, any comments on the automotive diesel engine's retrofitting with the right PMA running at idle speed or just above idling speed! Wouldn't that be most efficient from both aspects i.e. the fuel consumption as well as battery charging? Also easier maintenance, widely available parts, cheaper, etc.....