As I said I can't run another alternator so that's out!
I am aware of the 24-12 Droppers and have used them for low current supply.
What I need is to charge a 12v battery, from the 24v system, whilst the engine is running.
I have looked at DC-DC Converters but, are they "Clever enough" to regulate their output current/voltage
to the needs of the 12v battery?