Thanks for all the advice.
Basically, I spend a lot of time away from a reliable mains supply but with access to vehicles (12V and 24V) and batteries (12, 24 and 28V).
I need a regulator that can regulate any of these to 12V so that all I have to do is connect the regulator to any of the available sources and know that I will always have a regulated 12V supply regardless of the input.
As the batteries are lithium, they don't like to be completely discharged, so the ability to disconnect the load if the battery vltage falls below 18V would be good.
As I want to supply anything up to a car radio, I will need something of supplying up to 10A.
I'm happy to build the device myself, I'm just trying to source some circuit diagrams from those more in the know.
Cheers,
Cam