car generators/alternators  will go up to 16 volts if you are gunning  the engine.

I have  a link  for a dc to dc    somewhere someplace let me look for it.


 this is too much $$
and limits to 150 watts

https://www.trcelectronics.com/View/TRACO-Power/TEP%20150-2412WI.shtml


but  you need something like this