I have a 12v DC marine battery that is 105AH I need to be able to convert this to 5v 40A to drive electronics. I most likelly will only use 16A but would like the capability to scale to 40A if needed. Any help would be appriecated.

 

This 40a load, is it one load , or several things that add up to 40 amps?
The reason I ask this, it will be easier to come up with several individual lower current regulators than 1 40a regulator.

 

Your better off using a buck converter ic like LTC3854 with external slave Mosfets can give upto 20A
http://cds.linear.com/docs/Datasheet/3854fb.pdf

otherwise use an LM317 and 2n3055 slave transistors

 

. . . is this 40a load, is it one load , or several things that add up to 40 amps?

. . . There are several modules that add up to one load I will explain it this way. Each light module is a 2Amp draw. There is a total of 8 light modules on each grid for a total of 16 amps. Then 3 of those grids together for a total of 48Amps. The controller takes around 2Amps this creates a need for a 50 amp device. I made a mistake saying 40 amps was needed it is more like 50 Amps.

 

You need some kind of synchronous rectification buck converter. You will need to parallel FETs to keep power dissipation manageable. This design is possible, definitely not trivial.

To be honest, it would be a lot cheaper just to get a 6V battery or batteries and strap in parallel as required to get the current you need.