I'm working on a project involving a SBC, IR LED's and a screen. All of them together draws 3.5 to 4 Amps and voltage of-course is 5v. I'm hoping to run this using 18650 (3.7v), I have a couple of them lying around. I'm confused on what would give me the most efficiency - connecting batteries in series and using a buck converter or parallel batteries with boost circuit. Either way i'm not familiar these circuits, can anyone give me a lead.
parallell is usually easier with lithium batteries as their charge will self-balance. The efficiency you will get depends mostly on the DC-DC converter you use.
However: 6A (input current) is past the point where good boost converters start getting expensive, so using a series arrangement and a 4A or 5A (output current) buck converter may be easier.