I’m trying to decide how to setup the power block for my uBITX shack in a box. I’ve got 8 18650 cells and 2 of these boost-buck converter and here is the data sheet for the XL6009. I had been planning on setting it up with 2 packs in parallel of 4 cells 16.8v max, 14.8 nominal, and fully discharged ~11v. I’ll need a 13.8v and a 5v rail. The boost-buck converters could give me that with 2s4p as easily as 4s2p or I could get another cell and do 3s3p. I’m just not sure which would be a better option as far as efficiency etc.
Generally, for easy of design, you either want your buck or your boost to be in buck only or boost only mode 100% of the time. I’d choose a pack voltage that allows you to be in boost for your 13.8V and buck for your 5V regardless of your state of charge. Also, make sure you have UVLO because 18650s don’t like dropping too far.
I’d probably do 2s4p.