So I'm building a simple 12v router UPS and I have trouble with one concept - Nominal Voltage. So the build is very simple, some common 18650 cells, DC-DC voltage step up board, and a BMS (battery management system) board to charge and discharge (safely) the 18650 cells. Connecting them all is super easy too. Issue is, I need to determine how many cells I want to use. The build guide I followed used two batteries, but the only BMS board I could get my hands on needs 3 cells. Easy enough to adjust my build, but my question about the cells being fully charged and the effect of the higher voltage.
What I could research is that 18650 cells have a nominal voltage of 3.6v, so with some easy maths we know that 3 cells (in series) will produce 10.3v nominal voltage for the whole circuit. Easy enough, I can still use the step-up board to boost the voltage to the desired 12v. thing is, if the cells are fully charged they will be at 4.2 v, which will result them all combined to provided 12.6v (too much). So what does nominal voltage really mean? and will the step up board still work when the batteries are fully charged because then it technically wont be stepping up the voltage, its already past 12v.