Nominal Voltage with 18650 cells and DC-DC step up board
The typical step-up converter (see the 2nd circuit, "Boost", on top right in: https://en.wikipedia.org/wiki/Boost_converter) actually has a permanent series diode so even at 12.6V input the load will still see ~ 12V (maybe 12.1 if it's a very good Schottky diode).