I want to use this powerbank :"VOLTCRAFT LiPo 30000 mAh" for my project which includes various sensors. My main goal is a stable voltage supply of 5V.
Now my question would be, if there would be any gain in stability of the 5V by using a step down converter, and make use of the ability of the powerbank to output higher voltages?
If so, how does the powerbank "decide" which voltage should be output
Ok, but, as a secondary goal, how important is battery life?
The cells in the powerbank will likely be 3.7V li-ion, and they may be wired in parallel with a step-up/boost converter to output 5V, 12V etc. That boost converter will not be 100% efficient, probably 80%~95%. If you then use a step-down/buck converter, that too will be only 80%~95% efficient, so overall, the combination of converters could be only 65%~90% efficient, and waste up to a third of the powerbank's capacity.