@tiefpunkt@social.tchncs.de low power, here is what I'd do! 1S, get a dc-dc isolator, B0515S, this will *3 the voltage, which is enough even at cutoff, BUT its noisy! The output is isolated, so you can put this in SERIES with the battery for *4, then I would regulate it back using a 7809 for a nice, clean supply! wrap it all in tinfoil and nobody should ever hear it! the isolators are typically rated to 1w, which is pretty good, and they can operate at the low 3V point (lots of switching boost converters wont) of the battery and are good to 5v (well more actually), which would be long after the battery has exploded from overcharging!