Wonder if someone could help me please, trying to get a 12v rail
From a 12s battery, so voltage will go down as the battery gets used
Do buck converters go down in voltage with the battery?
Just wondering best way to get a stable 12v from a variable power supply?
No, they have a feedback from there output to set the voltage. At least, 99,9% of them do. So they act as voltage regulator
You have to be careful with this sometimes, as a buck converter can completely drain and damage a
battery before it stops generating output - its not a battery management system.
Most buck converters require an input voltage that is at least a volt or two above the output voltage. A buck-boost converter may do what you want, or you could use a battery with a higher voltage.
I did assume 12s wasn't a typo. But if that's a 12s lipo indeed, be sure to use a converter that can handle that voltage.
Look for a "buck-boost regulator" or "DC-to-DC converter".
Of course, it needs to rated for at-least the required current.
If you do need "significant" current, note that you'll be drawing more current at lower battery/supply voltage. Power (Watts) is calculated as Voltage x Current. So, if the boost regulator is putting-out 12V at 1 Amp and the battery drops to 6V, the battery has to supply 2 Amps into the regulator to get 1 Amp out. (That's assuming 100% efficiency and in the real world you'll need a little more.)
You still get "extended life" from the battery but it drains faster the more the battery gets drained.
The efficiency of a switchmode buck regulator works in your favor. With 24V in and 12V out, you only have to feed-in half as much current as you get out. That extends battery life compared to using a linear regulator.