I have a battery powered system and I want to preserve the test batteries during setup and calibration. I want the system to use the setup batteries if they are present and use the test batteries if not.
The test batteries are either LiOn (nominally 7.2V) or NiMh (nominally 6.0V) and the setup batteries are NiMh (nominally 6.0V). I'd prefer a simple system, and also one that doesn't necessarily depend on the exact voltages and charge levels (as it's possible the setup batteries could be higher voltage than the test batteries in some cases).
I'm guessing the following PMOS circuit would work, provided that the Vgs(th) is higher than the max difference between Vtest and Vsetup. Getting a PMOS FET with Vgs(th) of about 4V seems about right.
It seems like a JFET circuit might work here too (but I've never used a JFET yet).
Will my circuit work? Is there a better way?