I’ve got one never tried a flex volt battery but works with 10.8,14.4 &18v so no reason a flex volt shouldn’t .
Nice small radio but I’ve had problems with control panel buttons stop working on occasion.
Was fixed under warranty but then happened again after it ran out. Sorted it myself by taking it apart cleaning it securing screws on control panel then putting back together. Fingers crossed it’s still working.
If I was buying a new radio I’d swerve it and get another, mates got a little festool one there really good.