I tried my MTR3B with a 13.8 VDC battery, and on powering up the rig it immediately blew the fuse I put between the battery and the rig. I’m sure glad I used the 1A fuse as suggested in the 3B manual.
I’m no engineer, but I know the rig will suffer damage if powered by more than 12 VDC.
Greg
VE6KGH